Global Advances on Insect Pest Management Research in Oil Palm

Abstract

Here, we review the advances in research on management of key oil palm insect pests globally, including defoliators, leaf/fruit scrapers, borers and sap feeders. The common oil palm pest management methods include synthetic insecticides, biopesticides, semiochemical lures, cultural practices, and integrated approaches. However, effectiveness, affordability, availability and impact of these methods on human and environmental health vary considerably based on the target insect and geographical location. The use of quarantine regulations to prevent the spread of invasive pests has also been applied with remarkable results. There are notable research advances in semiochemicals, bioacoustic detectors, nanotechnology, insect growth regulators, and entomophagy for better management of oil palm pests. We suggest the following research areas for improving effectiveness of oil palm pest management interventions: (i) exploration of semiochemical attractants for the majority of pests with no previous semiochemical work, and their integration in attract-and-kill devices laced with pathogenic microbes; (ii) expanding the application of digital sensing, predictive modeling and nano-technology in pest control strategies; (iii) developing effective technologies for mass trapping of edible insect pests for food or feed, especially among communities with a tradition of entomophagy; and (iv) strengthening regulatory frameworks for the management of quarantine oil palm insect pests.

1. Introduction

Oil palm, Elaeis guineensis Jacq. (Arecales: Arecaceae), is a multipurpose plantation crop that is extensively grown in the humid tropics across Asia, Africa and the Americas for palm oil and other uses such as sauces, soap, wine, fertilizer (ashes), roofing (leaves), building material (trunk), medicines (roots), and ornamental purposes [1]. About 40% of all traded vegetable oil globally is palm oil [1], and this oleaginous species has the potential to produce 4–20 times more oil per hectare of land compared to other oil crops [2]. As such, oil palm production has rapidly grown more than 35-fold from only 2 million tonnes in 1970 to 71 million tonnes in 2018 [2]. There are more than 140 palm oil importing countries of which India, China and the United States of America lead the market with more than nine, five and one million tonnes of palm oil imports in 2017, respectively [2].

Although a wide range of tropical climates and soils in about 20 countries that lie between 10° N and 10° S of the equator are conducive for oil palm cultivation, Indonesia and Malaysia account for more than 84% of oil palm production globally [1,2]. Palm oil production in Latin America, which accounts for 6.4% of global production, has increased by almost 60% in the last ten years to 4.6 million tonnes in 2020/21 [3]. In Africa, the oil palm belt runs from the southern latitudes of Guinea, Sierra Leone, Liberia, Ivory Coast, Ghana, Togo, Benin, Nigeria, and Cameroon to Equatorial Guinea and the Congo [4]. Oil palm requires maximum and minimum temperature ranges of 29–33 °C and 22–24 °C, respectively; total annual rainfall of at least 2000 mm; and acidic sandy, fine clay or silty loam soils [5].

Although peak palm oil yields of 12 t ha⁻¹ yr⁻¹ have been achieved in small plantations against a theoretically simulated yield potential of 18.5 t ha⁻¹ yr⁻¹, average oil palm productivity worldwide has stagnated around 3 t ha⁻¹ yr⁻¹ [6]. The suboptimal oil palm productivity is attributed to climatic factors such as rainfall and temperature, nutrient deficiency (especially potassium and nitrogen), unsuitable ground vegetation, soil characteristics (e.g., depth and structure), genotypes, pests and diseases, among other factors [6,7]. The intensive monoculture of oil palm makes it highly susceptible to a range of indigenous insect pests due to reduced crop biodiversity which supports natural enemies of the pests [7,8,9,10]. In this study, we review research advances in the management of key insect pests of oil palm globally, highlighting new research directions towards more effective management of the pests for improved oil palm productivity.

2. Overview of Key Oil Palm Pests and Their Managements

Oil palm is attacked by numerous insect species globally, of which 58 notable species with their destructive stages, distribution, life cycles and extent of damage are highlighted in

Table 1. Notable pests of oil palm in different parts of the world.

