Host plant finding by Acraea acerata Hew. (Lepidoptera: Nymphalidae), the sweet potato butterfly: implications for pest management
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Phytophagous insects such as Lepidopteran species utilise both olfactory and visual cues to locate their host-plants used as mating or oviposition sites, shelter or food. Larvae of Acraea aceratafeed on sweet potato plant leaves causing more that 50 % loss of sweet potato tuber yield in some East African countries. Attempting to elaborate a management strategy to control A. aceratasuitable to a tropical resource- poor farming system, it was essential to investigate how the butterfly finds its host- plants. The results of a wind tunnel bioassay using glass-screened, muslin-screened and non-screened sweet potato plants suggested that sweet potato plant volatiles play an important role in attracting A. aceratato its host-plant. This was supported by both the distance moved by female A. aceratatowards muslin-screened plants (olfactory cues) and the percentage of butterflies which landed on the screen. Visual stimuli seemed to have a negative effect. The attractiveness of sweet potato plant volatiles to A. aceratawas later confirmed by the use of volatiles collected by headspace entrainment from sweet potato plantsThe main components of sweet potato plant volatiles were tentatively identified by GC-MS (Gas Chromatography coupled with Mass Spectrometry) analysis and electrophysiological responses were recorded for some of them. Compared to ethylbenzene, 3-carene and (-) trans-caryophyllene, 3-hexen-l-ol,(Z), a general green leaf alcohol, elicited far more substantial EAG (electroantennogram) responses in A.acerata.This result suggested that A. aceratamight well respond to a specific blend of volatiles made up of the different chemical components of sweet potato plant volatiles instead of one or two specific chemical components.Considering the important role of sweet potato plant volatiles in attracting A. acerata,a number of plants reported to be repellent to herbivorous insects were mixed with sweet potato plants and screened for repelling/disorienting of female A. aceratain olfactometer and wind tunnel bioassays. Two plant mixtures with opposite effects on the response of A. aceratato their volatiles were identified: sweet potato + Desmodiumplant volatiles were found to be more attractive to the butterfly than sweet potato plant volatiles alone, and sweet potato + onion plant volatiles which reduced considerably the attractiveness of sweet potato plant volatiles to A. acerata.As the trichomes of Desmodiumplants were reported to trap insects, a ‘push-pull’ management strategy for A. aceratainvolving the two intercrops was suggested: the intercrop sweet potato + onion plants would ‘push’ away ovipositing A. aceratawhereas the intercrop sweet potato + Desmodiumplants would attract the butterflies which would be trapped by Desmodiumtrichomes. The results of a preliminary field experiment carried out in Uganda suggested that the intercrop sweet potato + onion plants had a negative effect on the number of egg batches laid by A. acerataon sweet potato plants. There is, therefore, a need for comprehensive field experimentation of the whole strategy to validate these laboratory and field experimental findings.