Fruit Crops News

Biological Control of Mites in Midwest Apple Orchards
Part 3: Pesticide Impacts on Natural Enemies

Editor's note: this is the third in a series of articles on biological control of pest mites in apple orchards.

Insecticides and miticides are, of course, designed to kill insects and mites. Therefore, we shouldn't be surprised to learn that the use of these products can kill beneficial insects and mites as well as the pests. In fact, several studies have shown that the majority (but not all) of beneficials are actually more susceptible to insecticides than are the pests themselves. What may be surprising, however, is that some fungicides and herbicides are also harmful to beneficials, including the predators of spider mites in orchards. Therefore, an important key to an effective mite management program in apples is knowing how to cause the least disruption to predators when it becomes necessary to use various pesticides.

As discussed in the last issue, the most important group of predators are the predatory mites in the family Phytoseiidae, and much of our information comes from research on this group. However, some data are available for other important predators. For example, the other predatory mites, Agistemus and Zetzellia, are relatively unaffected by most organophosphate insecticides (such as Imidan, Guthion, and diazinon), but highly susceptible to sulfur-containing compounds, Omite, Morestan, Benlate, Sevin, and Karathane. Many predatory insects, including the predatory lady beetle Stethorous, are generally considered to be unaffected by most selective miticides, but moderately to seriously impacted by broad spectrum insecticides in the organophosphate, carbamate, and synthetic pyrethroid groups.

Interestingly, the important phytoseiids (Amblyseius and Typhlodromus) have developed resistance to some of the long-used organophosphate insecticides such as Guthion, Imidan, and malathion, and these products can be used in IPM programs without seriously affecting predatory mites. Endosulfan and methoxychlor are also considered to be relatively safe to phytoseiids. Insecticides that are generally considered to be highly to moderately toxic to phytoseiids include Ambush, Asana, diazinon, Lannate, Lorsban, Pounce, Sevin, and Vydate. Miticides show differing degrees of activity against phytoseiids. Oil, Omite, and Vendex, as well as some unregistered experimental miticides, are not, or only slightly toxic, but others, especially the carbamates Vydate and Carzol, and Kelthane, are highly toxic.

Benomyl, a commonly used fungicide, sterilizes phytoseiids, keeping them from reproducing. Captan, polyram, and dodine are relatively safe for phytoseiids. The herbicides paraquat and glyphosate are very harmful to phytoseiids. Because these herbicides are not sprayed on the trees, the greatest impact occurs when they are sprayed on the orchard floor early in the season before the predators have moved out of their overwintering sites and into the trees.

It is difficult to grow high-quality apples with a low cull rate without the use of pesticides. However, there are several things that can be done to reduce the impact of pesticides on predatory mites and insects. Of utmost importance is to develop a pest management strategy that relies on pest monitoring to make informed control decisions. With this strategy, pesticides will be used only when necessary. When it does become necessary to use insecticides and miticides, try to use only those that are known to have minimal impact on beneficials. For example, carbamate and synthetic pyrethroid insecticides are harmful to predatory insects and mites and should be used only if other products are not effective against the specific target pest. If a special spray is necessary, try to do it at a time when it will have the least impact on beneficials. For example, if you must use a carbamate to control leafminers, do this before bloom, when many predators have not yet moved into the trees. If it is necessary to treat for mites, use a selective miticide, such as oil, Omite, or Vendex, instead of a broad spectrum product such as Vydate, Carzol, or Kelthane. Avoid season-long use of fungicides that are known to be harmful to predators. If possible, spot spray only those areas that are actually infested; this will conserve natural enemies in adjacent unsprayed areas that will eventually repopulate the sprayed trees. Years of research and practical orchard experience demonstrate that mites can be effectively managed using biological control integrated with the use of selective miticides, insecticides, and other pesticides when these products are needed.

Sources of particular value for this article were the following three sources:

- Dan Mahr, University of Wisconsin - Madison

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