Biological Control of Fall Armyworm, Spodoptera frugiperda

: The fall armyworm (FAW), Spodoptera frugiperda , is one of the most important invasive pests worldwide, resulting in considerable losses in host crops. FAW comprises two genetic strains, such as the “rice strain”, which prefers rice and other grass species, and the “maize strain”, which feeds upon maize and sorghum. Potential control measures are generally more applicable to the farmers who lack ﬁnancial assets to buy chemical insecticides or costly pure seeds. The adverse effects of pesticides on the ecosystem and human’s health and the development of resistance to insect pests have exaggerated efforts to ﬁnd an alternative strategy that is cost-effective, low-risk and target-speciﬁc. Therefore, biological control is widely considered as one of the most important options for insect pest management. This comprehensive review amasses the information on biological control in all phases of their development, including predators, parasitoids, entomopathogenic fungi, viruses, nematodes, bacteria, and biopesticides, with a special focus on their effectiveness against FAW. The ﬁndings regarding biological control are brieﬂy discussed in light of improving management programs of the invasive pest S. frugiperda .


Introduction
The fall armyworm (FAW), Spodoptera frugiperda, is the most important pest worldwide, resulting in considerable yield losses in maize.FAW was first reported in 1797 as a devouring pest endemic to subtropical and tropical regions of America.It belongs to the family Noctuidae under the order Lepidoptera and was first reported in the African continent [1].FAW is a devastating pest that damages 186 plant species belonging to 42 families.Poaceae, Fabaceae, Solanaceae, Asteraceae, Rosaceae, Chenopodiaceous Brassicaceae, and Cyperaceae are mostly affected.It results in about 58% yield loss in maize [2,3].FAW is known to feed voraciously on more than 350 plant species, especially maize, rice, and sorghum, which might cause significant agricultural losses worldwide [3][4][5].The first confirmed reports of FAW invasion were documented from West Africa in early 2016, and then spread throughout sub-Saharan Africa and Southeast Asia [1,6].Now, this pest has been spread in more than 109 countries [3].In India, it was first identified in May 2018, causing major losses to farmers in Karnataka and other southern Indian states [7].FAW was first discovered in Nepal in May 2019 through morphological and genetic identification approaches [8].
FAW is an important notorious pest which attacks maize and various other crops belonging to the family Gramineae [9].It is a polyphagous insect pest with over 80 host plants that causes severe damage to cereal crops and vegetables [1,10,11].The young curl of leaves, ears, and tassels have been preferred, resulting in significant loss to maize crop [12].FAW travels approximately 500 km before starting oviposition [10].A single generation of FAW moths can disperse more than 500 km distance from the emergence location, owing to the wind, until they are sexually mature [11,13,14].
FAW is comprised of two genetic strains: the "rice strain", which prefers rice and other grass species, and the "maize strain", which feeds primarily on maize and occasionally on sorghum [15].When FAW arrives in large numbers, especially with an offensive effect, it is determined to pose a long-term and damaging threat to many important crops, as the surrounding circumstances provide a comfortable environment for a variety of host plant species pre-favorable weather conditions for reproduction in various areas [1,4,16].
Biological control strategies are more appropriate to farmers who do not have the financial capability to purchase chemical insecticides and expensive seeds [17].Microbial formulations are available in the market that are made from pathogens, arthropod natural enemies, and are more profitable in agricultural systems [3,18].Recently, the microbial formulation production costs have been significantly reduced because these are mainly mass produced in liquid medium [3,19].The repetitive use of synthetic pesticides in the field may prove detrimental to humans and the environment, have increased input cost, and, furthermore, initiate resistance and resurgence [20][21][22][23][24][25].The larvae of the FAW caterpillar are deeply embedded in the leaf curls and corn ears, resulting in control failures.However, it comes to feed on plants at night or dawn and twilight [26].

