Adaptive aggregation by spider mites under predation risk

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.

Cooperative Behaviors in Group-Living Spider Mites

“…Pioneers are later paid back by being released from costly individual web production and other aggregationrelated benefits such as enhanced mating opportunities and enhanced protection from predators (see also the section on web sharing under predation risk), and other abiotic and biotic hazards for themselves and their offspring. Joining other groups and tightening the levels of aggregation enhance the chances of survival under predation risk because of attack abatement, even when the webbing is light or absent (Dittmann and Schausberger, 2017). An intriguing example of non-kin interactions comes from Schausberger et al (2019), who showed that individuals of one population heavily benefited (without any direct interactions) from microhabitat manipulation by webbing or host plant biochemistry by pioneering colonizers from another population (Y coming to G environment), whereas in the reverse sequence, later arrivers were negatively affected (G coming to Y environment).…”

Integrating top‐down and bottom‐up effects of local density across scales and a complex life cycle

“…Large groups of prey may be more conspicuous to predators (Ioannou et al 2009), but this effect often saturates at high prey densities (Riipi et al 2001). This may lead to a benefit of prey aggregation, where the cost to prey of increased apparency is outweighed by the benefit of having fewer groups occupying the same search area (Ioannou et al 2011, Dittman and Schausberger 2017). Once a predator enters a prey patch, the risk for individual prey then depends on within‐patch search behavior and consumption.…”

Spatial structure arising from chase-escape interactions with crowding

“…An interesting extension of the model presented here would be to include interactions which affect the proliferation and mortality of individuals, such as predator-prey interactions or intra and interspecific competition 44,45 . The population dynamics resulting from such interactions will depend on the types of spatial structure examined in this work.…”