Feeding and oviposition preference of Spodoptera frugiperda (Lepidoptera: Noctuidae) for selected poaceous plant species
Van Antwerpen, Leani
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The Fall armyworm (FAW), Spodoptera frugiperda Smith (Lepidoptera: Noctuidae), is an agricultural pest native to the Americas. FAW is polyphagous with 274 larval host plant species being reported. Crop losses are caused by the larvae which are mainly controlled with insecticides. FAW is difficult to control by means of insecticide applications since the larvae feed inside the whorl of maize plants and are able to develop resistance within a relatively short period of time. Cultural control methods such as the push-pull strategy and trap cropping were investigated to determine if these could be used along with insecticides to control FAW. In push-pull systems plant volatiles play an important part in insect-plant interactions. Gravid moths may be drawn to the ‘pull’ component of the system or ‘attractant’ which emit more attractive volatiles than the main crop. If the larvae are unable to surbvive on this ‘pull’ crop, it may be regarded as a trap crop. The aim of this study was to evaluate the oviposition and feeding preference of Spodoptera frugiperda on selected poaceous host plants including maize, sorghum, Napier grass, Brachiaria spp and Vetiver grass for inclusion in a push-pull strategy. Volatile compounds were collected via entrainment of intact plants. The collected volatiles were analysed by means of gas chromatography (GC) and three main components occurring in all four plant species were identified viz. decanal, squalene and limonene. Electroantennography (EAG) was used to evaluate and record the olfactory responses of FAW moths. Antennae were exposed to the three compounds identified with GC. It was determined that maize and Napier emit more volatiles during the evening. Since FAW moths respond to an array of volatiles, gravid female moths may not profoundly respond to specific volatile cues. No-choice, two-choice and multiple-choice bioassays were conducted in Petri-dishes in which leaf material were provided to larvae. Host suitability for larval development was evaluated by rearing larvae on leaf tissue of the respective plant species in an insect rearing room. Maize was most preferred by larvae and higher numbers of larvae were able to reach the pupal stage when feeding on maize and their development to the pupal stage was significantly faster than on other plant species. Larval preference for Vetiver, Brachiaria and Napier grass was low. In the feeding studies, no larvae survived on Vetiver and Brachiaria while survival on Napier grass was also very low. These three grass species could therefore be considered as trap crops if moths prefer these plants as oviposition sites over maize. Moth oviposition preference studies suggested that gravid female moths are not particular in choosing a plant for oviposition and they often laid eggs on plants that were unsuitable for larval development and on substrates such as pots and the oviposition cages as well. It was concluded that gravid moth’s oviposition choice may be overridden by the dispersal behaviour of larvae.