What are the Biological and Biorational Pest Control Options?
The easiest way to manage the insect pest outbreak is to formulate a self-propelling and self-perpetuating system for the restoration or stimulation of self-sustainable biological control tactics.Biological control agents, such as predators, parasitoids, and pathogens, were supplied to keep close synchronization of the community of other organisms.Trichogramma and Telenomus are the most active biological control agents, which parasitized the FAW eggs and other key pests [27,28].Parasitoids are closely associated with one of the pest stages and have a higher level of specificity [29].Predators are rarely linked with a specific insect pest.They feed on the prey with a lower trophic level of specificity than other animals, e.g., the earwigs, ladybird beetles, podisus, and orius prey on different life stages of FAW.Entomopathogens include bacteria, fungi, viruses, protozoans, and nematodes that are the main cause of disease in insects.FAW was infected by several entomopathogens, including Bacillus thuringiensis (Bt), Metarhizium anisopliae, Beauveria bassiana, and Spodoptera frugiperda multiple nucleopolyhedroviruses (SfMNPV) (Figure 1).

Biological Control agent: a contemporary release against FAW
Telenomus or Trichogramma wasps are among the best options for potential development of biological control programs targeting FAW eggs [33], although FAW egg mass scale thickness could lower Trichogramma parasitoid performance on this host [34].Area-wide releases are made at tactical points (ranging from 20 to 40 per hectare) under 03-day intervals to obtain a constant presence of adult wasps for achieving proactive egg parasitism [35].This improved system is employed early in the season to limit the season-long spread of FAW in egg tracking and destruction [33].Synchronization in parasitoid release and the existence of FAW egg masses determine the efficacy of applied biocontrol [36].However, physically searching for FAW egg masses is a more tiresome activity compared with pheromone traps, regarded as the most effective approach to track the arrival of the FAW moth [37][38][39].
In the Natural Control of FAW, the importance of another beneficial insect Maize fields (in the Caribbean and America) include a diverse range of beneficial insects [40].Chelonus insularis is one of the major biological control agents that parasitize the eggs of FAW under natural conditions [41].However, FAW eggs hatch into the larvae and release the parasitized adult from the larva of FAW [41].Trichogramma and Telenomus wasps are smaller and less competitive than Chelonus wasps; thus, they have a better chance of surviving [41].The parasite larvae's food consumption is steadily reduced, with less than 10% of the biomass eaten by the borehole larvae [42].The presence of young larvae in the FAW release zone does not necessarily mean that there has been a failure in the biological control of FAW [42].Deployment of a conservation technique will sustain the natural biological control agents for obtaining a self-perpetuating and self-propelling organized agro-ecosystem.More information is given on the role of Chelonus in IPM [43,44].
In addition to C. insularis, many other parasitoid species are also observed in suppressing populations of FAW larvae [45].Campoletis flavicincta has been extensively used to suppress the FAW population [46].Insect predators consume both larvae and eggs that necessarily maintain the FAW population to a sub-economic level [47].For example, during the maize crop cycle, the predatory earwig Doru luteipes (Scudder) lays its eggs within the maize whorl [47].Doru luteipes nymphs consume 8-12 larvae per day, while adults consume 10-21 S. frugiperda larvae daily [47].The comprehensive demonstration of biocontrol is shown in Figure 1.Microorganisms, such as Baculovirus or Bacillus thuringiensis, should be considered in situations where the presence of biocontrol agents is not yet optimal and pesticide applications are needed [45,48].
