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

Petri dish experiments were conducted to study various aspects of prey sele ction in the predatory carabid Poecilius cupreus. The potential influence o f active and passive prey selection, relative prey frequencies and hunger s tate 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 we re 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 s hortest 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 l ive 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 preference s. Although starved P. cupreus consumed slightly more prey individuals, the different saturation level of the carabids had no influence on prey prefer ences. In the preference experiment with dead prey (active choice), rank or der of prey selection was reserved: p. cupreus now selected prey types in t he order A. domestica > D. melanogaster > R. padi. The results indicate tha t for P. cupreus, the cricket A. domestica is the most attractive prey, Dro sophila is at an intermediate position, and R. padi is the least attractive one. Nevertheless, in live prey tests P. cupreus preferred the aphid R. pa di, probably because this prey type was easier to catch. The results sugges t that the outcome of predator-prey encounters P. cupreus is determined by prey vulnerability rather than by active choice of the carabid predator. Th erefore, it is argued that the predation impact of carabid beetles on aphid s depends not only on the presence and value of alternative prey, but also strongly on the escape efficiencies of the different prey species. Hence, i n studying prey preferences of carabids with the aim of evaluating their ef fectiveness against aphids, a closer examination is necessary of the behavi oural parameters affecting the outcome of specific carabid-prey encounters.