Bacillus thuringiensis is certainly a proven performer. This microbial insecticide is effective against foliage and fruit feeding caterpillars such as cankerworms, tent caterpillars, fall webworms, leafrollers, and fruitworms. The residual period of activity after application is very short, and multiple applications may be necessary. Sprays should be timed to control young larvae. Bt will not be useful against interior-feeding caterpillars, such as leafminers, or codling moth larvae inside of fruit.
Insect parasitic nematodes may provide some benefit against insects that spend some of their life in the soil (such as plum curculio and apple maggot), but there has been little research on such applications in orchards.
Of the commercially available parasites, Trichogramma has the greatest potential. Several species are available; T. platneri is often most recommended for use against tree-dwelling pests, but ask your supplier for recommendations. Potential targets include codling moth and leafrollers. High release rates are necessary because the tiny parasitic wasps can rapidly disperse from release sites -- a problem when trying to cover small areas. Also, remember that the egg is the target stage and therefore releases have to be properly timed.
Green lacewings are generalist predators that will feed on many pests, including aphids, scale crawlers, spider mites, and small caterpillars. I recommend applying them as eggs, which are the easiest stage to handle. Two or three releases during the growing season will help control many types of pests.
Spider mites are often not problems in home orchards, but if these are of concern, predatory phytoseiid mites are effective predators. They are rather expensive to use -- check with suppliers for suggested species, release rates and prices. Releases should be made shortly after the end of the blossom period. Most of the above suggestions relate to the control of leaf-feeding pests.
Regrettably there are very few options for biological control of serious fruit-feeding insects such as codling moth, plum curculio, and apple maggot. Some non-chemical methods are available for these pests, such as limb-jarring for plum curculio and trapping of apple maggot (information on these methods are available from many extension offices). If it is necessary to use broad spectrum insecticides, remember that these may interfere with the activity of predatory or parasitic insects that have been released into the orchard.
- Dan Mahr, University of Wisconsin - Madison