Within colony parasitism of woolly apple aphid by Aphelinus mali

Parasitised aphids were found from the end of January in each season until aphids were no longer visible on the trees at the end of winter. All the developmental stages of woolly apple aphid (WAA) were parasitised by A. mali. However, the third, fourth and adult stages were preferred. The percentage of parasitised live aphids reached a peak at the end of winter when nearly 70% parasitism occurred within the colonies. The total percentage parasitism including mummies still containing parasitoids reached 90% during some seasons. Many of the earlier instars escaped parasitism as they were under other aphids and mummified aphids. Live first instar crawlers were also found inside the empty mummified aphid bodies during winter. First and second instar aphids were parasitised when parasitoid activity was high and insufficient numbers of the preferred older aphids were available. There was a good correlation (R²=0.606, P<0.001) between the working logit average number of parasitised E. lanigerum colonies counted in the trees and the working logit of the average number of individuals parasitised within colonies. Early in the season parasitism was low in both the parasitised colonies in the trees and the number of individuals parasitised in each colony and both reached high numbers at approximately the same time during the season.