Topical Collection: Natural Enemies and Biological Control of Plant Pests

Natural enemies have an extensive history as biological control agents against crop pests worldwide. Predatory insects and mites, parasitic wasps and flies, pathogenic bacteria, fungi, and viruses have been used against crop pests with varying degrees of success. This editorial aims to highlight articles that reveal recent advances or discoveries in fundamental and applied research on natural enemies and biological control.

One article tested the effects of UV-absorbing (photoselective) nets on the dispersion potential of a syrphid (Sphaerophoria rueppellii Wiedemnann) and its prey, aphids [1]. The authors concluded that the deployment of these nets and augmentative releases of S. rueppellii were compatible strategies in IPM aphid control programs [1]. In a review of the literature on volatile and non-volatile plant-derived organic compounds as potential oviposition stimulants for aphidophagous predators, the author indicated that volatile compounds were effective oviposition stimulants for syrphids (e.g., Episyrphus balteatus DeGeer) more so than for coccinellids or chrysopids [2]. Organic compounds with low-to-moderate molecular weights and moderate-to-high vapour pressures were more effective oviposition stimulants [2].

An article revealed complementary action of a generalist predatory mite Anystis baccarum (L.), family Anystidae, and augmentative releases of a hymenopterous aphid parasitoid Aphidius ervi Haliday, family Aphidiidae, to control the foxglove aphid Aulacorthum solani (Keltennbach) on sweet peppers cultivated in greenhouses [3]. The predatory mite became established in the greenhouse and benefited from attacking other insects such as western flower thrips Frankliniella occidentalis (Pergande), family Thripidae [3].

One article stressed the importance of rigorous morphological investigations to accurately identify predatory coccinellids (Chilocorus species) before release for classical biological control of coccids [4]. The authors found that Chilocorus kuwanae Silvestri and Chilocorus renipustulatus (Scriba) were the same species. Consequently, releases of C. kuwanae in Europe and the Caucasus did not represent classical biological control, since the same species (misidentified as C. renipustulatus) were native to these regions [4].

To facilitate cost-effective augmentative biological control, research has centered on mass production and artificial diet development for another coccinellid, i.e., Stethorus gilvifrons, a specialist predator on tetranychid spider mites [5]. A diet consisting of sucrose, honey, royal jelly, agar, yeast, date palm pollen supplemented with Mediterranean flour moth (Ephestia kuehniella Zeller) eggs and 2,4-dihydroxybenzoic acid was most effective and supported the development and reproduction of S. gilvifrons in the laboratory [5].

Mirid bugs can be effective predators of soft-bodied plant pests [6], but ongoing research is necessary to test the compatibility of mirids with other control technologies. In one study, the mirid Nesidiocoris tenuis (Reuter) was exposed to a plant-derived toxin, methyl benzoate (MB) in laboratory and greenhouse bioassays [7]. MB is a volatile organic compound with acute toxicity to aphids, whiteflies, and spider mites. A 1% MB concentration had little lethal or sublethal effect on N. tenuis adults [7]. This research suggested that MB and N. tenuis could be implemented together for integrated control of pests. In another article, researchers tested for side effects of ds-RNA technology on the mirid N. tenuis [8]. Results indicated that ds-RNA (RNA interference) designed to target the αCOP gene of a gelechiid moth Tuta absoluta (Meyrick) had no lethal or sublethal effects on N. tenuis in laboratory bioassays. The authors suggested that dsRNA and N. tenuis could be combined for integrated pest management of T. absoluta [8].

Other research tested the compatibility of two intraguild predators, a coccinellid Cryptolaemus montrouzieri (Mulsant) and a chrysopid Chrysoperla carnea (Stephens), and their potential to control mealybugs [9]. Intraguild predation was common between the two predators, usually in favor of C. carnea. However, the addition of extraguild prey, i.e., mealybug nymphs and E. kuehniella eggs, significantly reduced encounters between C. montrouzieri and C. carnea in laboratory arenas [9]. One article examined the predation potential of another coccinellid Harmonia axyridis (Pallas) and an anthocorid Orius sauteri (Poppius) against the noctuid Spodoptera frugiperda (JE Smith) in functional response experiments in laboratory arenas [10]. The theoretical maximum daily prey consumption rate, instantaneous attack rate, and handling time estimates indicated that H. axyridis (rather than O. sauteri) was a voracious predator of S. frugiperda eggs and young larvae [10].

