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.

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This study explores the food plant spectrum of the sugar beet weevil (Asproparthenis punctiventris Germar; Coleoptera: Curculionidae), one of the most important pests of sugar beet (Beta vulgaris subsp. vulgaris Altissima group). It examines the potential of various weeds and other plants to maintain populations of A. punctiventris adults and larvae outside sugar beet fields. To this end, leaf consumption of females and males on twelve Amaranthaceae and six Polygonaceae species was compared over a 24-h period in a laboratory environment. Both sexes consumed the greatest amount of leaf mass from Beta spp. and on average about a third less from Atriplex spp., indicating that these plants have the highest nutritional value for A. punctiventris. Weevils consumed between 30 and 60% of the amount of A. retroflexus and Chenopodium spp. than they fed on sugar beet leaves. Like Spinacia oleracea (Amaranthaceae), plant species of the Polygonaceae family were hardly or not at all fed on. Mated females generally consumed more leaf mass than unmated, especially from Chenopodium spp. and A. retroflexus, i.e. plants with low feeding value. Experiments with potted plants revealed that the most and heaviest 4th instar larvae developed on sugar beet, while fewer individuals with lesser weight were found on B. vulgaris subsp. maritima and A. hortensis. Very few larvae were able to develop on C. album, and none on A. retroflexus. To prevent promotion of pest population, special attention should be paid to the control of weeds from the Amaranthaceae family in sugar beet growing areas.