Invasive species are a major driver of global biodiversity loss. However, a mechanistic understanding why some non-native species become invasive is often missing. Our research project aims at a deeper understanding of species interactions between invasive predators and native prey. Since there is a lack of co-evolutionary history between invasive predators and native prey, higher consumption rates of invasive predators and reduced antipredator response by native prey are expected. Here, we compared predation rates between native European lady beetle species (Coccinella septempunctata, Hippodamia variegata, Adalia bipunctata, and Propylea quatuordecimpunctata) and the invasive, multi-colored Asian lady beetle Harmonia axyridis, using the native pea aphid Acyrthosiphon pisum for prey. In the same experiment, we quantified initial dropping behavior of A. pisum from broad bean plants (Vicia faba) after encountering of lady beetles. In an additional plant-choice experiment, we tested for predator cue avoidance of aphids. This experiment allowed us to evaluate the role of chemical lady beetle cues mediating avoidance behavior of aphids in the absence of lady beetles.
Results/Conclusions
Our results indicate that invasive H. axyridis and native C. septempunctata consumed the highest amount of aphids. If we corrected predation rates for body width, the differences in predation rates between the different lady beetle species disappeared. Thus, predation rates depend on beetle size rather than on possible advantages of H. axyridis, due to missing co-evolution. Initial dropping behavior of aphids differed between lady beetle species. However, it does not support the hypothesis of lower antipredator response of A. pisum towards H. axyridis. We suggest that the strength of dropping behavior in A. pisum is influenced by multiple factors such as the activity of the predator rather than predator recognition alone. In accordance with our expectations, A. pisum avoided plants bearing C. septempunctata cues but not plants bearing cues of the invasive H. axyridis. Although, predation rates of H. axyridis and C. septempunctata did not differ in our laboratory experiment, lower aphid predation rate of C. septempunctata is expected under more natural conditions, due to stronger cue avoidance.