Type of Pest		Order		Family		Pest		Stage		Distribution		Life Cycle		Extent of Damage		Reference
Borers		Coleoptera		Curculionidae		Rhynchophorus ferrugineus Olivier		Larva		Native to Southeast Asia but with a recently expanded range to Europe, North Africa, and North and South America,		45–180 days		The attacked palm tree trunk is structurally weakened, making the plant liable to collapse.		[10,11,12]
						Rhynchophorus palmarum L.		Larva		North, Central and South America		45–180 days		The attacked palm tree trunk is structurally weakened, making the plant liable to collapse. The insect is a vector of the Red-Ring disease and is also associated with the Bud-Rot disease.		[11,12,13]
						Rhynchophorus phoenicis Fabricius		Larva		Central, eastern and southern Africa		45–180 days		The attacked palm tree trunk is structurally weakened, making the plant liable to collapse.		[10,11,12]
						Rhynchophorus quadrangulus (Quedenfeld)		Larva		West-Central Africa		45–180 days		The attacked palm tree trunk is structurally weakened, making the plant liable to collapse.		[10,11,12]
						Metamasius inaequalis Gyllenhal		Adult		Central and South America		–		Attacks the pruned frond bases.		[14]
						Rhynchophorus bilineatus (Montrouzier)		Larva		Eastern Indonesia, Papua New Guinea		–		Larvae live and feed inside oil palm trunk near the bud area. The attacked palm tree trunk is structurally weakened, making the plant liable to collapse.		[15]
						Temnoshoia quadrimaculata Scopoli		Adult		Democratic Republic of Congo and Ghana		33–52 days		Injury symptoms include premature withering of fronds and necrosis of the terminal shoot. Young palms can be killed by damage to the crown.		[16,17]
				Dryophthoridae		Sparganobasis subcruciata Marshall		Larva		Eastern Indonesia, Papua New Guinea		–		Larvae live and feed inside the basal part of the oil palm trunk making the plant liable to collapse.		[15,18]
				Scarabaeidae		Oryctes rhinoceros L.		Adult		Southern and Southeast Asia and the western Pacific Islands		115–162 days		Adults penetrate 10–50 cm down the center of the spear cluster to feed on juice from host tissue. In addition to the damage by the beetle, the burrows may provide secondary access by pathogens that can kill the palm.		[8]
						Strategus aloeus L.		Adult		South America		308 days		Burrow up to 50 cm in the soil just under the stem base of 1–3-year-old palms and then tunnel up into the base of the plant, sometimes moving into the stem above the soil line.		[19,20]
		Lepidoptera		Brachodidae		Sagalassa valida Walker		Larva		Brazil, Colombia, Ecuador and Venezuela		78–81 days		Root-boring results in premature frond death and poor rooting. Causes significant damage to the roots of young palms (2–6-year-old).		[8,21]
				Castniidae		Cyparissius daedalus Cramer		Larva		Brazil, Colombia, Peru and Venezuela		217–493 days		Later instars make shallow galleries between the petioles and the stem, causing premature frond abscission. In a few cases, palm death occurs when larvae bore the growing point of the palm. Oil palm trees become susceptible at about 5 years after planting.		[8,16,22]
				Crambidae		Pimelephila ghesquierei (Tams)		Larvae		West Africa		–		Two or three neonates penetrate the leaflets of the growing, unopened spears, forming galleries and mostly destroying the bases of young palms.		[8,17,23]
Defoliators		Coleoptera		Buprestidae		Taphrocerus cocois Bondar		Larva		Brazil		49 days		The larvae make mines in the fronds.		[24]
				Chrysomelidae		Alurnus humeralis Rosenberg		Larva/Adult		Colombia and Ecuador		334–532 days		This insect attacks spear leaves and young opened leaves.		[8,22]
						Gyllenhalius palmarum Maulik		Larva		Ghana and Nigeria		–		-		[8]
						Spaethiella tristis Boheman		Larva/Adult		Brazil, Colombia and Suriname		45 days		Adults feed on leaflets, making small longitudinal grooves. Larvae scrape only the undersides of leaflets. Damage can reach between 50–60% of leaf surface		[8,22]
				Scarabaeidae		Adoretus compressus Webb.		Adult		Southeast Asia		–		Adult feeds on leaflets of oil palm seedlings		[25]
						Apogonia expeditionis Ritsema		Adult		Southeast Asia		–		Adult feeds on leaflets of oil palm seedlings		[25]
						Leucothyreus femoratus Burmeister		Larva/Adult		Colombia		170 days		Adult cut irregular square or rectangular holes on leaves. Leaf consumption per adult is 13mm2/day. Larvae also feed on roots.		[21,26]
		Lepidoptera		Depressariidae		Acria meyricki Shashank and Ramamurthy		Larva		India		33 days		Early instars feed on green parenchymatous tissues of the leaflets from the underside, leaving a thin parchment-like upper epidermis undamaged, while the later stages defoliate the leaves.		[27]
				Elachistidae		Loxotoma elegans Zeller		Larva		Brazil, Bolivia, Colombia and Panama		100–124 days		Damage begins with the lower leaves and can reach all the levels. Infestation symptoms include defoliation of the apical part and the center of the leaves.		[21,28]
						Stenoma cecropia Meyrick		Larva		Colombia		74–89 days		Leaf consumption by each larva is 40–50 cm2		[21]
						Stenoma impressella Busck		Larva		Colombia, Ecuador, Guatemala, Honduras, Panama, Peru and Venezuela		60–67 days		A single larva consumes a mean of 40–50 cm2 of leaf material. In later feeding, the entire leaf tissue, except for midveins, is consumed.		[8,29]
						Antaeotricha phaeuneura Meyrick		Larva		Argentina, Brazil, Colombia and Ecuador		47 days		Larvae scrape the upper and undersides of the leaflet, causing progressive dryness, but the overall damage is minimal.		[7,22]
				Limacodidae		Acharia fusca Stoll		Larva		Colombia, Ecuador, Guyana, Honduras, Peru, Suriname and Venezuela		76–94 days		A single larva can consume 400 cm2 of leaf area in a few days.		[30]
						Darna bradleyi Holloway		Larva		Southeast Asia		44–48 days		Causes severe damage to oil palm canopy at population density of 30 larva per frond		[25]
						Darna trima Moore		Larva		Southeast Asia		60 days		Causes severe damage to oil palm canopy at population density of 10–20 larva per frond		[25]
						Euclea diversa Druce		Larva		Brazil, Costa Rica, Colombia, Honduras and Peru		61–88 days		A single larva can consume 50–60 cm2 of leaf area in a few days. Under favorable conditions, the pest can swarm and cause more damage		[7,22]
						Euprosterna elaeasa Dyar		Larva		Brazil, Colombia, Ecuador, Guyana, Mexico, Panamá, Peru, Suriname, Trinidad and Tobago, and Venezuela		64 days		A single larva consumes ~66 cm2 in a few days, and the pest causes ~80% loss of plant canopy.		[31]
						Natada subpectinata Dyar		Larva		Argentina, Brazil, Colombia, Costa Rica, Paraguay and Suriname		35–106 days		Damage is uniform and, in some cases, they damage the whole blade leaving only the midrib of the leaflet.		[7,21]
						Sibine fusca Stoll		Larva		Colombia and Peru		78–103 days		A single larva can consume 400-600 cm2 of leaflets.		[22]
						Talima straminea Schaus		Larva		Brazil, Colombia, Ecuador, Guyana and Mexico		120–160 days		Sporadic leaf scrapping, with minimal harm to the tree.		[7]
				Nymphalidae		Amathusia phidippus L.		Larva		Southeast Asia and the Malay Archipelago		60 days		Larvae are voracious feeders on the underside of the leaf, eating backwards towards the base from the tip of the leaf.		[8,32]
						Brassolis sophorae L.		Larva		Argentina, Brazil, Bolivia, Colombia and Ecuador		81–125 days		A single larva can consume 500-600 cm2 of leaf area in a few days.		[7,21]
						Opsiphanes cassina Felder		Larva		Central America, Colombia, Ecuador and Peru		59–77 days		An individual larva may consume up to 800 cm2 of foliar tissue		[8,21,33]
						Opsiphanes invirae Huebner		Larva		Brazil		59–77 days		Initially, the infestation occurs at the plantation edges, with subsequent spread to the entire area. Larvae disperse by passing from one plant to another, infesting all the area.		[7,11]
				Psychidae		Clania tertia Templeton		Larva		Indonesia and Malaysia		120 days		A single larva can consume 4.8–5.4 cm2 leaf area per day.		[34]
						Mahasena corbetti Tams		Larva		Southeast Asia		143–166 days		The severe defoliation can be lethal to infested trees and can cause 40–50% yield loss of oil palm in two years.		[35]
						Manatha conglacia Haettenschwiler		Larva		Indonesia and Papua New Guinea		–		Young larvae scrape the parenchyma.		[36]
						Metisa plana Walker		Larva		Indonesia and Malaysia		103.5 days		Causes high yield losses up to 43% in years		[37]
						Oiketicus kirbyi Guilding		Larva		Colombia, Costa Rica, Ecuador, Mexico and Trinidad and Tobago		235–320 days		First to third instars scrape the parenchyma but from fourth instar onwards, the larvae consume the entire leaf in circular patterns.		[7,8]
						Pteroma pendula Joannis		Larva		Indonesia and Malaysia		43–46 days		Cuts small holes which result in leaf desiccation with high infestation, with almost all fronds being affected and plantation dieback at severe infestation levels.		[25,38,39]
				Saturniidae		Automeris liberia Cramer		Larva		Central and South America		78–80 days		Larvae destroy the underside of the leaflets at any stage of the palm. Their stinging hairs are hazardous to nursery and landscape workers.		[7,21]
						Dirphia gragatus Bouvier		Larva		Colombia, Ecuador and Peru		60–166 days		A single larva can consume 400–600 cm2 in a few days.		[21,40]
		Orthoptera		Tettigoniidae		Segestes decoratus Redtenbacher		Nymph/Adult		Papua New Guinea		-		Often attack the younger fronds first, but dense populations can effectively defoliate the entire palm. Their damage is noticeable before the insects themselves are seen.		[8]
						Segestidea defoliaria Uvarok		Nymph/Adult		Papua New Guinea		-		Same as S. defoliaria		[8]
						Segestidea novaeguineae (Brancsik)		Nymph/Adult		Papua New Guinea		-		Same as S. defoliaria		[8]
						Sexava coriaceae L.		Nymph/Adult		Eastern Indonesia		-		Similar to S. defoliaria		[41]
Sap suckers		Hemiptera		Cixiidae		Haplaxius crudus Van Duzee		Adult		Colombia, Mexico and USA		45 days		Vector of lethal yellowing disease.		[8,21]
				Miridae		Carvalhoia arecae Miller and China		Adult		India		-		Occupies the leaf axils of the youngest fronds and attack the spear leaf, which develops chlorotic streaks in the newly unfolded frond.		[42,43]
				Tingidae		Leptopharsa gibbicarina Froeschner		Adult		Colombia and Venezuela		62–75 days		Vector of Pestalotiopsis fungal complex through piercing and sucking of sap from leaves.		[44]
						Stephanitis typica (Distant)		Nymph/Adult		New Guinea and Tropical Asia		50 days		Stylets are inserted through the stomata, rupturing cell walls, terminating in the phloem, and penetrating to a maximum of 600 μm.		[17,45]
Scrapers and Miner		Coleoptera		Chrysomelidae		Cephaloleia vagelineata Piceus		Larva/ Adult		Brazil, Colombia, Costa Rica, Guatemala, Honduras, Mexico and Venezuela		130 days		Adults scrape leaflets of young fronds before they are fully unfolded, forming fossae parallel to major veins; whereas larvae feed on surfaces of the rachis of spear leaves and young fronds causing necrosis.		[8,21,46]
						Delocrania cossyphoides Guérin		Larva/ Adult		South America to Panama		-		The larvae and adult beetles occur together on young foliage, both of them feeding on the abaxial epidermis of palm leaflets, starting near the midvein and extending laterally.		[47]
						Demotispa neivai Bondar		Larva/ Adult		Brazil, Colombia, Panama and Venezuela		270 days		Larvae and adults scrape green fruits causing wounds that turn greyish.		[22,48]
						Hispoleptis subfasciata Piceus		Larva/ Adult		Brazil and Colombia		104 days		Adult feeding makes grooves parallel to the midrib of the leaflet. Larvae mine inside the leaflet forming a gallery.		[8,21]

