Abstract
Seasonal abundance and within-plant distribution of Acyrthosiphon spp. and their natural enemies were investigated on alfalfa in Obihiro, Hokkaido, Japan in 2001. Densities of A. pisum and A. kondoi peaked in early June. Coccinellid and syrphid predators appeared to respond to aphid numbers and were synchronized with aphid populations. Two species of braconid (aphidiine) parasitoids (Aphidius ervi and Praon barbatum) were collected. Parasitism rates for each aphid species were variable through the season, but the estimated number of parasitized aphids reached a peak at the same time as overall aphid densities. After the first harvesting of alfalfa (14 June) aphid densities became very low, and coccinellid and syrphid predators were not found. In contrast to the coccinellid and syrphid predators, the heteropteran predators, Nabis stenoferus and Orius spp. became common during summer. Aphids, immature parasitoids (live parasitized aphids and mummies), Coccinella septempunctata and syrphids occurred more on the upper parts of alfalfa stems than on the lower parts, but this trend was not found for Hippodamia tredecimpunctata. The temporal and spatial distributions of members of the aphidphagous guild are likely to influence natural enemy–aphid relationships and intraguild interactions.
