Identification of Semiochemicals from Cowpea, Vigna unguiculata, for Low-input Management of the Legume Pod Borer, Maruca vitrata

Abstract

Cowpea, Vigna unguiculata L. Walp. (Fabaceae), is one of the most important food legumes grown on the African continent, as it provides an affordable source of dietary protein. Yields of cowpea are significantly reduced through damage by legume pod-borer, Maruca vitrata (Lepidoptera: Crambidae), caterpillars to flowers, tender leaves and pods. Semiochemical-based strategies are considered as environmentally benign and affordable for pest management, particularly on smallholder farms. In this study, we investigated the importance of cowpea flower volatiles as host location cues for egg-laying M. vitrata, and herbivore-induced plant volatiles (HIPVs) as M. vitrata repellents and natural enemy (Apanteles taragamae and Phanerotoma syleptae parasitoid) attractants. In oviposition choice assays, M. vitrata laid more eggs on flowering cowpea plants than non-flowering plants. Coupled gas chromatography-electrophysiology (GC-EAG) analysis using the antennae of female M. vitrata and an extract of flower volatiles collected by dynamic headspace collection revealed the presence of five EAG-active components that were identified by coupled GC-mass spectrometry (GC-MS) analysis as benzaldehyde, benzyl alcohol, acetophenone, a vinylbenzaldehyde isomer and (E)-cinnamaldehyde. A synthetic blend of the identified compounds, prepared using 3-vinylbenzaldehyde, induced M. vitrata to lay as many eggs on non-flowering cowpea as on flowering plants. The moths also preferred laying eggs on intact plants compared to M. vitrata-infested plants. As the emission of EAG-active floral compounds was determined to be lower in the headspace of infested cowpea flowers, the role of HIPVs emitted by M. vitrata-damaged leaves was also investigated. Of the compounds induced by larval damage, (E)-DMNT, indole, n-hexyl acetate, 1-octen-3-ol and linalool were shown by GC-EAG to possess electrophysiological activity. A synthetic blend of the EAG-active compounds, using racemic 1-octen-3-ol and linalool, significantly reduced egg numbers on flowering cowpea. Larval and egg parasitoids, i.e. A. taragamae and Ph. syleptae, respectively, of M. vitrata both preferred the Y-tube olfactometer arm treated with synthetic (E)-DMNT, whereas preference for racemic linalool and (E)-nerolidol was dose-dependent in A. taragamae. Our results provide the platform for the development of future semiochemical-based pest management strategies against M. vitrata on smallholder farms in West Africa.

Keywords: Cowpea; IPM; Legume pod borer; Maruca vitrata; Natural enemies; Semiochemicals; Vigna unguiculata.

Fig. 1

Mean (±SEM) number of eggs laid a) on flowering and non-flowering cowpea plants in cage choice bioassays (n = 5); b) on intact flowering cowpea plants and on those infested by M. vitrata larvae (n = 13); c) on flowering cowpea plants and on non-flowering plants treated with a blend of synthetic floral volatiles (n = 6); d) on flowering cowpea plants and on flowering plants treated with a blend of synthetic HIPVs (n = 11)

Fig. 2

Behavioural responses of 3-day-old naive mated female A. taragamae to different doses of synthetic (E)-ocimene, (E)-nerolidol, (RS)-linalool and (E)-DMNT in Y-tube olfactometer bioassays. N = 20

Fig. 3

Behavioural responses of naive mated female P. syleptae to synthetic (E)-ocimene, (E)-nerolidol, (RS)-linalool and (E)-DMNT in Y-tube olfactometer bioassays. N = 50