To control the increasing pest population of S. frugiperda in America, mass breeding and release of predators and parasitoids have been used to manage other pests [2,[37][38][39] (Tables 1 and 2).In Sub-Saharan Africa, the implementation of classical biocontrol due to its high cost is required by the Government to control S. frugiperda [37].Native parasitoids having a better level of parasitism have been observed from different vicinities of SSA [33,[50][51][52].The best way to control FAW is augmentative biocontrol, releasing predators to overcome the increasing pest population of FAW [37] (Table 1).In America, Trichogramma parasitoids have been used to efficiently control the eggs of S. frugiperda [10,39] (Table 2).Scientists at ICIPE in Kenya and Agboyi et al. [33] observed that Trichogramma and Telenomus parasitoid can efficiently augmentative biocontrol against S. frugiperda.Before FAW neonates emerge, parasitoids (Trichogramma and Telenomus) are introduced into maize fields, search for FAW egg masses, and lay their eggs on them to limit the FAW population at the egg stage [36].
In Africa, Lepidopterous species have been parasitized by C. luteum, the lepidopterous species including Crypsotidia mesosema (Hampson), Spodoptera exigua (Hübner), Prophanti ssp.Spodoptera exempta (Walker), Condica capensis (Guenée), and Cydia ptychora (Meyrick) [55].Coccygidium luteum has been reported in Africa and many other countries, such as Nigeria, Madagascar, Kenya, Guinea, Congo, Ethiopia, Namibia, Mauritius, Rodrigues Island, Tanzania, Uganda, Somalia, South Africa, and Cameroon [33].Coccygidium luteum is a solitary koinobiont parasitoid that belongs to the Braconid subfamily Agathidinae, which includes more than 46 species [56,57].As biocontrol agents, Agathidinae species in this subfamily are not well known regarding their efficacy against insect pests and are rarely studied [58].In China, C. luteum controls the eggs of many Spodoptera species [59,60].The comprehensive demonstration of biocontrol is shown in Figure 1.Parasitoids can complete several generations in 90 days, leading to the emergence of early-maturing varieties of maize in West Africa [61].Populations of natural enemies are affected due to the occurrence of variation in parasitism [62].In the United States, parasitism levels were lower on average than previously reported levels, i.e., 15.5% [63], 35% [64], 8.1% [65], 28.3% [41], 18.3% [66], and 13.8% [67].Agboyi et al. [33] reported parasitoids of 10 different species from the various localities of Ghana and Benin.These species are Trichogramma spp.Meteoridea cf, Charops spp, Drino quadrizonula (Thomson), Metopius discolour (Tosquinet), Telenomus remus, Chelonus bifoveolatus (Szpligeti), Coccygidium luteum, Pristomerus pallidus (Kriechbaumer), and Cotesia icipe [33].Introduction, conservation, and augmentation are the three basic techniques for promotion of a biological control system in an ecosystem.Hymenoptera insects act as egg and larval parasitoids that are collected from FAW-infested areas.Further assistance might be given to stabilize the system either through inoculative or inundative releases.This manipulation chiefly increased the effectiveness of natural enemies.Extremely abundant species are mostly used in biological control programs.They prey on small arthropods, such as aphids, thrips, whiteflies, mites, and lepidopteran eggs [70] Ground beetle Calosoma calidum (Fabricius) Carabidae Coleoptera After mating, the females lay their eggs on the soil's surface or just below it.Before pupation in the ground, the immature stage goes through 3 instars [71] Spined soldier bug Podisus maculiventris (Say) Pentatomidae Heteroptera Lepidopteran larvae are the primary food source for nymphs and adults.It pierces its prey and injects a toxin that causes it to become paralyzed in a short period.The predator sucks out the prey's internal fluids, killing it [72] Assassin bug Pristhesancus plagipennis (Walker)