. Twenty-five out of thirty-six of the species which defoliate oil palm are lepidopteran larvae, dominated by limacodids, nymphalids, psychids and elachistids. Other oil palm defoliators are coleopterans and tettigoniids. Oil palm borers are dominated by larvae of curculionids in the genus Rhynchophorus, but it is also bored by adult scarabids and larval brachodids and castniids. Oil palm sap suckers are dominated by members of the hemipteran family, Tingidae. Leaves of the oil palm are scraped and/or mined by chrysomelids.

2.1. Defoliators

Oil palm leaves contain a high content of indigestible cellulose fibers and a low nutritional value, therefore, a restricted number of insect orders such as Orthoptera, Phasmatoptera, Lepidoptera, Coleoptera, and Hymenoptera have evolved as palm defoliators [8]. The larval stages of these insects adapt to the low nutritive value of oil palm leaves by developing relatively slowly, always for more than a month. For example, larvae of the leaf defoliator Leucothyreus femoratus Burmeister (Coleoptera: Scarabaeidae) takes 92–113 days to develop [26]; those of Acharia fusca Stoll (Lepidoptera: Limacodidae) take 76–94 days [30]; and those of Metisa plana Walker (Lepidoptera: Psychidae) take 72 days [37]. There are, however, some oil palm defoliators with short larval durations, e.g., Acria meyricki Shashank and Ramamurthy (Lepidoptera: Depressariidae) with approximately 20 days [27]. The damage by defoliators is exacerbated by dry conditions, probably due to suppression of the activities of natural enemies such as entomopathogens, parasitoids and predators during dry spells [8].

2.1.1. Nettle and Slug Caterpillars

The nettle and slug caterpillars (Lepidoptera: Limacodidae) are the most-widespread oil palm defoliators in all oil palm growing areas of the world, with at least 22 species reportedly reaching outbreak levels and attacking different parts of the crown [49]. These caterpillars are controlled using a variety of approaches. Damage by Euprosterna elaeasa, and other Neotropical lepidopteran defoliators also facilitates the spread of oil palm Pestalotiopsis fungal pathogen [29]. The lifecycle of E. elaeasa has nine larval instars which feed on the mesophyll until the fourth instar, and then on the entire leaf blade thereafter [31]. Euprosterna elaeasa is commonly controlled using synthetic insecticides, although insecticide-resistance has been noted [50]. Essential plant oils, notably from Cymbopogon martinii [51], insect growth regulators such as fenoxycarb, methoxyfenozide, pyriproxyfen, and tebufenozid [52], the nucleopolyhedrosis virus EuelNPV [53], and Bacillus thuringiensis Berliner (Bt) strains [52] have demonstrated toxicity and/or antifeedant activity against E. elaeasa at levels that are comparable or superior to conventional insecticides, under controlled trials, and are recommended for evaluation as components of integrated approaches against the pest in the field [44,53].

2.1.2. Bagworms

The bagworm family (Lepidoptera: Psychidae) which includes approximately 1000 described species in 300 genera and ten subfamilies is well reviewed by Rhainds and colleagues [54]. Embryonic development in bagworms is usually completed within a month under tropical and subtropical conditions. Upon hatching, phytophagous neonates either remain to feed on the mother host plant or disperse by wind, vehicles, animals, or humans, aided by silk threads which they secrete from their posterior openings. Before starting to feed, first instars construct a protective self-enclosing bag using plant tissue or organic and inorganic debris within which the larval development is completed. The destructive larval stage lasts for about 1–7 months under tropical and subtropical conditions. Prior to pupation, the larvae tightly attach the anterior portion of their bag onto a substrate. Most species of bagworms feed on a broad range of plants during their larval stages. Several species of bagworms have been reported attacking oil palm in Asia since the start of the plantation industry [49], and others such as Clania tertia Templeton and Manatha conglacia Haettenschwiler continue to expand their host range to become new serious pests of oil palm [34,36].

Mahasena corbetti Tams is one of the large bagworm species which is endemic to Malaysia, but it has spread across the Paleotropical region [54]. The pest reportedly causes 40–50% losses in oil palm yields in Malaysia and Indonesia if not managed, with severe infestation of preferred host plants such as Fabaceae and Arecaceae (including oil palm) being fatal [35]. Owing to its invasiveness and a wide host range comprising 37 plant genera from 21 families, M. corbetti is considered a quarantine pest in many countries such as India, Singapore, Sri Lanka and Malaysia, among others [55,56,57,58]. Adult males of M. corbetti are black to brown with white scales on some of them; while females are wingless, cylindrical, and creamy-white with either no or greatly minimized appendages [56]. Larval feeding causes yellowing and subsequent dying and falling of leaves [59]. Mahasena corbetti is highly prolific, with each female laying over 3000 eggs, hence making the pest efficient at creating outbreaks [55]. It is quite difficult to control large M. corbetti outbreaks using aerial sprays of contact insecticides because its larvae cryptically feed on the abaxial surface of fronds which the insecticides cannot easily reach [23]. Therefore, palm trunk injections with systemic insecticides are required to effectively control the pest. Application of Bt to control M. corbetti has also not been effective [60]. Though laborious, hand-picking of M. corbetti larval bugs which actively move during bright sunlight can be effective against the pest [56].

