Abstract
Grouping together is a commonly observed anti-predator strategy. Possible anti-predator benefits of aggregation include the encounter/avoidance effect for visually hunting predators and the dilution effect, together dubbed attack abatement. Possible costs opposing the dilution effect are easier detection of aggregated than scattered individuals. The benefits of attack abatement, and opposing costs after group detection, are poorly understood for chemosensory predator-prey interactions. We tackled this issue by assessing the aggregation behavior of spider mites Tetranychus urticae under predation risk emanating from predatory mites Phytoseiulus persimilis. We examined whether adult spider mite females aggregate more tightly when perceiving predator cues (traces left and eggs), representing graded risk levels, and whether grouping enhances survival in physical predator presence. The spider mites aggregated more tightly and were more active in presence than absence of predator cues. Grouped spider mites were less likely and later detected and attacked than scattered spider mites. Moreover, encounter and attack of one group member did not increase the risk of other members to be attacked, as compared to scattered spider mites. To the best of our knowledge, our study is the first rigorous documentation of the adaptive benefit of tightened prey aggregation towards a purely chemosensorily hunting predator.
Highlights
Grouping together is a commonly observed anti-predator strategy
The first objective of our study was determining if adult T. urticae females respond to predation risk posed by cues of P. persimilis with tighter aggregation and, if so, if the aggregative response varies with the level of predation risk
The inter-individual distances of the spider mites varied significantly with presence/absence of predatory mite cues on the leaf discs (Fig. 1; generalized estimating equations (GEE): Wald ӽ32 = 17.368, P = 0.001) but remained constant across time and treatments
Summary
Possible anti-predator benefits of aggregation include the encounter/avoidance effect for visually hunting predators and the dilution effect, together dubbed attack abatement. The benefits of attack abatement, and opposing costs after group detection, are poorly understood for chemosensory predator-prey interactions We tackled this issue by assessing the aggregation behavior of spider mites Tetranychus urticae under predation risk emanating from predatory mites Phytoseiulus persimilis. The encounter or avoidance effect applies primarily to visually hunting predators and assumes that a group, being more conspicuous, is often, but not necessarily always[4, 12, 13], less likely detected than the same number of scattered individuals in the same area[4, 11, 14]. To enhance group formation, T. urticae follow the threads of conspecifics; group members benefit from Allee effects[34, 39,40,41]
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