Woolly apple aphid - biological control

Biological control of woolly apple aphid is very effective, provided broad spectrum chemicals toxic to its natural enemies are avoided. There are two major biological control components - parasites and predators - and these are best combined with cultural control through the use of aphid-resistant rootstocks. Following its introduction to New Zealand last century, woolly apple aphid increased rapidly and caused severe damage to apples, but the introduction of the parasitoid wasp, Aphelinus mali, was one of the most successful programmes of classical biological control undertaken in this country. This wasp is today common in organic orchards and in orchards practising IFP, where it can cause high levels of aphid parasitism, particularly late in the season. However, care should be exercised to avoid toxic pesticides. The wasp (and other natural enemies) is susceptible to organophosphate and some carbamate insecticides, whereas the new insect growth regulator chemicals and pirimicarb are more selective. For growers changing to organic production or removing the use of broad-spectrum chemicals, there is risk of woolly apple aphid causing damage before the natural enemies increase and provide biological control. This risk can be reduced by collecting overwintered mummified aphids (parasitised by A. mali) from unsprayed or organically-managed apple trees and releasing them into the orchard in spring. This process can be repeated over the summer.

Despite the major reductions of woolly apple aphid damage after the introduction of A. mali, recent research in many parts of the world indicates that it cannot control woolly apple aphid alone and that predators play an important role. A total of 73 predatory insect species have been recorded worldwide. In New Zealand, these include the European or common earwig, the brown lacewing, hover flies, and several ladybirds. A number of other general predators (e.g. predatory bugs) probably also contribute. The only identified fungal pathogen of woolly apple aphid (in Australia) is Verticillium lecanii (Zimm.). It is spasmodic in occurrence and makes only a small contribution to control.