Fruit Crops

Biological Control of Apple Aphids

It would be nice if using natural enemies in biological control could be as straightforward as using pesticides in chemical control. Read the directions, release the natural enemies at the right time and the correct rate, and get highly predictable results. Unfortunately, biological control isn't always that easy. We are dealing with living organisms, and often subtle changes in their environment may result in less than optimum results. A great deal of biological control research is aimed at making the results as predictable as possible.

A recent paper on biological control of green apple aphid (GAA) provides a good example. The work was conducted in Washington state in the dry apple growing area east of the Cascades. The researchers evaluated the impact of native predators and parasites on GAA. In other plots, releases were made of two commercially available predators, the native aphid midge, Aphidoletes aphidimyza (at the rate of 200-250 pupae per tree per week), and a non-native green lacewing, Chrysoperla rufilabris, which is generally recommended as an arboreal predator (at the rate of 200-400 eggs or first instar larvae per tree per week). The research was conducted at three locations consisting of one- and two-year-old apple trees. In two locations, the vegetative growth of the young trees was vigorous, which was ideal conditions for rapid population buildup of GAA. In these locations, the native natural enemies, consisting of native green lacewings, Aphidoletes, syrphid fly larvae, lady beetles, and parasitoids, suppressed the aphid populations, but not sufficiently to adequately control the damage. In the third location, vegetative growth was less, the aphids did not increase as rapidly, and the native predators provided adequate control. In none of the three trials did the released, commercial natural enemies provide control. The researchers suggested that C. rufilabris may not have been adapted to the location, or may have preferred to feed on prey other than GAA. On the other hand, Aphidoletes, which was released as pupae, may have dispersed out of the relatively small research plots as soon as they emerged as adults. The researchers also concluded that the overall plant composition of the orchards may not have provided adequate habitat or food resources for the natural enemies. Two other possibilities are that a different species of Chrysoperla may have been a better GAA predator, and larger plots may have resulted in better results with Aphidoletes.

This research illustrates the difficulties of developing biological control methods that are consistently and predictably effective. We can't conclude that GAA biological control is impossible; we can only say that the specific natural enemies and methods used in this study in eastern Washington did not result in adequate control under all conditions of tree growth. In mature trees, where less vigorous growth is desirable, or in orchards with better natural enemy habitat, the native natural enemies may provide good control. Different environmental conditions in different locations may mean the difference between successful and unsuccessful biological control.

The Natural Enemies of Aphids
It's amazing that there are any aphids at all in the world, because they are so heavily attacked by natural enemies. Almost every type of aphid is attacked by one or more species of tiny parasitic wasp. More commonly recognized are the predators, including bugs such as damsel bugs and minute pirate bugs; beetles of many types, including many species of lady beetles; green lacewings; a small family of flies called aphid flies (Chamaemyiidae), and the larvae of several types of hover flies. The predatory midge Aphidoletes is not only naturally important, but is also commercially available. Even dragonflies, which are aerial predators, take large numbers of aphids during flight. In addition to these and many other types of predators, there are many aphid-pathogenic fungi that are capable of devastating aphid populations during periods of warm, moist weather. In making aphid-management decisions, it is helpful to be able to recognize the activities of this large complex of aphid natural enemies.

- Dan Mahr, University of Wisconsin - Madison

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