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Seedlings are fed upon within the whorl. Larger larvae can cut the base of the plant. Mature plants suffer attack on reproductive structures. On tomato plants, buds and growing points may be eaten and fruits pierced. Maize leaves are eaten and the whorl (funnel) may be a mass of holes, ragged edges and larval frass. Young larvae skeletonize the leaf lamina in a typical 'window-pane' damage. 'Window-paning' is the most common damage symptom at early whorl; however, this is sometimes indistinguishable from damage that is due to other stem borers. Usually many young larvae will be present on the same plant, but normally one or two older larvae may be found on a single plant, as others will migrate and feed on neighbouring plants. Later larval instars make larger holes, causing ragged whorl leaves, and produce sawdust-like larval droppings, while fresh feeding produces big lumps. Badly infested fields may look as if they have been hit by a severe hailstorm. Fall armyworm can also destroy silks and developing tassels, thereby limiting fertilization of the ear. Maize plants may have the cobs attacked by larvae boring through the kernels. Damage to cobs may lead to fungal infection and aflatoxins, and loss of grain quality. At high densities, large larvae may act as armyworms and disperse in swarms, but they often remain in the locality on wild grasses, if available.
The literature on this pest is extensive (Ashley et al., 1989). This is in part due to the importance of maize, the importance of lepidopteran pests, the quest for alternative control methods following the development of insect resistance to pesticides, and the development of host-plant resistance breeding programmes. On maize, if 5% of seedlings are cut or 20% of whorls of small plants (during the first 30 days) are infested, it is recommended that an insecticide be applied (King and Saunders, 1984); on sorghum the pest threshold level is regarded as one (or two) larvae per leaf whorl and two per head (Pitre, 1985).
Control is largely achieved in the northern range through a winter kill by exposing larvae and pupae within the upper soil surface. Freezing temperatures cause high larval mortality. Therefore, clean cultivation and weeding are recommended. Some locally adaptable methods have also been tried such as soil, charcoal, ash, detergents, paraffin and engine oil. Various plant extracts are often included, such as chilli, neem, Tephrosia, Tithonia, Lantana and garlic. Handpicking egg masses and caterpillars has been tried in Africa. The efficacy of these methods is not well documented.
Harrison et al. (2019) have reviewed evidence for the efficacy of potential agro-ecological measures for controlling fall armyworm. These include (i) sustainable soil fertility management, especially measures that maintain or restore soil organic carbon; (ii) intercropping with appropriately selected companion plants; and (iii) diversifying the farm environment through management of (semi)natural habitats at multiple spatial scales. The 'push-pull' system has been shown to reduce fall armyworm damage due to various pests in maize (Midega et al., 2018). A study in Uganda showed that intercropping maize with food legume crops can reduce fall armyworm damage levels by 30% with bean, 21% with soyabean and 31% with groundnut (Hailu et al., 2018).
A large number of parasitic Hymenoptera, acting as larval parasitoids, have been reared from S. frugiperda, and many predators are recorded including recent work by Molina-Ochoa et al. (2003), Hay-Roe et al. (2016), Meagher et al. (2016), Birhanu Sisay et al. (2018), Shylesha et al. (2018) and Kenis et al. (2019); it appears that natural controls are of considerable importance. Natural levels of larval parasitism are often very high (20-70%), mostly by braconid wasps. Some 10-15% of larvae are often killed by pathogens.
The compound N-(17-hydroxylinolenoyl)-L-glutamine called volicitin was isolated from oral secretions of Spodoptera exigua larvae. When applied to damaged leaves of maize seedlings, volicitin induced the seedlings to emit volatile compounds that attracted females of the parasitoid Cotesia marginiventris. Mechanical damage of the leaves, without application of this compound, did not trigger release of the same blend of volatiles. Volicitin appears to regulate tritrophic interactions among plants, insect herbivores and natural enemies of S. exigua (Alborn et al., 1997).
Virus-based insecticides, which are mostly in the Baculovirus group, such as the multiple nucleopolyhedrovirus (SfMNPV) have potential for use in the management of fall armyworm (Behle and Popham, 2012; Gómez et al., 2013; Haase et al., 2015). They are highly host specific, non-pathogenic to beneficial insects and other non-target organisms, and are attractive candidates for integrated pest management. SfMNPV is specific to only fall armyworm. The pest is infected by ingesting the baculovirus. The symptoms of Baculovirus infection include appearance of blemishes, yellowing of the skin, and decline in feeding.
Metarhizium anisopliae and Beauveria bassiana have also shown efficacy against eggs and second-instar larvae of fall armyworm (Komivi et al., 2019). B. bassiana caused moderate mortality of 30% to second-instar larvae. M. anisopliae caused egg mortalities of 79.5-87.0% under laboratory conditions. Cumulative mortality of eggs and neonates with M. anisopliae reached as high as 96% with some fungal isolates. Bateman et al. (2018) reviewed products registered in 30 countries, 11 in the fall armyworm native range and 19 in Africa, and 50 biopesticide active ingredients were identified for use on this pest.
Azadirachtin (neem) is effective against fall armyworm. Oxymatrine and matrine (found in Sophora spp.) are reported to be effective against fall armyworm in the field and laboratory bioassays, respectively, in the Americas. Pyrethrins (from Chrysanthemum cinerariaefolium, formerly Pyrethrum) are effective against fall armyworm and registered in many countries, but have non-target risks that require mitigation. In Mexico, recent studies have shown that extracts of Couroupita guianensis and Myrtillocactus geometrizans could be good candidates for the control of Spodoptera due to their larvicidal activity. Also, extracts from Synedrella nodiflora and Lupinus stipulatus have shown to have biological effects on mature insects of the genus Spodoptera.
Spodoptera spp. resistance breeding programmes have developed field crop varieties with improved resistance, one example being maize (Mihm et al., 1988). One resistance mechanism that appears to be operating in maize is increased leaf toughness vis-à-vis a thicker epidermis (Davis et al., 1995).
Transgenic maize containing genes encoding delta-endotoxins from Bacillus thuringiensis kurstaki have been commercialized in the USA and Brazil. Vegetative insecticidal proteins (vip) have been isolated from Bacillus thuringiensis (Bt) during the vegetative phase of growth which show a wide spectrum of activities against lepidopteran pests, especially Spodoptera spp. (Estruch et al., 1996). Spodoptera spp. appear to be controlled by these toxins, but the development of resistance is a concern (Moar et al., 1995). Field-evolved resistance to the Bt maize expressing the Cry1Ab protein is reducing it efficacy in Brazil (Omoto et al., 2016). Fatoretto et al. (2017) reported that most Bt maize hybrids lost their ability to control fall armyworm within 3 years of introduction in Brazil.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: