Visual and olfactory cues used in host location by the blister beetle Hycleus apicicornis (Coleoptera: Meloidae), a pest of Desmodium (Fabaceae) species

Abstract

Desmodium spp. are leguminous plants mainly used as livestock fodder. In Kenya and neighbouring countries they are also used in a „push-pull‟ strategy by smallholder farmers to protect maize (Zea mays) against two major groups of pests, stemborers (Lepidoptera) and witchweed, Striga spp., by repelling the former away from the cereal crop and suppressing growth of the latter. However, smallholder seed production of Desmodium spp. is compromised by blister beetles Hycleus spp. (Coleoptera: Meloidae) which feed on the flower petals and adversely affect seed setting. The objective of this study was to determine the behavioural responses of Hycleus apicicornis (Guér.) (Coleoptera: Meloidae) towards olfactory and visual cues from host plants, and the impact of these cues on resource-finding by this beetle. The present findings should be of future use in developing a trapping system to manage this pest. A farmer questionnaire and a field survey were used to determine the pest status and host range of blister beetles. Desmodium spp. were identified as the most preferred followed by beans (Phaseolus vulgaris) and sweetpotato (Ipomoea batatas). Fewer farmers mentioned cowpea (Vigna unguiculata) and maize as additional host plants. Based on the questionnaire, the field survey and a literature review on potential host plants for blister beetles, D. uncinatum, I. batatas, P. vulgaris, V. unguiculata, Abelmoschus esculentus, and Ipomoea hildebrandtii (a local wild host) were chosen for subsequent experiments. Behavioural studies were undertaken to determine i) the impact of visual cues on their host locating and landing behaviours, ii) responses of beetles towards volatiles released by individual host plants, iii) responses of beetles towards extracts of flowers of host plants and two synthetic compounds and iv) orientation and settling preferences of Hycleus apicicornis towards different host plants in the laboratory. Beetles were significantly attracted to sky blue (with maximum spectral reflectance of 73% around 450 nm), which was preferred over other colours tested (black, red, yellow, white and green) and other shades of blue (turquoise and dark blue). Beetles also showed a high preference for odours of cut flowers of D. uncinatum, I. batatas and P. vulgaris, and 2-phenylethanol and phenylacetaldehyde but not for intact flowers in olfactometer bioassays. In the orientation and settling experiment, cut flowers of A. esculentus were attractive in addition to P. vulgaris, D. uncinatum and I. batatas. Volatiles were collected from A. esculentus, D. uncinatum, P vulgaris and V. unguiculata through air-entrainment techniques and analysed with gas chromatography coupled with mass spectrometry (GC-MS) to identify volatile compounds that may be inducing behavioural responses of adult beetles. Qualitative and quantitative differences were observed between and within the chemical profiles of the four host plant species. This study provides baseline information required for the development of a trapping system for the management of blister beetles on Desmodium spp. or other potential host crops by making use of visual and olfactory cues. A stronger response was obtained with the visual cues compared to olfactory cues suggesting that H. apicicornis may be using visual cues for long range while the olfactory is used at close range during host location.