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Abstract
Pea aphids (Acyrthosiphon pisum) have been reported to produce winged offspring in the presence of predatory ladybirds. These offspring may leave host plants by flight after they have developed into winged adults. The inter-generational nature of this response raises the question about the chances of survival of aphids developing in attacked colonies. We studied the behaviour of predatory ladybirds on host plants by releasing adult 7-spot ladybirds (Coccinella septempunctata) on bean plants hosting either no prey individuals or colonies of 10 or 30 pea aphids. Interactions between predator and prey were recorded until the ladybird left the plant. Ladybird patch residence time increased with the number of aphids present on a plant but beetles generally left a plant before all aphids were eaten. The time budget of the ladybirds revealed a high proportion of time not spent in feeding activities. Predation rate was about one aphid killed per 10min residence time in both treatments with aphid-infested plants. Aphids that survived an attack by the predator or that were alarmed when a conspecific was attacked often emigrated from the host plant, and their number was of the same magnitude as the number of aphids killed by the predator. On average, pea aphid numbers at the end of an experiment were reduced to less than a third of the initial value. The results of the experiment show that attack by single ladybirds does on average not cause immediate extinction of small aphid colonies. The short patch residence times of on average less than two hours show that a predator individual that induces winged-offspring production in an aphid colony will not any longer be present in the colony when the induced offspring mature. To understand the adaptiveness of predator-induced wing development in pea aphids the probabilities of subsequent attacks on an aphid colony need to be investigated.
Highlights
Aphid life-cycles are complex and typically consist of several asexual generations alternating with a single sexual generation (Dixon, 1998)
We studied the behaviour of predatory ladybirds on host plants by releasing adult 7-spot ladybirds (Coccinella septempunctata) on bean plants hosting either no prey individuals or colonies of 10 or 30 pea aphids
Aphids that survived an attack by the predator or that were alarmed when a conspecific was attacked often emigrated from the host plant, and their number was of the same magnitude as the number of aphids killed by the predator
Summary
Aphid life-cycles are complex and typically consist of several asexual generations alternating with a single sexual generation (Dixon, 1998). The winged morphs develop in response to deteriorating conditions, i.e. when aphids are crowded or feed on plants of declining quality (Hille Ris Lambers, 1966; Dixon, 1998). It has been shown for pea aphids (Acyrthosi phon pisum Harris) that predatory ladybirds can induce the production of winged offspring (Weisser et al, 1999; Dixon & Agarwala, 1999). Producing winged offspring in the presence of a ladybird is an inter-generational response of the aphids and not a strategy to reduce the immediate risk of being eaten by the predator (Weisser, 2000). Do ladybirds stay in an aphid colony until the induced offspring have matured? If this is the case, the winged aphids that leave a plant by flight would escape from the same predator indi vidual that induced their phenotype
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