Other bagworms, such as M. plana and Pteroma pendula Joannis are significant oil palm defoliators across Southeast Asia [37,61,62]. Application of both Bt-based biopesticide and synthetic insecticides (chlorantraniliprole, cypermethrin and flubendiamide) in Malaysia using a motorized backpack mist blower caused 70–83% mortality of M. plana and were able to suppress the pest population for a month after treatment [62]. In addition, several species of hymenopteran parasitoids such as Apanteles sp., Dolichogenidea metesae (Nixon), Brachymeria carinata Joseph, Goryphus bunoh Gauld, Pediobius anomalus (Gahan), Apanteles aluella (Sumatra), Cotesia sp. and Glypapanteles sp.; as well as predators such as Callimerus arcufer Chapin (Coleoptera: Cleridae) and Cosmolestes picticeps Stål (Hemiptera: Reduviidae) reportedly attack the bagworms in Malaysia [38], indicating a high potential of their use in biological control of the devastating oil palm pest. However, further research is needed to develop efficient protocols for the mass production of these natural enemies. The success of using these natural enemies against the bagworms will also require addressing other bottlenecks such as changing the attitudes of farmers who prefer chemical insecticides for their fast action and effectiveness, despite their hazardous impacts on human and environmental health. The effectiveness of the biocontrol agents against the bagworms may also be hampered by hyperparasitism, e.g., Pediobius imbreus (Walker) (Hymenoptera: Eulophidae) against D. metasae [38] and their susceptibility to chemical insecticides [62], among other factors.

2.1.3. Other Defoliating Lepidopterans

Females of the palm moth Stenoma impressella Busck (Lepidoptera: Elachistidae) lay eggs on the adaxial frond surfaces [8]. Early feeding by S. impressella larvae damages the abaxial frond surface, except for secondary veins. In later feeding, the entire leaf tissue, except for midveins, is consumed. Rhysipolis sp. (Hymenoptera: Braconidae) was identified as the most important natural enemy of S. impressella in South America, but the rate of parasitism is only 7–18%. Synthetic chemicals, e.g., teflubenzuron, chlorantraniliprole and flubendiamide cause 95–100% mortality of S. impressella in oil palm plantations [8,63,64].

The palm king Amathusia phidippus (L.) (Lepidoptera: Satyrinae) is a common defoliator of oil palm in Southeast Asia [8]. The eggs are laid in a row, where larvae hatch in 6–7 days and voraciously feed on the underside of the leaf, from the tip towards the base. The larvae are greenish-grey to brown with longitudinal bands and a thick pile of reddish- setae, tufts of longer setae on the second and third abdominal segments, paired hornlike processes on the head and paired processes projecting from the anal segment. Chalcidoid wasps appear to be important parasitoids of A. phidippus [8].

The split-banded owlet Opsiphanes cassina Felder (Lepidoptera: Nymphalidae) is an important pest of oil palm in Central America, Colombia, Ecuador and Peru [22]. A natural enemy complex of hymenopterous parasitoids and a nuclear polyhedrosis disease apparently regulate populations of O. cassina. Its larval populations have been controlled by applications of carbaryl. Although the control of adults with insecticide-laced honey baits is reportedly effective against O. cassina, the technique could be hazardous to many kinds of non-target organisms [22]. The larva of another nymphalid species, Opsiphanes invirae Hübner has a bright green body marked by two thin longitudinal stripes of yellow-ochre, a pink head with two pointed extensions facing backward, and the last abdominal segment ending in a long, bifid and coniform tail [65]. In Brazil, the mass production of the parasitoid wasp Palmistichus elaeisis Delvare & LaSalle (Hymenoptera: Eulophidae) from alternative hosts has allowed inundative biological control of O. invirae populations in oil palm crops [66].

2.1.4. Leaf Beetles

Leaf beetles (Coleoptera: Chrysomelidae), with 37,000 described species and possibly up to 23,000 more to be described, are the third largest family in the order Coleoptera after Curculionidae and Staphylinidae [67]. Chrysomelids are small to medium-sized beetles, often brightly colored, boldly patterned, or metallic, with hairs or scales in some species. Several species of leaf beetles attack oil palms in different parts of the world (Table 1) and are controlled using different methods. The entomopathogenic fungus Metarhizium sp. was used to control Alurnus humeralis Rosenberg in Ecuador [68]. Dense populations of C. vagelineata have been observed to diminish after heavy rains [22,46]. Barrios et al. [46] found field-collected C. vagelineata to be infected by Lecanicillium lecanii and Metarhizium anisopliae, which could be developed into biopesticides against the pest.

2.1.5. Grasshoppers

Segestidea novaeguineae (Brancsik), Segestidea defoliaria Uvarok and Segestes decoratus Redtenbacher (Orthoptera: Tettigoniidae) are pests of oil palm in Papua New Guinea [8]. Meanwhile, Valanga nigricornis Burmeister (Orthoptera: Acrididae), Sexava nubila and S. coriacea (Orthoptera: Tettigoniidae) were reported on oil palm in Indonesia [41]. They often attack the younger fronds first, but dense populations can effectively defoliate the entire palm. Often, their damage is noticed before the insects themselves are seen [8]. The nymphs and adults of endoparasite Stichotrema dallatorreanum Hofeneder (Strepsiptera: Myrmecolacidae) are potential classical biocontrol agents against these grasshoppers [69].

2.2. Fruit Scrapers

The oil palm fruit scraper, Demotispa neivai Bondar (Coleoptera: Chrysomelidae) is an important pest in commercial plantations and distributed in Central and South American countries [48]. Adults of D. neivai are reddish-brown, oval-shaped, dorsally flattened, and convex laterally [26]. Adults damage oil palm fruits, with a consumption rate of 12.35 mm² d⁻¹ per adult on exocarp [48]. The scrapping by D. neivai feeding on the exocarp causes a gray corky appearance. The resultant drying of fruits affects palm oil production. The use of natural plant extracts such as Ricinus communis L., Citrus sinensis Oesbek, Nicotiana tabacum L., and Capsicum annuum L. are a valuable tool for controlling D. neivai [70]. Moreover, entomopathogenic fungal isolates of B. bassiana and M. anisopliae are effective against D. neivai with the potential to be used as biological control agents, hence reducing reliance on hazardous chemical insecticides [71].

2.3. Sap Feeders

Prominent oil palm sap feeders are true bugs (Heteroptera) in the families Miridae and Tingidae [8]. For instance, the spindle bug Carvalhoia arecae Miller and China (Hemiptera: Miridae) attacks young oil palms in nurseries [43,72]. Inorganic insecticides such as malathion 5% dust, phorate 10% granules, monocrotophos 0.15% spray, and lambda cyhalothrin 0.10% are effective in controlling the pest [73].

Among tingids, the lace bug Stephanitis typica (Distant) is the best-known sapsucker associated with palms in Asia through to New Guinea [8]. Females typically lay eggs concealed in a lipid substance and insert them deep in the abaxial surfaces of fronds. Nymphs hatch after about 12 days. Stephanitis typica populations surge during dry periods. Stylets are inserted through the stomata, rupturing cell walls and terminating in the phloem [17]. Nymphs and adults feed on the lower surfaces of leaves, causing whitish-to-yellowish spots on the corresponding upper leaf surfaces. The feeding on the lower leaf surfaces leaves dark brown or black marks. Chemical insecticides are commonly used to control outbreaks of S. typica, but their effectiveness is questionable [8]. Stethoconus praefectus (Distant) (Hemiptera: Miridae) an obligate predator, and, two egg parasitoids namely Erythmelus panis and Anagrus sp. (Hymenoptera: Mymaridae) have been reported as natural enemies of S. typica in India [74].

