Seasonal abundance of amaranth lepidopteran defoliators and the role of indigenous parasitoids and phenylacetaldehyde in their control in Nairobi County, Kenya
Omburo, Othim Stephen Tarmogin
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Amaranth is one of the African indigenous vegetables that is gaining popularity due to its nutritional, medicinal and economic values. Its potential as a source of food security in East Africa and most parts of the world has heightened demands for the once neglected crop. In Kenya and Tanzania, the Lepidopteran defoliators are important pests of the crop which can cause up to 100% yield loss. Little information on the population dynamics of these pests make their management difficult. Indiscriminate use of synthetic chemicals have raised environmental and health concerns creating a need for other environmentally safe and sustainable control strategies. The objectives of this study were to assess the seasonal abundance of the leaf webbers and their associated parasitoids, investigate the efficacy of Phenylacetaldehyde (PAA) as lepidopteran attractant and effect of amaranth lines on pest abundance. Damage by lepidopteran defoliators and performance of endoparasitoid Apanteles hemara on the two leaf webber species were also evaluated. Field experiments were set up at Kenyatta University and the International Centre of Insect Physiology and Ecology in a randomized complete block design with six replicates. Performance studies were carried out in the laboratory at ICIPE to assess the acceptability and suitability of Spoladea recurvalis and Udea ferrugalis to A. hemara. Abundance of leaf webbers (P=0.537) and leaf worms (P=1.0) and their associated parasitoids (P=0.083) did not differ significantly between the wet (Nov, 2014-Jan, 2015) and dry (Jul-Sep, 2014) seasons. Phenylacetaldehyde plots had significantly higher number of leaf webbers than the control plots (P=0.014). Phenylacetaldehyde traps also attracted significantly higher number of moths than the traps in the controls in both dry and wet seasons (P<0.001). Both S. recurvalis and U. ferrugalis were accepted by and suitable for the parasitoid A. hemara. Successful oviposition was significantly higher (P=0.018) when A. hemara was reared on S. recurvalis and exposed to the same host than when reared on U. ferrugalis and exposed to S. recurvalis. Rearing host did not, however, significantly affect successful oviposition (P=0.782) when tested on U. ferrugalis. The sex ratio of the parasitoid was female biased when reared on S. recurvalis while on U. ferrugalis, it was male biased. Parasitism rate was significantly higher (P=0.025) in S. recurvalis (64.4%) than U. ferrugalis (48.6%). Non-reproductive mortality was not significantly different from natural host larval mortality in both S. recurvalis (P=0.782) and U. ferrugalis (P=0.115). These results show that lepidopteran defoliators of amaranths occur throughout the crop cycle calling for efficient and adequate management strategies. Abuk2 amaranths were shown to exhibit certain levels of non-preference by these pests hence should be targeted by breeding programs to produce resistant lines. High levels of parasitism exhibited by A. hemara on both S. recurvalis and U. ferrugalis makes it a suitable candidate for biological control of these leafwebbers in amaranth production.