Apparent niche differences between native and exotic lady beetles

Abstract

Generalist predators can enhance biological control by filling unique niches and complementing other natural enemies, or disrupt it by preying upon other predators and parasitoids, complicating their net value for biological control. We searched for evidence of complementarity and/or interference between native (primarily Eriopis chilensis and E. eschscholtzi) and exotic (primarily Hippodamia variegata and Harmonia axyridis) lady beetles attacking aphids in alfalfa (Medicago sativa) fields near Santiago, Chile. Across 60 fields and three growing seasons, exotic lady beetle adults and larvae reached peak abundance relatively earlier in the growing season than the natives. Furthermore, exotic lady beetle peaks aligned more closely with aphid peak abundance than did native lady beetle peaks. Abundances of native lady beetle adults and larvae were weakly positively correlated with those of exotics, and peak aphid densities were generally lowest in fields where native and non-native lady beetle adults were similarly abundant. Therefore, we did not see evidence of strong interference between exotics and natives. Natural-agricultural habitat edges reduced native, but not exotic, larval lady beetle lag behind aphid peaks and was associated with greater lady beetle and lower aphid densities. Moreover, farms embedded in landscapes with greater natural habitat cover supported more exotic, but not native, lady beetles, and increased the lag between larval lady beetles and aphids for natives but not exotics. These findings support the idea that farms embedded within mosaics of natural and agricultural habitats may provide greater interaction potential between native lady beetles and aphid pests on farms as they move across the landscape, leading to greater top-down control. In contrast, exotic lady beetle lags were unaffected by landscape composition or configuration, but their greater densities in farms surrounded by more natural habitats might reflect a 'concentration effect' whereby exotic lady beetle abundance becomes concentrated in the agricultural habitats that they prefer. Our findings suggest that some lady beetles introduced for biocontrol may be better adapted to intensively managed habitats than natives, possibly leading to spatiotemporal complementarity between native and exotic lady beetles.