Another notable tingid associated with oil palm is Leptopharsa gibbicarina Froeschner [8]. It is also the main vector of the Pestalotiopsis fungal complex in oil palm in the Americas [52]. The females insert their eggs into leaf tissue along a major vein of the abaxial surface, usually covering them with excrement. Upon hatching, larvae mingle with the adults, maturing in 6 weeks. The adults live for more than a month. They are relatively more abundant in the dry season [75]. The bugs prefer upper and middle fronds, rarely attacking the older fronds. Leaf surfaces injured by lace bug feeding may be an entry point for pathogenic fungi, such as Pestalotiopsis spp. Natural enemies of L. gibbicarina include Neuroptera, ants (especially Crematogaster spp.) and B. bassiana [76]. When these are not effective in maintaining low populations of the lacebug, an option is to treat the palm with a systemic insecticide [8]. Martínez et al. [52] reported that novaluron, teflubenzuron, and triflumuron are highly effective against L. gibbicarina.

Besides insects, at least 31 species of tetranychid mites attack palms [8]. For instance, Retracrus elaeis Keifer (Acari: Eryophidae), feeds on the abaxial frond surfaces of oil palms in tropical America [75]. This mite has been associated with severe yellowing of palm leaves. Foliar spraying with sulfur reduces its severity [77]. Another mite recorded on oil palm foliage in the American tropics is the red spider mite Tetranychus mexicanus (McGregor) (Acari: Tetranychidae), but its damage is minimal [8].

2.4. Borers

A palm borer makes a tunnel by chewing or burrowing into the stem, crown, unopened inflorescences, flowers, fruits, peduncles, petioles, fronds or roots of the palm tree. The major oil palm borers belong to orders Isoptera, Coleoptera and Lepidoptera [16].

2.4.1. Palm Weevils

Among the coleopterans, weevils (Curculionidae) are characterized by an elongated rostrum or snout with mandibles at the distal end for chewing host tissue or excavating oviposition sites [78]. The larvae, which are the most destructive stage, are protected from most predators, parasites and external abiotic factors as they cryptically feed inside the host tissues. The success of palm weevil borers may be due to their specialization as borers within the Arecaceae and sometimes including other monocotyledons, such as sugarcane (Poaceae), banana (Musaceae) and pineapple (Bromeliaceae) [16]. Palm weevil borers fall into seven subfamilies: Dryophthorinae (Rhynchophorinae), Cholinae, Baridinae, Erirhininae, Petalochilinae, Scolytinae and Platypodinae. In particular, palm-associated members of the Rhynchophorinae are the most damaging to palms worldwide. Four tribes within the Rhynchophorinae, i.e., Rhynchophorini, Sphenophorini, Diocalandrini and Orthognathini are the major borers of palms in general [16].

Species in the weevil genera Rhynchophorus and Dynamis are most often referred to as ‘palm weevils’ and are relatively large insects, with adult length and width ranging from 3.5 cm to 5 cm and 1 cm to 2 cm, respectively, while the larvae are approximately 3.5–6.4 cm long and 1–2.5 cm wide [16]. Adults of Dynamis species are usually glossy black, in contrast to Rhynchophorus species which can exist in varying colors [11]. Dynamis and Rhynchophorus lay between 30 and 832 eggs during a 42-day oviposition period [11].

No species of Dynamis has been reported on oil palm. However, oil palm is attacked by Rhynchophorus palmarum, R. bilineatus, R. phoenicis, R. quadrangulus and R. ferrugineus [11,12,15,79,80,81,82]. The main pathway through which the weevils spread outside their native regions is through the movement of infested palms [83]. Early signs of oil palm infestation by weevils include notches at the base of fronds with frass and pupal cocoons, eccentric crown growth, holes at the base of the cut palms and symptoms resembling those of drought stress, e.g., wilting and yellowing [84]. Cryptic larvae and adults that spend their entire life inside the palm tree may destroy the interior of the palm causing its collapse. Although visual inspection may allow early detection of signs of weevil attack on palms, palm infestation with Rhynchophorus is difficult to accurately detect prior to serious damage to the apical meristem. The females are attracted to and deposit eggs in palm sheaths and stems, where they lay eggs in damaged parts along the trunk or in petioles. Upon hatching, the apodal larvae begin feeding towards the interior of the palm and migrate towards the crown region where the larvae primarily develop while voraciously feeding on and irreparably destroying tissues in the trunk and subsequently leading to tree collapse [11]. About 20 larvae are needed to cause a lethal infestation. A delay in destroying the palms allows weevils to emerge and spread to other palm trees [16].

Rhynchophorini are highly devastating pests. For example, the Gulf region of the Middle East, which accounts for nearly 30% of global palm production, has been threatened by R. ferrugineus since the mid-1980s [85]. The economic burden due to the eradication of 1 and 5% of severely infested palms of 259,172 hectares was estimated to range from $5.18 to $25.92 million, respectively. Besides this, the indirect losses would increase several folds. Further, it is also estimated that savings due to the curative treatment of palms in the early stage of attack at the above infestation levels and hectarage range from $20.73 to $103.66 million, respectively [85]. Considering the above economic loss, it is important that countries where oil palm is grown strengthen the on-going management programmes against palm weevils.

The major components of integrated pest management (IPM) programme for palm weevil control include surveillance of the pest; maintaining plant and field sanitation; trapping adult weevils; preventive chemical treatment of wounds; filling the leaf axils of young palms with a mixture of insecticide and sand; curative chemical treatment of infested palms; and cutting and burning of severely infested palms [82]. Adult populations of palm weevils can be monitored by pheromone traps, acoustic detection or infra-red systems [10,81,84]. Abandoning oil palm fields over time makes them reservoirs for the weevils [86]. Flood irrigation causes dampness at the base of the palm and its offshoots, creating a favorable environment for the weevils to lay their eggs [87]. The chemicals used to control palm weevils include methidathion, oxydemeton-methyl, carbaryl, cypermethrin, deltamethrin dimethoate, chlorpyrifos, fipronil, imidacloprid and trichlorphon [88]. Post-application monitoring is required to confirm successful control of the weevils which is characterized by cessation of oozing of sap from weevil-damaged parts [84]. In regions with very high relative humidity, deeply damaged stems are treated by stem cleaning followed by filling these cavities with wet sand mixed with pesticide dust and the treated parts with a polyethylene sheet to retain humidity [88].

Early detection of Rhynchophorus weevil infestation followed by insecticide treatment may help palms to recover. However, palms in the latter stages of attack exhibit extensive tissue damage in the region of the apical meristem, often harboring several overlapping generations of the borers. These palms are difficult to treat, and they usually die. The lethal nature of this pest, coupled with the high value of the attacked palm species, warrants early action against the weevils [10].

Another weevil attacking bases of pruned oil palm fronds is Metamasius inaequalis (Gyllenhal) (Coleoptera: Curculionidae) [14]. Pitfall traps and palm tissue baited with aggregation pheromone and treated with insecticides are reportedly effective in controlling M. inaequalis [8]. Additionally, Temnoschoita quadrimaculata Gly. (Coleoptera: Curculionidae) is a borer of oil palms in nurseries and young plantations in Ghana [89]. Inflorescences of older palms can be severely damaged, leading to significant tunneling through both dead and living tissue near the point of entry. Damage includes premature withering of fronds and necrosis of the terminal shoot. Young palms can be killed by damage to the crown and apical meristem by the activity of T. quadrimaculata [16]. The base-borer weevil, Sparganobasis subcruciata Marshall (Coleoptera: Curculionidae: Dryopthorinae) is also a lethal pest of oil palm in Papua New Guinea and Indonesia [15,18]. Although S. subcruciata attacks the bases of only mature oil palms older than 10 years at low incidence, the affected trees harbor hundreds of the weevil larvae, resulting in massive damage of the internal basal stem tissue and subsequent tree collapse.

