Predator hunting mode and host plant quality shape attack-abatement patterns of predation risk in an insect herbivore.

Group formation reduces individual predation risk when the proportion of prey taken per predator encounter declines faster than the increase in group encounter rate (attack-abatement). Despite attack-abatement being an important component of group formation ecology, several key aspects have not been empirically studied, that is, interactions with the hunting mode of the predator and how these relationships are modified by local habitat quality. In 79 cage trials, we examined individual egg predation risk in different-sized egg clutches from the blue willow beetle Phratora vulgatissima for two predators with different hunting modes (consumption of full group [Orthotylus marginalis] vs. part group [Anthocoris nemorum]). Because these predators also take nutrients from plant sap, we could examine how the quality of alternative food sources (high-vs. low-quality host plant sap) influenced attack-abatement patterns in the presence of different hunting strategies. For the O. marginalis predator, individual egg predation risk was largely independent of group size. For A. nemorum, egg predation risk clearly declined with increasing group size. However, approximately one-third of the grouping benefit was lost to an increase in group detectability. There were clear differences in attack-abatement patterns between plants with high-vs. low-quality sap. When O. marginalis was the predator, there was no clear change in attack-abatement in relation to host plant quality. However, for A. nemorum there was a clear reduction in overall predation risk and a stronger attack-abatement pattern with increasing group size when plant sap quality increased. This implies that the relative benefits of prey grouping behavior for any species might show diurnal or seasonal changes as other aspects of resource/habitat quality change for the focal predator. Modulation of attack-abatement by bottom-up effects such as plant-based food resources is yet to be incorporated into general theory, despite the ubiquity of omnivorous predators and with omnivory being important for shaping food webs, ecosystem functions, and in biological control. Thus, ongoing refinement of attack-abatement theory by focusing on bottom-up vs. top-down processes could have significant impacts on many important contemporary fields of study. [ABSTRACT FROM AUTHOR]