Abstract
The behaviour of insects is dictated by a combination of factors and may vary considerably between individuals, but small insects are often considered en masse and thus these differences can be overlooked. For example, the cowpea bruchid Callosobruchus maculatus F. exists naturally in two adult forms: the active (flight) form for dispersal, and the inactive (flightless), more fecund but shorter-lived form. Given that these morphs show dissimilar biology, it is possible that they differ in odour-mediated orientation and yet studies of this species frequently neglect to distinguish morph type, or are carried out only on the inactive morph. Along with sex and age of individual, adult morph could be an important variable determining the biology of this and similar species, informing studies on evolution, ecology and pest management. We used an olfactometer with motion-tracking to investigate whether the olfactory behaviour and orientation of C. maculatus towards infested and uninfested cowpeas and a plant-derived repellent compound, methyl salicylate, differed between morphs or sexes. We found significant differences between the behaviour of male and female beetles and beetles of different ages, as well as interactive effects of sex, morph and age, in response to both host and repellent odours. This study demonstrates that behavioural experiments on insects should control for sex and age, while also considering differences between adult morphs where present in insect species. This finding has broad implications for fundamental entomological research, particularly when exploring the relationships between physiology, behaviour and evolutionary biology, and the application of crop protection strategies.
Highlights
Chemical ecology is used increasingly in crop protection and vector control programmes, and exploiting the orientation behaviour of insects seeking food, mates or oviposition sites has provided environmentally sustainable solutions for some of mankind’s major pest problems [1,2]
In this study we investigated differences in the response of C. maculatus adults to uninfested and infested host material and to the plant-derived repellent compound methyl salicylate [28], focusing on how these differences were associated with sex, age, morph and the level of infestation
The highest level of attraction was in inactive females (63.566.8%) and the lowest was in inactive males (44.265.7%). These results indicate significant differences in the odourmediated preferences of the inactive and active forms of C. maculatus when orienting towards host material, something which has not been previously reported
Summary
Chemical ecology is used increasingly in crop protection and vector control programmes, and exploiting the orientation behaviour of insects seeking food, mates or oviposition sites has provided environmentally sustainable solutions for some of mankind’s major pest problems [1,2]. In C. maculatus conditions consisting of low larval densities, plentiful food and high moisture content, intermediate photoperiods and lower temperatures, cause the inactive form to develop [20,21] This is darker in colour, has a shorter life-span and does not fly, but is more fecund, with the females laying around 50 eggs. The active morph may represent less of a control target in established populations but individuals could be key pioneers able to start new populations, so control of these individuals is an important preventative strategy With such different activity stages in the life cycle, behavioural responses to odour cues may not be uniform. This has particular implications for management of stored product pests which employ technologies based on odour repellence. Using an olfactometer assay and multivariate analyses, we showed that these factors interact in determining behavioural responses
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