Several hypotheses have been proposed to explain invasion success of species spreading beyond their natural ranges. The predator-prey naivete hypothesis suggests that non-native predators benefit from being unknown to prey, resulting in inappropriate behavioral response by native prey. Naivete is not restricted to predator-prey interaction but can also occur at the same trophic level (e.g. between predators). This novelty advantage should depend on the similarity of recognition cues between non-native and native predators. We focus on the invasion success of the Asian ladybeetle Harmonia axyridis, which was introduced in many parts of the world as a biocontrol agent against aphids. Ladybeetles are not only engaged in predator-prey interactions but also compete with ants for their shared food source. Here, we compare ant aggression and ladybeetle response in four native European ladybeetle species and the non-native Asian ladybeetle species H. axyridis. In addition, we manipulated cuticular hydrocarbons on the elytra of ladybeetles to test, whether these chemical cues are involved in species recognition. For this, we conducted behavioral assays in the laboratory confronting two common ant species with living ladybeetles but also with ladybeetle elytra bearing or lacking the chemical cues of the respective ladybeetle species.
Results/Conclusions
Consistent with the predator-predator naivete hypothesis, our behavioral experiments revealed weaker aggression of ants towards the invasive ladybeetle H. axyridis compared to most native ladybeetle species. Similarly, H. axyridis exhibited fewer behavioral responses in the presence of ants compared to native ladybeetle species. Thereby, differences in ant aggression and behavioral response to ants were most pronounced between H. axyridis and Coccinella septempunctata. The removal of cuticular hydrocarbons from ladybeetle elytra greatly reduced aggression by ants. If cues of respective ladybeetle species were added on cue-free elytra, natural levels of ant aggression could be restored for most ladybeetle species. This experiment demonstrates that chemical cues on the surface of ladybeetles are indeed important determinants of intraguild interactions. However, preliminary results indicate that chemical cue differences are not sufficient to explain differences in ant aggression. In this context, differences in ant aggression against H. axyridis and C. septempunctata are of particular ecological importance because these are the most effective (largest) aphid predators in European agro-ecosystems. Reduced ant aggression in H. axyridis is likely to improve their ability to deplete the shared food resource and thus outcompete native ladybeetles in the field.