Woolly apple aphid - cultural control with resistant rootstocks

The use of rootstocks resistant to woolly apple aphid is one of the most effective methods of control for this pest. It can be integrated with biological control by natural enemies, such as in organic production or IFP, and/or with the use of insecticides. Woolly apple aphid infests the roots as well as the aerial parts of apple trees and resistant rootstocks greatly limit the aphid populations which develop below ground and at the base of the trunk. This reduces the reinfestation of aerial parts of the plant from the roots in spring. An early resistant rootstock was cv. 'Northern Spy' but this has since been replaced by the Malling Merton series which were bred at East Malling Research Station in the United Kingdom. This includes MM 106, MM111, and MM793. From time to time, small aphid colonies may be found on Malling-Merton roostocks. In contrast, susceptible rootsocks such as for example M9, MARK, and M26, can develop severe infestations, especially on light soils and soils with an open structure.

Rootstocks which are resistant to woolly apple aphid should be selected at the time of planting. If suitable aphid-resistant rootstocks are not available, then interstems should be used to control excessive vigour rather than plant aphid-susceptible rootstocks. Aphid-resistant rootstocks have been used primarily at a time when broad-spectrum insecticides have been used for insect control, including the control of woolly apple aphid. With the change to IFP and organic production, biological control with the natural enemies of woolly apple aphid is playing an increasing role. Further research is needed to determine whether these natural enemies are able to control woolly apple aphid when trees are grown on aphid-susceptible rootstocks.

Biotypes (strains) of woolly apple aphid able to overcome the resistance of MM rootstocks have been reported overseas.