It is common and good practice to grow cover crops during winter. These crops, especially leguminous types, favour the build-up of aphids and mites. To limit the number which transfer to the foliage of the tree crop in spring the cover crop should be cultivated or slashed well before the trees leaves emerge. Control by these natural enemies is not as rapid as with pesticides as there is a lag time between when the pest becomes a problem and when predators build up in sufficient numbers to control the pest. Unfortunately many of these beneficial insects are also killed by chemicals used for pest and disease control.
Some chemicals are more harmful than others to certain predatory insects.
In an orchard where chemicals have been used on a regular basis it may take several years of careful management before beneficial insects re-establish themselves. A number of predatory insects are commercially available for release into orchards for the control of two spotted mites and light brown apple moth. Some of these can establish a small, but permanent population in the orchard which will respond to pest outbreaks and control them. Others need to be released each year as they are unable to survive the winter period. The successful use of any predatory insect requires increased management input into monitoring and chemical selection where application is needed.