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Abstract
The nature of stimuli, emanating from other locusts, which are effective in inducing
gregarization in the desert locust was investigated. Isolated-reared fifth-instar
nymphs were subjected to tactile, visual and olfactory stimuli, presented singly and
in combination, and the effect on the behavioural phase state was quantified using
logistic regression analysis. Tactile stimulation provided by rolling paper spheres
proved to be highly gregarizing, whether presented alone or in combination with the
other stimuli. Olfactory and visual stimuli together caused partial behavioural gregarization.
Visual stimulation alone was weakly gregarizing after prolonged exposure, while olfactory
stimuli alone were ineffective. Nymphs and pre-reproductive and reproductive adults
of both sexes were also treated with synthetic adult male `aggregation' pheromone
blend (Torto et al., 1994, Journal of Chemical Ecology 20, 1749). No effect of this
blend was found on the behavioural phase state, even when visual stimuli were present.
Non-locust related stimuli, including wheat odour and flashing lights, were also tested
on nymphs. Neither induced any change in the behavioural phase state, indicating that
increased sensory flow is not a sufficient explanation for locust-induced behavioural
phase change.