Abstract
BackgroundAnimals can gain protection against predators and parasites by living in groups. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to group size (i.e. encounter effect), so that predators do not offset the encounter effect by attacking more members of the group (i.e. dilution effect). In this paper, we propose a novel mechanism by which prey insects could gain by producing decoys that act as multiple targets for predators or parasitoids if these decoys are recognised as preys or hosts and negatively affect the patch foraging strategy of these predators and parasitoids. Such a decoy mechanism could be present in aphid colonies in which aphid exuviae are recognised and attacked by Aphidiine wasps.ResultsWe conducted a behavioural study to evaluate the effect of exuviae on parasitoid patch residence time and egg allocation in experimental aphid patches with or without exuviae. We showed that exuviae are recognised and attacked at the same level as aphids when both are present in the patch. While parasitism rate was not significantly lower in patches with exuviae when the parasitoid left the patch, the time wasted by parasitoids to handle exuviae did influence the patch residence time. As a consequence, the attack rate on the live aphids was lower in patches that contain exuviae.ConclusionAphids had more time available to flee and thus each individual might gain protection against parasitoids by leaving their exuviae near and within the colony. These results demonstrate that the encounter-dilution effect provided by living in a group can be enhanced by extra-materials that act as decoy for natural enemies.
Highlights
Animals can gain protection against predators and parasites by living in groups
This study shows that extra-host material such as exuviae could play a role in group protection against parasitoids through a decoy mechanism
Aphids are under selection pressure (1) to reduce the amount of kairomone they produce and/or (2) to stay away from these traces by moving from them or by removing them. It follows that when aphids leave their exuviae in their colony they face a trade-off between their effect as decoys that disturb parasitoid patch exploitation and their effect as cues used by parasitoid females to locate the aphid patch
Summary
Animals can gain protection against predators and parasites by living in groups. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to group size (i.e. encounter effect), so that predators do not offset the encounter effect by attacking more members of the group (i.e. dilution effect). We propose a novel mechanism by which prey insects could gain by producing decoys that act as multiple targets for predators or parasitoids if these decoys are recognised as preys or hosts and negatively affect the patch foraging strategy of these predators and parasitoids Such a decoy mechanism could be present in aphid colonies in which aphid exuviae are recognised and attacked by Aphidiine wasps. The group provides protection when the probability of detection of the group does not increase in proportion to its size (i.e. encounter effect), and when the predator does not offset the encounter effect by attacking more members of the group (i.e. dilution effect). Experimental data on selfish herding are available for prey-predator relationships [5], but only a few studies have discussed host-parasitoid systems [6]
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