Reduviidae Hemiptera
The genus Zelus is the most common killer bug in maize.Females lay their eggs in clusters on plant leaves or even on the ground.The nymphs are wingless and look like adults [73] Pirate bug Orius sauteri (Poppius) Anthocoridae Hemiptera An important parasitoid.Moth eggs, aphids, and small Lepidoptera larvae are all prey for O. sauteri [74] Table 2. Parasitoids against Spodoptera frugiperda.

Scientific Name Family Order Description Reference
Trichogramma pretiosum (Riley) Trichogrammatidae Hymenoptera Egg parasitoids, used in the control of eggs of FAW and Helicoverpa spp. [75] Trichogrammatoidea armigera (Nagaraja) Trichogrammatidea Hymenoptera Egg parasitoids, used in the control of eggs of Helicoverpa armigera and FAW [76] Telenomus remus (Nixon) Scelionidae Hymenoptera Egg parasitoid, shows more than 45% parasitism level [31].Telenomus remus completes its life cycle within 12-13 days at 25 • C, and its female parasitizes more than 250 eggs in [77] [75] Chelonus insularis (Cresson) Braconidae Hymenoptera Egg-larval parasitoids, The female Chelonus lays eggs inside the eggs of fall armyworm, and the larva, which is parasitized, steadily curtails down its feeding until its death happens [78] Chelonus bifoveolatus (Szpligeti) Braconidae Hymenoptera Egg-larval koinobiont endoparasitoid, mostly attack the insects belonging to the family of Pyralidae and Noctuidae.The parasitoid attacks by ovipositing into the host eggs [66] [51] Campoletis sonorensis (Cameron) Ichneumonidae Hymenoptera Larval parasitoids, females lay eggs within the first, second, and third instars of the FAW, and the larva completes its life cycle by feeding on the pest's internal contents.For the parasitoid, the third-instar larvae are the most favorable stage [78] Charops sp.Ichneumonidae Hymenoptera Larval parasitoids, used to control FAW population on the larval stage.[33] Cotesia icipe Braconidae Hymenoptera Larval parasitoids, also Lepidopterous larvae and eggs of FAW have been parasitized by C. icipe [79], and the larval instars of 1st and 2nd stage have been parasitized too [80]. [81] Habrobracon hebetor (Say) Braconidae Hymenoptera Larval parasitoids, under laboratory conditions, a small wasp that has been used to combat the pearl millet head miner, often targets FAW larvae. [33] Coccygidium luteum (Brullé) Braconidae Hymenoptera Larval parasitoids internally attack larvae belonging to the Noctuidae family [82].An idiobiont ectoparasitoid Bracon sp. of C. luteum hiddenly attacks the larvae of cereal stored products and cereal stem borers and is also used against S. frugiperda [31,50,83,84]. [33]

Winthemia trinitatis (Thompson) Tachinidae Diptera
Larval parasitoids, the female lays her eggs near the head of a fifth-or sixth-instar FAW, making it difficult to detach.The parasitoid's larvae enter the larva's body, delaying the pupation and resulting in up to 30% parasitism.
Hatching takes place, and the young larvae emerge.When these larvae come into contact with a host and invade the host's body, parasitism occurs. [50] Lespesia archippivora (Riley) Tachinidae Diptera Larval-pupal parasitoids, on the back end of the caterpillar, the female oviposits.Lespesia archippivora has three instars that feed on the host caterpillar, and when they reach maturity, the parasitoid emerges from the larva and pupates in the soil. [87]

Entomopathogenic Viruses
Virus-based insecticides among microbial control agents within the Baculovirus group have been recognized with higher potential as bioinsecticides for growth and development because of their specific characteristic nature, higher virulence against the host, and increased safety towards vertebrates [88,89].Two variants of Baculovirus have been studied to control S. frugiperda, named granulovirus "SfGV", which is Betabaculovirus, and multiple nucleopolyhedroviruses "SfMNPV", which is Alphabaculovirus.However, the SfMNPV has a good ability to manage FAW [90][91][92].SfMNPV is specific to FAW larvae.The pest becomes infected by ingesting contaminated (maize leaf) food in natural conditions [93].After ingestion, polyhedral inclusion bodies (PIB) dissolve in an alkaline midgut, while infectious virions are released.The comprehensive demonstration of biocontrol is shown in Figure 1.These virions cause infection in epithelium cells of the midgut and start multiplying in the nucleus [10].Further, these viruses are distributed in the body cavity and start infecting other tissues, i.e., tracheal matrix, adipose tissue, epidermal, even including Malpighian tube, salivary glands, and blood cells [94], which leads to its death after ingestion in 6 to 8 days compared to a healthy caterpillar; an infected one with nucleopolyhedroviral only eats 7% of food [94].In Baculovirus infection, the main symptoms include yellowing of the skin, the appearance of blemishes, and a decrease in feeding [94].Usually, infected larvae start moving upper parts and are attached to the plants in a hanging-down position upon their death, while some prolegs remain attached to plants [94].Such larvae are soft with dark color and found with released body fluids rich in polyhedrons, aiding the further spread of the virus [94].Virus efficacy and speed of killing FAW larvae depends on larva age at the time of infection, ingested virus amount, and its virulence with current climatic conditions, i.e., temperature, humidity, and solar radiation [95,96].In addition to this, time of spray, equipment type, and formulations are influential in the efficiency of the virus [95,96].

