A sublethal effect on native Anthocoris nemoralis through competitive interactions with invasive Harmonia axyridis

1. The invasive ladybird Harmonia axyridisPallas (Coleoptera: Coccinellidae) is implicated in declines of native ladybirds with intraguild predation identified as an underlying mechanism, however, less studied are the effects on non-coccinellid predators. Intraguild predation between Anthocoris nemoralisFabricius (Heteroptera: Anthocoridae) and H. axyridis and the relative effects of inter- and intraspecific competition on predators on potted Tilia cordata Mill. was investigated, at two aphid densities. 2. Intraguild predation was unidirectional in favour of H. axyridis in Petri dishes although a low level of ladybird egg predation by A. nemoralis occurred. However, on trees intraguild predation was rare. 3. A positive effect of aphid density on H. axyridis weight gain, and increased survival when reared with A. nemoralis on potted trees indicated stronger effects of intraspecific competition on H. axyridis. Consistent suppression of aphids by ladybirds revealed their superior competition for aphids. 4. Anthocoris nemoralis weight gain was reduced when reared with H. axyridis on potted trees, indicating a potential for a sublethal effect through interspecific competition. Survivorship of A. nemoralis did not differ between con- and heterospecific treatments, but high aphid density enhanced A. nemoralis survival. Overall, high mortality among nymphs across treatments suggested effects related to both inter- and intraspecific competition for resources. 5. Results provide evidence of a sublethal effect by invasive H. axyridis through resource competition, suggesting that where these predators co-occur, competitive interactions for shared prey are more likely to affect A. nemoralis populations than intraguild predation by H. axyridis.