The influence of mechanical, visual and contact chemical stimulation on the behavioural phase state of solitarious desert locusts ( Schistocerca gregaria)

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We investigated the influence of mechanical, visual and contact chemical stimulation on behavioural gregarization of fifth-instar solitarious nymphs of the desert locust. The stimuli were applied in two 2×2 factorial experimental designs, the first with contact chemical and mechanical stimuli, and the second with contact chemical and visual stimulation. Stimulus treatments were applied for a 4–5 h period, after which the behavioural phase state of individual locusts was measured using an assay based on multiple logistic regression analysis of behavioural variables. Mechanical stimulation was provided by showering test insects with millet seeds, thereby excluding the possibility of contact chemical self-stimulation by repeated contact with the same objects. Visual stimulation consisted of the sight of crowd-reared locusts, while contact chemical stimulation was a dichloromethane extract of cuticular hydrocarbons from gregarious nymphs applied to the perch of the test insect. Mechanical stimulation was powerfully gregarizing, whether alone or in combination with contact chemical stimuli. Application of cuticular extract to the perch had no measurable effect on behavioural phase state, either alone or when presented with mechanical or visual stimuli. Visual stimulation alone partly gregarized test locusts. These results appear to conflict with other reports of the gregarizing effect of cuticular hydrocarbons and possible reasons for this discrepancy are discussed.