Entomopathogenic Fungi
Entomopathogenic fungi (EPF) can infect several insect species on different stages at a wide range, causing epizootics under certain natural conditions [97].Fungal spores of such EPF species infect through integument, and then multiply within the insect's body.After multiplying, EPF releases certain toxins that destroy tissues and the insect dies.The introduction of epizootics depends upon climatic conditions and the frequency of insect contact [97].When insects become infected with EPF, they become discolored (cream, green, reddish, or brown), stop eating, and, finally, die as a hard calcareous cadaver, in which the fungus begins to sporulate [98].Moisture has an important role for fungi as a biocontrol agent.Among the potentially beneficial fungi against insect pests, Beauveria bassiana, Metarhizium anisopliae, and Nomuraea rileyi are the most commonly used for Spodoptera control [98].FAW larvae are susceptible to B. bassiana compared to other lepidopteran pests [99].In vitro studies showed 30% and 87% mortality when B. bassiana and M. anisopliae was applied on second instars and eggs, respectively.

Entomopathogenic Bacteria
Bacillus thuringiensis (Bt) Berliner is one of the most widely used biopesticides for insect control [100].These bacteria are Gram-positive, soil-dwelling bacteria that aid in producing crystal proteins known as delta-endotoxins, which are naturally insecticidal [101].Only a few Bt products are effective against FAW available in the market for managing lepidopteran pests [101].
FAW has increased susceptibility to Bt aizawai and Bt thuringiensis [100] compared to Bt kurstaki, which is efficient against numerous lepidopteran pests [102].There are few limitations in its wide adoption and use, including UV susceptibility of endotoxin, inability to reach pest for inducing toxin consumption, and the high cost of production [102].Various research groups are trying to determine more effective Bt strains against FAW.In contrast, variation in susceptibility towards different Cyt toxins (also known as Cry toxins) has been observed among populations of FAW [103].To control FAW, Bt-based biopesticides must be considered throughout the selection process in various locations.Seven Bt strains were highly effective among nineteen in screening against second-instar FAW larvae at ICIPE in Africa, causing 100% mortality within 7 days of post-treatment, having lethal time mortality (LT 50 ) with standard ranges of 2.33 ± 0.33 and 6.50 ± 0.76 days.Bt-based biopesticides have been considered for mass production through fermentation technology using solid-, semi-solid-, or liquid-state fermentation [104].Not only Cry toxins, but also FAW show susceptibility towards vegetative insecticidal proteins that could be usually seen in Bt culture supernatants [105].

Entomopathogenic Nematodes EPNs
Entomopathogenic nematodes, i.e., Steinernema feltiae, Steinernema carpocapsae, Heterorhabditis indica, and Heterorhabditis bacteriophora, are used as effective biological control agents [106].EPNs are eco-friendly and play an essential role in suppressing soil-dwelling insect pests, notably armyworms [106].The susceptibility rate of FAW to such beneficial nematodes is 23,000 per sq.ft. to target mature and young larvae [10].Early morning or late night are the most efficient times to apply beneficial nematodes because they are more active at these times and are less exposed to ultraviolet (UV) light, which helps them find armyworm larvae more easily [107].
In a petri dish, 280 infective juveniles of Steinernema sp. can kill 100% of third-instar FAW, while 400 infective juveniles of H. indica kill 75% of FAW [108].Usage of hydraulic spraying jets, which require 100 filtrating mesh elements, could decrease the concentration of infective juveniles of H. indica and Steinernema species up to 28% and 53%, respectively [10].The comprehensive demonstration of biocontrol is shown in Figure 1.Molina-Ochoa et al. [109] informed earlier that S. carpocapsae and S. riobravis efficiently manage FAW at the prepupal stage.According to various researchers, EPNs with resistant maize silk could increase FAW mortality at the prepupal stage [110].These nematode species, S. carpocapsae, S. glaseri, and H. indica, have shown compatibility with various commercial insecticides under laboratory conditions [110].Heterorhabditis indica shows increased efficacy when mixed with lufenuron against FAW [111].Moreover, a compatibility evaluation study of insecticides or biopesticides with EPNs is necessary before recommending it to use an IPM for FAW [111].

