Effects of prey density and spatial distribution on prey consumption of the adult predatory ladybird beetle

The effects of prey density and spatial distribution on prey consumption of the adult predatory ladybird, Harmonia axyridis, were investigated by usin g 2 x 2 factorial design in large scale cages. Prey density influenced prey consumption of the ladybirds, and the frequency with which predation occur red was quire different between the prey distributions. The ladybirds consu med a relatively constant and small number of aphids when the prey were uni formly distributed, whereas the number of prey consumed per day when predat ion occurred was large and much more variable when the prey were contagious ly distributed. At high prey density, the number of prey consumed was highe st during the first day of the experiment; thereafter, only 10-20 aphids we re consumed during the following 3 days. However, these patterns of prey co nsumption were not observed at low prey density. The percentage of aphids t hat remained on the host plants when the experiments were terminated was hi gher at low prey density than at high prey density, suggesting that predato r foraging efficiency at low prey density was lower than at high prey densi ty. Ladybirds foraging for high prey density were more frequently observed on the plants with aphids than ladybirds foraging for low prey density. Pre y distribution also influenced the frequency of residence of ladybirds on t he plants. The different predation patterns observed in the two spatial dis tributions, in which prey consumption was much more variable for the contag ious distribution, might be explained by the difference in prey encounter r ate of the predator between the distributions. This study indicated that th e ladybirds had limited ability to search out prey over large spatial scale s.