2.4.2. Rhinoceros Beetles

Another destructive oil palm borer is the rhinoceros beetle Oryctes rhinoceros (L.) (Coleoptera: Scarabaeidae). The female lays up to 512 eggs in rotting stumps and rubbish piles. The eggs hatch in 8–12 days into white grubs taking 12–200 days to turn into pupae and then adults which can have a longevity of 100–270 days [90]. Young adults of O. rhinoceros bore and damage the crowns of healthy palms, penetrating 10–50 cm towards the center of the spear cluster and feeding on juice from host tissue. The major sign of palm infestation by O. rhinoceros is crushed tissue pushed out of the entrance of the burrow. The pest inflicts major damage to palms 1–3 years old (sometimes leading to death), while in healthy older palms the damage caused can be minor [16]. Field sanitation such as removal, burning, burial or destruction of dead standing palm logs, stumps and rubbish piles which are breeding sites for the beetles are helpful in managing the pest [91]. Although insecticides such as lambdacyhalothrin, cypermethrin, fenvelarate and chlorpyrifos are commonly used against O. rhinoceros, their effectiveness is limited due to the cryptic nature of the beetles inside the plant tissue [91]. A male-produced aggregation pheromone, ethyl-4-methyloctanoate [92], is widely used in mass trapping, monitoring and augmentation with biopesticides [93,94]. The use of biological control agents associated with O. rhinoceros is immense, including predators, parasitoids and entomopathogenic fungi, bacteria, and nematodes, but only a few biopesticides products are available in the market [91,95]. These reports highlight Rhabdionvirus oryctes as a landmark success story of classical biological control of O. rhinoceros. Similarly, several fungal-based biopesticides such as M. anisopliae and Beauveria brogniartii (Sacc.) Petch, and entomopathogenic nematodes such as Heterorhabditis sp. are marketed for the control of O. rhinoceros [95]. However, the bulk of reports on natural enemies of O. rhinoceros are largely about their identification and efficacy bioassays under controlled conditions, but with limited commercial field application. For biopesticide products in the market, their potency may last long and are relatively easier and less costly to apply compared to chemical pesticides, but their scarcity raises their costs way above that of conventional pesticides [94].

The rhinoceros beetle, Strategus aloeus L. is a pest in oil palm plantations in the Americas. During the replanting of old palm trees by new palms, adult S. aloeus colonize palm trees. Adults attack young palms by tunneling into the soil near the palm trees, boring their way into the meristem of the plant. The collection of the immature stages on dead palms [20] and chemical insecticides are the main methods of controlling S. aloeus [96].

2.4.3. Lepidopteran Borers

Aside from coleopterans, Cyparissius daedalus (Cramer) (Lepidoptera: Castniidae) is a major borer of oil palm in northern South America and the Amazon basin [8]. Each female lays about 265 eggs, and it takes about 17 days for larvae to hatch. The larval stage, consisting of 14 instars, takes up to 1 year and the pupal stage lasts about 35 days. The early instars bore into the fruits and peduncles of oil palm causing rotting of the affected parts. Later instars make shallow galleries between the petioles and the stem, causing premature frond abscission. In a few cases, palm death occurs when larvae bore the growing point of the palm. Oil palm becomes susceptible at about 5 years after planting. An egg parasitoid, Ooencyrtus sp. (Hymenoptera: Encyrtidae), was isolated from eggs of C. daedalus in Peru, where it occasionally regulates its populations. Scouting plantations, pruning and destroying infested fronds and rotten fruit stalks are considered to be good preventive measures against C. daedalus.

Sagalassa valida Walker (Lepidoptera: Brachodidae) is a small, brown-banded moth, whose larvae cause significant damage to the roots of young oil palms causing up to 70% yield losses [16,51]. The larvae consume the entire central core of the infested quaternary and tertiary roots, starting from their apex. The short-lived adults move between surrounding forests and oil palm crops; hence, the greatest damage is at the forest-plantation borders [97]. Insecticide application is recommended for infestation levels, where 20% of the primary roots are attacked [49]. Cultural control methods involve keeping and maintaining clean borders between the plantation and the surrounding forest [16]. Mulching with empty bunches prevents the caterpillar from getting into the roots [49]. Biological control with the entomopathogenic nematode Steinernema carpocapsae is reportedly effective against S. valida in the laboratory and field trials [16], but reports of its practical use in managing the pest are scarce. The presence of the predatory ant Pachycondyla harpax (Fabricius) (Hymenoptera, Formicidae) reduces infestation by S. valida [49].

The African spear borer Pimelephila ghesquierei (Tams) (Lepidoptera: Crambidae) is a pest of oil palm in West Africa [8]. Two or three neonates penetrate the leaflets of the growing, unopened spears, forming galleries and mostly destroying the bases of young palms [17]. The caterpillars hatch from eggs laid at the base of the spear leaf and bore into it. The fronds may break off where the rachis has been weakened. Damage by P. ghesquierei greatly impacts young palms in nurseries or in recent field plantings [23]. Collection and destruction of pupae have been recommended to prevent population build-up in nurseries or young plantations [23]. The pest mostly attacks stressed and/or shaded palms [23].

3. Selected Research Advances on Sustainable Management of Oil Palm Pests

3.1. Semiochemicals

Semiochemicals are chemical signals which meditate behavior of organisms such as attraction to mates or host plants, and avoidance of non-hosts or natural enemies by insects [98]. They are widely used in insect pest control, e.g., as lures for monitoring, mass trapping and attract-and-kill strategies. These natural chemical signals are largely species-specific and less persistent in the environment; therefore, they are harmless to non-target insects and do not pollute the environment.

The use of semiochemicals in oil palm pest management programs as lures, repellents and mating disrupters is gaining popularity [10,86]. For instance, males of palm weevils produce aggregation pheromones that are applied in weevil trapping [14]. Palm weevil pheromones comprise 8, 9, or 10 carbon, methyl-branched, secondary alcohols. The major components of the aggregation pheromones of R. ferrugineus, R. phoenicis, and R. palmarum are (4S,5S)-4-Methyl-5-nonanol (ferrugineol), (3S,4S)-3-methyl-4-octanol (phoenicol), and (4S,2E)-6-methyl-2-hepten-4-ol (rhynchophorol), respectively [14]. Fermentation host-plant volatiles such as ethyl esters and ethanol strongly enhance the attractiveness of pheromones [14]. However, the pheromones attract more weevils than host-derived volatiles alone [81]. Pheromone-based mass trapping using one trap per acre has been successful in controlling R. palmarum in oil palm plantations in Central and South America [99]. Semiochemical lures appear to be superior in detecting the occurrence of palm weevils over the use of host–plant infestation symptoms [82,83]. Most palm weevil attractants lure more females than males [81], which could be attributed to the higher sensitivity of female antennae to the odors than those of males [100].

Besides palm weevils, Oryctes rhinoceros males produce an aggregation pheromone, (4S)-ethyl 4-methyloctanoate, which is synergized with fermenting oil palm fruit bunches to attract males and females [92]. Trapping adults of O. rhinoceros using one trap per two hectares and serviced biweekly lowers the weevil damage by over 90% and is less expensive than the application of insecticides [101]. The pheromone trap efficiency improves when the traps are elevated above the oil palm canopy level [102].

