Behavioural mechanisms behind aggregation in a tritrophic perspective

Gregarious organisms need to handle the trade-off between positive effects of group living and increasing food competition. The gregarious Phratora vulgatissima, a specialist leaf beetle on willow in Europe and Asia, frequently reaches outbreak densities in natural stands and short rotation coppices. Outbreaks threaten the yield and plantations therefore rely on omnivorous predators as biocontrol agents, like Anthocoris nemorum and Orthotylus marginalis. I aimed to elucidate behavioural mechanisms of the beetle and the predators to understand how and why species aggregate. The beetle’s aggregation behaviour was studied by looking at key reproductive traits like oviposition rate, clutch size, and oviposition site choice on willow shoots. Both predators were characterized further by examining where on the vertical shoot they preferentially hunt for the beetle’s eggs and how the quality of alternative food (different plant genotypes) alters their effects on the beetle. I so revealed how the reproduction of the beetle is modulated by lateral (conspecific density), bottom-up (plant genotypes), and top-down (omnivorous predators) effects. To lower exploitative competition among larvae females increase the distances between clutches on a plant and lower their oviposition rate if too many, or too few (too few confirmations of own decision) conspecifics visited a shoot. Observed bottom-up effects include lowering clutch size and number of eggs on a shoot with plant genotype unsuitability, initially selecting large leaves for oviposition/feeding, and increasing clutch distances due to larger leaf area of a willow genotype. Three top-down effects in form of predator avoidance behaviours were observed. Females lowered median clutch size and oviposition rate leading to fewer eggs in presence of the predators/their combination. Females also preferentially feed in the shoot canopy but oviposit in the lower part. In combination with the observation that both predators show contrasting vertical preferences on the shoot I argue that females try to avoid the area were the predators are consuming more eggs. In general I contributed to the ecological concept of habitat domain, attack-abatement, clutch size, cognitive maps, and nonconsumptive effects. I hope the results facilitate our understanding of insect outbreaks and species aggregation, possibly leading to better control of those in economic relevant systems