The effects of prey distribution and digestion on functional response of Harmonia axyridis (Coleoptera: Coccinellidae)

The effects of prey distribution and digestion on the functional response of an invertebrate predator Harmonia axyridis (Pallas) to varying densities of Aphis citricola (van der Goot) were investigated. Experiments were conducted in petri dish and wind tunnels with multiple plants to present conditions in which the predator could behave more naturally. To discriminate if the functional response of H. axyridis is digestion or handling limited, numbers of aphids consumed were measured at five time-points during a 24 h period in all assays. In the wind tunnel, aphid consumption was measured with two types of prey distributions (aggregate and uniform). Type 2 functional responses were observed in petri dish and three wind tunnel assays with aphid uniformly distributed (6, 12 and 24 h). Based on the Akaike Information Criterion (AIC) value, type 1 response was most appropriate for two assays (1 and 2 h) with aphid uniformly distributed and all assays with aphid aggregate in wind tunnels. There was a significant effect of aphid distribution and its interactions with experiment duration and prey density on the number of aphids consumed in wind tunnel. The functional responses between uniform and aggregate treatments were mostly overlapped except when prey densities were above 90. This indicates that prey distribution could affect H. axyridis foraging under relative high prey densities. Prey risk raised as the increase of experiment duration for all treatments, while attack rates and handling times differed significantly among the assays with different experimental lengths, thus digestion process could limit predation process of H. axyridis. Our results indicate that prey distribution and predator digestion could alter predator-prey interactions and thus the capacity of this generalist predator to suppress aphids.