Furthermore, the female produced sex-pheromone components of two nettle caterpillars that defoliate oil palm in Southeast Asia, Darna trima (Moore) [(S)-2-methylbutyl (E)-7,9-decadienoate and (E)-2-hexenyl (E)-7,9-decadienoate] and Darna bradleyi Holloway [(E)-7,9-decadienoate and isobutyl (E)-7,9-decadienoate] are synergistic attractants of males [103]. However, the application of these sex pheromones in the management of the nettle caterpillars is still scarcely documented.

The success of semiochemicals in pest management critically depends on the optimization of the technique to remove the target pests faster than they can reproduce [100]. This entails proper release mechanisms, catch retention mechanisms and proper trap densities and placement distances. The key limitations to the use of semiochemicals in pest management include lower efficiency in eliminating pests compared to conventional insecticides and costly equipment needed for their extraction and characterization.

Semiochemicals are also gaining prominence in pest control for their role in the dissemination of entomopathogenic microbes, e.g., fungi, bacteria and viruses through a mechanism termed attract-and-infect or auto-dissemination [104]. This concept has been attempted with the attraction of the oil palm pest O. rhinoceros and its infection with the entomopathogenic fungus M. anisopliae [49]. The medium-term research area is the evaluation of this technique in the dissemination of different pathogens against oil palm pests whose attractants have been identified. This can also be followed with other pests whose semiochemical attractants will be identified in the long run.

3.2. Bioacoustics and Other Signal Sensors

Insects that bore and hide inside trees exhibit different acoustical characteristics. Every activity of the insect produces some acoustic trait which is detectable using sensors. Oil Palm borers, especially larvae of Rhynchophorus spp. develop within the tree stem, damaging the vascular system and leading to the death of the infested tree. Early detection is normally a challenge as the infestation is concealed until the tree is severely damaged. The larval feeding/chewing activity and movements inside the palm tree produce sound with specific frequencies and spectra, which can help with the early detection of infestation for timely remedial actions to avert severe damage [105]. Manual filtering of external stimuli such as wind and ambient noise enables the detection of palm weevil infestation in an unshielded natural environment [106].

An acoustic R. ferrugineus detection system for field application in coconut accurately detected larval infestation by 97%, with a false-positive rate of about 8% [107]. Human and machine detection systems of R. ferrugineus larval infestation sound were 75% and 80% accurate, respectively [106]. The sensitivity was lower during the early phase of infestation (39% and 33%, respectively), and significantly improved as the larvae developed. Both larvae and adults of Rhynchophorus spp. can be detected acoustically [108]. Different types of acoustic detection systems have been used successfully to detect R. ferrugineus larval sounds in the laboratory and the field [109,110,111,112].

Besides acoustic sensing, Thermal Imaging Detection, which entails sensing temperature changes associated with pest infestation using an infrared camera, could be used in pest detection on the grounds that the temperature increases proportionately with the increase in the number of larvae inside the trunk [105]. Another promising automated pest infestation signal is the changes in volatiles emanating from infested trees. For instance, the odor of Rhynchophorus-infested trees produces a scented signal that dogs can be trained to detect but requires frequent retraining [111]. Moreover, it is possible to employ modern chemical sensors such as an electronic nose, which comprises a sensor array that is responsible for sensing the chemicals and algorithms which provide an analyzing software model in the system [112].

3.3. Entomophagy

Several insect pests are highly nutritious and are delicacies in many communities around the world, especially in Asia, Africa and Latin America. For example, Rhynchophorus species are among the most consumed insects worldwide because they are large, tasty, nutritious and abundant [8]. Rhynchophorus phoenicis and R. quadrangulus are a delicacy in African countries such as Cameroon, Ghana, Nigeria and Uganda and are rich in proteins, fats and minerals [113,114,115]. In Suriname and Ecuador, R. palmarum is a delicacy, which is eaten either raw or cooked [8]. Oryctes spp. are also widely consumed in Africa, South America and Asia [116,117,118]. Among numerous edible insects in Peru are larvae of C. daedalus which are gathered by the Ashaninka communities during seasonal outbreaks [118].

Based on the above accounts of harvesting wild insect pests of oil palm in some communities around the world, we envisage that promoting the culture of insect consumption in these communities could serve multiple purposes of crop protection and improvement of human nutrition. However, these edible insect pests are harvested from the wild using rudimentary methods. The practice of cutting and splitting up infested palm trees to extract edible insects from traditional collectors is destructive and unsustainable, hence necessitating the development of techniques for luring the insects into a trap for either consumption or captive mass rearing to produce the edible stage of the insects [81].

3.4. Host Plant Resistance

Host-plant resistance entails the intentional use of resistant crop varieties to reduce the impact of a pest on crop yield or quality. Four major steps are involved in developing and dissemination of pest resistant crop varieties namely (i) evaluation of crop germplasm for resistant genotypes; (ii) assignment of resistance phenomena to the desired categories such as antibiosis, antixenosis, and tolerance; (iii) introduction of genes responsible for resistance; and (iv) integration of resistant varieties into pest management strategies [119].

de Oliveira et al. [7] reported that oil palm genotypes influenced its susceptibility to defoliators, with interspecific crosses between oil palm and its close relative Elaeis oleifera (Kunth) such as genotype Deli × Lamé being more susceptible to the leaf-eaters compared to crosses involving different varieties of oil palm. The high susceptibility of the genotype Deli × Lamé was of great concern as the variety is popularly grown worldwide due to its high yields. Further research on the mechanisms of resistance of some oil palm genotypes to pests will be helpful in developing resistant varieties with additional good attributes such as high yield. This can be enhanced by transgenic breeding, which has already been initiated for the speedy development of oil palm varieties with desirable traits including pest-resistance [120]. Host plant resistance will lead to a reduction in pesticide use, hence reducing pesticide residues in the environment and leading to increased activity of beneficial organisms

3.5. Nanotechnology

Nanotechnology research and development involves understanding and controlling matter at the nanoscale (i.e., approximately 1–100 nm) for novel applications in different fields such as information technology, security, medicine, transportation, energy, food safety, environmental science and agriculture [121]. Although its utilization in the oil palm industry is still limited, nanotechnology has immense promise for early pest detection, monitoring of pest spread and improvement in pesticide formulations for more efficiency and reduced toxicological impacts [122]. For instance, encapsulation of pesticides with nanoparticles reportedly reduces the rate of application by 10–15 times compared to classical formulations, hence proportionately reducing the amount of pesticides used to control pests [123].

Although 90% of nano-based patents and products still come from only seven countries, namely, Germany, France China, Japan, South Korea, Switzerland, and USA, there are already several insecticides encapsulated with nanoparticles from silver, silica, copper, iron and carbon for the control of insect pests/vectors such as mosquitoes, beetles, weevils, aphids and white flies [124]. It is also hypothesized that nanomaterials can be combined with a global positioning system (GPS) and remote sensing to develop crop pest detectors and monitoring systems based on spectral images [121]. The longer persistence, enhanced transport and higher toxicity of nanoparticles may however result in harmful impacts on the environment such as contamination and detrimental impacts to non-target organisms [124], which need to be continuously evaluated.