Several articles tested the utilization of entomopathogens as natural enemies of crop pests. Seven species of entomopathogenic nematodes in genera Heterorhabditis and Steinernema were tested against late larval stages and pupae of S. frugiperda in the soil [11]. Three nematode species Heterorhabditis indica (Poinar), Steinernema carpocapsae (Weiser), and Steinernema longicaudum Shen & Wang were most virulent in soil column and pot bioassays [11]. Another article tested the pathogenicity of a novel Metarhizium fungus, i.e., Metarhizium sp. BCC 4849, on tetranychid spider mites (Tetranychus truncatus Ehara and Eutetranychus africanus Tucker) and several insect pests [12]. The mortality of T. truncatus exceeded 80% five days post-inoculation with this fungus. The fungus caused 92–99% mortality of cassava mealybug (Phenacoccus manihoti Matile-Ferrero), bean aphid (Aphis craccivora Koch), Oriental fruit fly (Bactrocera dorsalis Hendel), and sweet potato weevil (Cylas formicarius F.) within three-to-six days, post-inoculation [12].

Metarhizium were also tested for compatibility with a braconid parasitoid Diachasmimorpha longicaudata (Ashmead) to control a tephritid fruit fly Anastrepha ludens Loew [13]. Metarhizium robertsii strain V3-160 and Metarhizium anisopliae strain MAAP1 caused moderate levels of mortality in D. longicaudata adults. However, M. robertsii had no adverse effects on D. longicaudata larvae developing inside treated hosts, A. ludens [13]. In another compatibility study, entomopathogenic fungi Beauveria bassiana (two strains) and trichogrammatid parasitoids in genus Trichogramma were evaluated for combined utilization against the European pepper moth Duponchelia fovealis (Zeller), family Crambidae [14]. Both B. bassiana strains had minimal adverse effects on Trichogramma species. The authors suggested that Beauveria and Trichogramma could be combined to control D. fovealis under natural conditions [14].

Trichogrammatids have a long history of utilization to manage the egg stage of lepidopteran pests, and articles in this collection also tested the capacity of Trichogramma species to control a gelechiid T. absoluta [15], a crambid Ostrinia furnacalis Güenée [16], and a noctuid Helicoverpa armigera (Hübner) [17]. Nine European Trichogramma species were evaluated as candidates for biological control of T. absoluta, which was introduced to Europe in 2006. Results indicated that three species, Trichogramma nerudai Pintureau & Gerding, Trichogramma pintoi Voegele, and Trichogramma cacoeciae Marchal, were just as effective as commercially available Trichogramma achaeae Nagaraja & Nagarkatti in laboratory bioassays [15]. In another article, research demonstrated that Trichogramma ostriniae Pang & Chen and Trichogramma dendrolimi Matsumura were capable of parasitizing O. furnacalis eggs [16]. Trichogramma ostriniae attacked young and old host eggs, whereas T. dendrolimi preferentially attacked young host eggs. Since host egg age had no effect on T. ostriniae parasitism rate, authors suggested that T. ostriniae would be the best candidate for biological control of O. furnacalis [16]. One final article tested the suitability of H. armigera as a rearing host for Trichogramma euproctidis (Girault) [17]. Authors determined that host egg age was important; young rather than old H. armigera eggs were best for optimal development and reproduction of T. euproctidis in the laboratory [17].

In conclusion, this editorial summarizes research collected on a topic of considerable interest and importance: natural enemies and biological control of plant pests. This topical collection demonstrates that researchers continue to search for new methods and techniques to manage natural enemies, singly or combined with other species, with the ultimate goal of integrating them into pest management systems that rely less and less on harmful pesticides. Future research should continue to discover new and improved technologies to mass produce and release natural enemies, manipulate their densities in agroecosystems, and conserve their populations within agricultural landscapes throughout the world.