Asymmetric larval interactions between introduced and indigenous ladybirds in North America.

Understanding the mechanisms that result in the success of introduced species will contribute to predicting future invasions and managing invaded systems. We examined interactions between larvae of two predatory ladybird species recently introduced to North America, Coccinella septempunctata (CS) and Harmonia axyridis (HA), and two indigenous ladybirds, Coccinella transversoguttata (CT) and Hippodamia convergens (HC). By pairing young and old larvae in the laboratory at low and high levels of aphid availability, we assessed the degree of asymmetry in intraguild predation (IGP), the strength of competitive effects on growth and development of larvae escaping predation, and the nature of attack and escape behavior among the species. Interactions were generally asymmetric, with larvae of introduced species acting most frequently as intraguild predators and larvae of indigenous species serving most frequently as intraguild prey (the two Coccinella spp., however, preyed on each other at similar rates). Because they were especially aggressive and because other larvae were least successful in escaping their attacks, larvae of HA had stronger negative effects on larvae of the two indigenous species than did larvae of CS. Such negative effects, expressed most strongly when aphid availability was low, were especially adverse for the smaller of the two indigenous species, HC. In general, older larvae interacted with each other more strongly than young larvae did, and older larvae had especially strong negative effects on young larvae when interactions occurred between age classes. Our results suggest that HA more than CS may represent a threat to indigenous ladybirds as an intraguild predator, and that IGP in turn may play a stronger role for HA than for C. septempunctata in promoting the successful invasion of North America.