3.6. Insect Growth Regulators

Developing compounds that provide more environmentally friendly insect pest control has been a challenge. Insect growth regulators (IGRs) are synthetic insect hormones that are employed as insecticides to regulate the populations of harmful insect pests by interfering with their growth and development processes such as molting. They are less toxic to non-target organisms [125]. For instance, fenoxycarb, methoxyfenozide, pyriproxyfen, and tebufenozide are lethal to E. elaeasa larvae, with more than 55% mortality [44]. Other IGRs derivate of benzoylphenyl ureas were also used for the control of L. gibbicarina [52]. Novaluron, teflubenzuron, and triflumuron are reportedly highly effective against L. gibbicarina nymphs [52]. The reports demonstrate that IGRs can be investigated and applied in the management of other oil palm pests. However, similarities in biochemical processes among vertebrates and invertebrates may result in the limited development of insect growth regulators [126], as there could be a possibility of the synthetic hormones interfering with the biochemical processes of non-target organisms.

3.7. Geospatial Predictive Modeling

The use of predictive techniques such as Geographic Information System (GIS), Remote Sensing (RS) and Global Navigation Satellite System (GNSS) to collect, map, and analyze data on the current and potential future distribution of insect pests is a growing agricultural practice in modern times [127]. Proactive identification of locations and/or seasons that are suitable for pest establishment or outbreak enables timely and appropriate decision-making on preventive or rapid response interventions, e.g., surveillance, monitoring, quarantine or curative control. Using ecological niche modeling approaches, Fiaboe et al. [79] successfully predicted the known distribution of R. ferrugineus, including the single North American occurrence point of Laguna Beach, California. Based on these models, the authors predicted all areas that are suitable for R. ferrugineus establishment around the world. This approach is worth emulating in the prediction of the spread and outbreaks of other oil palm pests for more effective management.

In Colombia, the use of GIS and spatio-temporal analysis concepts in the “campaign of defoliators”, have offered advantages in the implementation of IPM to monitor oil palm pest populations [50]. In this sense, the detection of foci or areas of regional influence, identification and categorization of damage patterns, use of different control methods, evaluation and forecast of insect movements, population demographic explosions, and localized application of chemical insecticides have been integrated into the GIS technology. This is generating regional action plans, demonstrating that control of defoliating insect pests can serve to generate dividends, which are reflected in the reduction in pest management costs.

4. Regulatory Management of Oil Palm Pests

The movement of infested palms plays a key role in aiding the spread of invasive pests to new regions [10,85]. For instance, the rapid spread of R. ferrugineus in North Africa was attributed to ineffective quarantine regimes, weak enforcement, and difficulty in early detection of the pest [128]. Therefore, strict pre- and post-entry quarantine regulations such as stopping the importation of palm offshoots from already infested countries are highly important in preventing the spread of invasive oil palm pests.

Regulatory strategies helped to prevent the spread of R. ferrugineus in Mauritania and Morocco, restricting it to the original foci of the infestation [128]. In 1996, Spain published an order detailing provisional measures for preventing the spread of R. ferrugineus in the country [129]. These included the prohibition of the importation of all Palmaceae from non-European Union (EU) countries, compulsory use of the EU Plant Passport for any movement of Palmaceae originating within the EU and eradication measures, including chemical treatments, pheromone trapping and destruction of infested specimens both in public and private gardens. In 2000, these restrictions were extended to all Rhynchophorus species; and referred to all specimens belonging to the Palmaceae family with an upper diameter of greater than 5 cm, with the exception of seeds and fruits [130].

The explosive spread of R. ferrugineus in Europe in 2004 prompted the EU to publish the Commission Decision 2007/365/EC on emergency measures for curbing its spread [131]. The decree prohibited the introduction and spread of R. ferrugineus into the EU and laid down strict requirements for importing plants that are susceptible to the pest. It required member states to adopt measures to protect themselves against the introduction and spread of R. ferrugineus such as conducting official annual surveillance and notifying the Commission and the other Member States about their findings. Penalties were set out that were to be applied in case of infringement. This decision was modified in October 2008 [132] and August 2010 [133]. The main modifications were: (i) palms to be only imported from areas where they have been grown throughout their life either in a country where R. ferrugineus is not known to occur, or in R. ferrugineus-free areas. Otherwise, palms should have been grown during a period of at least one year prior to export in a place of production subjected to official inspections certifying that no signs of R. ferrugineus presence have been observed; (ii) plants to be accompanied by a plant passport and, if originating from an infested area, they should have been grown for two years prior to the movement in a site with complete physical protection against R. ferrugineus, and no signs of its presence should have been observed during official inspections; and (iii) establishment of demarcated areas within the EU infested countries including the infested zone plus a buffer zone of at least 10 km beyond the boundary of the infested zone. Extensive monitoring and appropriate measures against R. ferrugineus aimed at its eradication should be carried out within these areas. The above measures helped in the successful eradication of R. ferrugineus in the Canary Islands [10] and Spain [128]. However, the pest has still been continuously detected within the EU probably due to (i) poor techniques for early detection of infested palms; (ii) shortage of a sound quarantine treatment against the weevil; (iii) the problem of involving homeowners within the process; (iv) the risks related to the employment of mass trapping in uninfested areas; and (v) the dearth of highly effective, environmentally safe plant protection strategies (biological control, semiochemicals, soft pesticides) suitable for public areas such as gardens, parks and avenues [10]. The lessons learned from the EU regulations against the spread of R. ferrugineus need to be considered in improving the efficiency of regulatory measures for the management of invasive oil palm pests in the region and developing regulatory frameworks for other oil palm growing regions of the world.

5. Conclusions and Future Research Directions

In this review, different control methods against key oil palm pests have been highlighted and advances in research on their management analyzed. We conclude that a large diversity of insect pests are serious bottlenecks to optimal productivity of oil palm globally and few have been well managed and studied. Therefore, knowledge of the biology and ecology of these insects is necessary to establish control programs, particularly for those species that can cause quantitative/qualitative damage during plant vegetative and reproductive phases. The traditional techniques used to manage these pests include:

• Application of synthetic insecticides;
• Biological control techniques such as entomopathogens (e.g., fungi, bacteria and nematodes), predators and parasitoids;
• Monitoring and mass trapping with semiochemicals;
• Cultural practices, and occasionally,
• A combination of the different techniques in an integrated pest management approach.

Although some of these approaches are effective, others may be ineffective, unaffordable, unavailable or negatively impact on human and environmental health. In addition to re-emphasizing the need for intensification of the integrated pest management approaches for more effective management of these pests, research and development effort is especially required in:

• Exploration of semiochemical attractants for the majority of pests with no previous semiochemical work, and integration of the chemical lures with microbial pathogens in the attract-and-infect technique;
• Expansion of the application of modern pest management techniques such as digital sensing, predictive modeling and nano-technology;
• Developing effective technologies for mass trapping of edible oil palm insect pests for food or feed, especially among communities with a tradition of entomophagy.;
• Strengthening quarantine regulatory frameworks and building requisite human resource capacity for their implementation in the management of invasive oil palm insect pests.

In the future, the challenge will be to implement new alternatives that promote biological control, identify new molecules that act as semiochemicals or growth disruptors, produce pest-resistant materials, and develop new biotechnological tools to produce high-quality products with greater efficiency in controlling oil palm pests in sufficient quantities at low cost. Moreover, due to their incorporation into agroecosystems, it is important to evaluate the compatibility of these methods with others for the conservation, inoculation or release of biological control agents.