Prey vulnerability and active predator choice as determinants of prey selection: a carabid beetle and its aphid prey

Abstract

Petri dish experiments were conducted to study various aspects of prey selection in the predatory carabid Poecilus cupreus. The potential influence of active and passive prey selection, relative prey frequencies and hunger state of the carabid beetle were investigated with respect to the predation of aphids. For prey the focus was on the aphid Rhopalosiphum padi, and the fruit fly Drosophila melanogaster and the house cricket Acheta domestica were used as prey for comparison. Both search time and handling time for live prey items was longest for A. domestica, shorter for D. melanogaster and shortest for R. padi. Absolute maximum consumption in numbers was lowest for A. domestica, whereas the consumed numbers of D. melanogaster and R. padi were not significantly different. In the prey preference experiments with live prey (passive selection), P. cupreus ranked prey types in the order R. padi > D. melanogaster > A. domestica. Due to the predation activity of P. cupreus, prey frequencies changed in the course of the experiments; however, the relative abundance of alternative prey did not affect prey preferences. Although starved P. cupreus consumed slightly more prey individuals, the different saturation level of the carabids had no influence on prey preferences. In the preference experiment with dead prey (active choice), rank order of prey selection was reversed: P. cupreus now selected prey types in the order A. domestica > D. melanogaster > R. padi. The results indicate that for P. cupreus, the cricket A. domestica is the most attractive prey, Drosophila is at an intermediate position, and R. padi is the least attractive one. Nevertheless, in live prey tests P. cupreus preferred the aphid R. padi, probably because this prey type was easier to catch. The results suggest that the outcome of predator–prey encounters in P. cupreus is determined by prey vulnerability rather than by active choice of the carabid predator. Therefore, it is argued that the predation impact of carabid beetles on aphids depends not only on the presence and value of alternative prey, but also strongly on the escape efficiencies of the different prey species. Hence, in studying prey preferences of carabids with the aim of evaluating their effectiveness against aphids, a closer examination is necessary of the behavioural parameters affecting the outcome of specific carabid–prey encounters.