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Résumé :
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1. The effects of predator species, aphid density, aphid age, diel period, and habitat complexity on the dropping behaviour of the pea aphid Acyrthosiphon pisum were assessed in a series of laboratory and field-cage experiments. 2. The presence of foliar-foraging predators significantly increased the proportion of aphids that dropped from alfalfa plants. In the absence of predators, less than 7% of the aphids dropped. Dropping more than doubled (14%) when one of three hemipteran predators, N. americoferus, G. punctipes or O. insidiosus, was present. Nearly 60% of the aphids dropped when the ladybird beetle, Coccinella septempunctata, was present. 3. Adult aphids showed a significantly higher propensity to drop than immature aphids, regardless of the presence or absence of predators. Aphid density had no effect on dropping behaviour. 4. Neither diel period nor habitat complexity had an effect on aphid dropping behaviour. Aphids were significantly more likely to drop in the presence of predators during either the day or night and from either early or late regrowth alfalfa. 5. A review of the factors affecting dropping behaviour, including those elucidated in this study, indicates that the propensity to drop from a plant is influenced by three factors: the risk of predation on the plant, the quality of the resource to be abandoned, and the risk of mortality in the new microhabitat.
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