Woolly apple aphid - Population dynamics

The woolly apple aphid occurs worldwide and studies of its ecology in New Zealand are generally consistent with overseas research on its population dynamics. Special attention has been paid to the reproduction and growth rates of woolly apple aphid populations compared to those of its major natural enemy, the wasp Aphelinus mali. This research has consistently shown that woolly apple aphid has a greater capacity for population increase than A. mali under the conditions mainly prevailing in New Zealand, and that the generation time of the wasp is longer than that of its host up to at least 25°C (this relationship reverses at higher temperatures). In most studies, the lower temperature threshold for development is higher in A. mali than in woolly apple aphid. Woolly apple aphid is able to complete more generations per year than A. mali, and whereas the aphid produces over 100 female young (see fecundity), A. mali produces fewer young (average about 85) of which only slightly more than half are female. These differences are not fully compensated by the ability of A. mali to produce more female young as aphid numbers increase, and to produce more females from the larger aphid hosts. European simulation modelling has indicated that there would need to be on average 23 times as many A. mali as woolly apple aphid in spring for the wasp to achieve control! The combination of these studies with field evidence of A. mali performance have led to the conclusion that A. mali cannot control woolly apple aphid on its own. Predators have been found to play an important part, particularly in the spring, including predation by earwigs and ladybirds, assisted by lacewings, hover flies and predatory bugs. The need for these predators has been confirmed by woolly apple aphid outbreaks associated with their exclusion, or their destruction by pesticides.

The first nymphal stage (instar) of woolly apple aphid is unusually long, lasting longer than the later nymphal stages even in summer. This may in part be related to time taken for stylet (feeding tube) penetration (about 3 days), but it appears also to be a survival strategy linked to the long-lived overwintering role of the first stage nymph. This is the most cold resistant stage in the population and is vital for population survival in New Zealand because the woolly apple aphid here lacks the overwintering stages (sexual stages and eggs) on elm. The use of aphid-resistant rootstocks also places greater reliance on the overwintering of the young nymphs on the exposed arboreal parts of the apple trees. Resistant rootstocks may have a significant influence on the population dynamics of woolly apple aphid. They have certainly been an important part of aphid control in combination with broad-spectrum insecticides. With the change to more selective chemicals and the use of organic production, natural enemies are increasingly important in the control of woolly apple aphid. Research is needed to understand the interactions between natural enemies, rootstock resistance/susceptibility, and woolly apple aphid population dynamics.