Woolly apple aphid - Life history

Woolly apple aphid overwinters as nymphs, mainly first stage (instar) crawlers, settled in cracks and crevices on the bark of the trees. Nymphs also overwinter on the roots although this is limited in apple orchards by the use of aphid-resistant rootstocks. Pruning wounds and damage to the bark provide the main refuges for nymphs during the winter. As temperatures warm in spring the nymphs mature through four instars and produce live young. The population is entirely female and 8-10 generations are completed over the annual life cycle in New Zealand. The time for completion of nymphal development decreases from about 6 weeks in early spring to 3 weeks in midsummer, with the first instar remaining the longest in duration. Winged females are produced from February onwards but these aphids do not migrate to elm, as they do in North America, and they produce few young. The wingless females are, therefore, responsible for maintaining and spreading the population and they have a fecundity of over 100 young under good conditions. The aphids in summer favour settlement not only on the bark wound sites but also the leaf axils, particularly on terminals and water shoots. Crawlers disperse over the tree to establish these colonies which may constitute half of all colonies by mid-summer. Because the aphids form dense colonies and have limited ability to disperse between trees, the distribution of the population is highly aggregated and pockets of infestation may develop in orchards.

Larval development can be completed as quickly as in 8-10 days (e.g. 9 days at 26°C in U.S.A. research) and is faster under diurnal alternating temperatures. The estimated lower temperature threshold for development is 5.2°C and the upper development limit is 32°C (Australia). In this study, the mean day degrees required for total nymphal development above the threshold was 268. Studies in Israel have indicated a lower threshold of 4.2°C and a requirement of 250 day degrees for development. This compares with a lower threshold of 8.3°C or 8.5°C (in two separate studies) for Aphelinus mali, the most important natural enemy of woolly apple aphid. These results contrast with those from Washington State, U.S.A. where the thresholds for the aphid and the wasp were similar (about 7-8°C).