Botanical Pesticides
Local farmers have claimed that botanical extracts from local plants are beneficial [10].Botanical pesticides are a better alternative to synthetic insecticides, which could be more harmful to the environment, increase consumer cost, and delay recovery [112][113][114][115] (Table 3).Such pesticides are also responsible for increased pest resistance [116][117][118][119]. Few botanical extracts include Chrysanthemum cinerariifollium, Jatropha curcas, Nicotina tabacum, Milletia ferruginea, Phytolacea docendra, and Croton macrostachyus, which could be used as insect pest control [120] (Table 3).Around 50 botanical pesticides were found to be registered for controlling FAW in more than 30 countries (Table 3).Among those, 23 are recommended for field trials and bioassays [2,121].Solaris 6 SC ® was shown to be the most effective insecticide against immature fall armyworms, followed by botanical extracts of garlic and neem, as well as detergent [10].
Botanical pesticides caused 80% mortality under laboratory conditions [122].According to different reports and studies, these botanicals are effective against FAW [123,124] (Table 3).Neem extracts have shown 70% mortality in FAW [125,126].Eucalyptus urograndis was found to be more helpful in saving maize from pests [124].The seed powder of Carica papaya was discovered as an efficient chemical insecticide [127].Neem oil containing 0.17-0.33%concentration reduces FAW damage in maize [123].
Botanical insecticides are target-specific, non-hazardous for the environment, and safe for natural enemies as compared to chemical pesticides [128].Thus, their application promotes FAW natural parasitism up to 60% in comparison with pesticide-treated areas [41].A recent study conducted in Ghana recommends farmers to use intercropping technique during the first nine weeks of crop establishment because foliage of a crop remains too soft for FAW neonates; therefore, moths are attracted by crop to lay eggs [137].Under this duration, intercropping encouraged natural biological control agents to establish under the prevailing conditions.It resultantly checked the FAW population [13].Push-pull technology (PPT) is now suggested for FAW management [36].International Centre of Insect Physiology and Ecology (ICIPE) introduced this method to control stem borers in maize.The push-pull technology comprises intercropping maize with drought-tolerant greenleaf Desmodium intortum (Mill.)Urb., while Brachiaria cv Mulato II is planted as a border crop surrounding the intercrop.Desmodium plays an important role in protecting maize crops by repelling moths away with the emission of semiochemicals [16].At least 80% of FAW infestations can be eliminated using this technique [16,36].In Uganda, FAW infestation levels on maize using PPT were 36-38%, compared to 95% when single cropping was used.PPT is better than maize-legume intercropping for controlling FAW infestation [138].Botanical or biopesticides are recommended as an alternative to hazardous synthetic insecticides, such as pyrethroids and organophosphorus compounds, which can influence and interfere with environmental conditions, and increase expense, resurgence, and insect resistance.

Conclusions
Spodoptera frugiperda is a worldwide invasive pest, causing significant losses in host crops.FAW comprises two genetic strains: the "rice strain", which prefers rice and other grass species, and the "maize strain", which feeds primarily on maize and occasionally on sorghum.Small and marginal farmers that lack the financial resources to purchase chemical pesticides or expensive seeds are more likely to benefit from potential control methods.Maintaining natural antagonist diversity and intensity is crucial for biological control strategies.It may be accomplished by providing favorable conditions for biological control agents near crops.Shelter crops, alternate food resources, and multiplication favoring environments for such biological species are all important steps in regulating the population of FAWs.Growing Mexican sunflower or Crotalaria on crop edges in maizegrowing areas is a good way of boosting beneficial insect biodiversity, even if they are not related to FAW, and achieve landscape management goals.The push-pull technology
(Source: adopted from Fall Armyworm in Africa: A Guide for Integrated Pest Management).