Integrated Mite Management

Galandromus occidentalis Western Predatory Mite

Few success stories in the history of tree fruit Integrated Pest Management have been more dramatic and had more impact than that of integrated mite control in apple orchards. In the early 1960s a crisis of mite control resulted in trees with brown leaves in late summer, even though several chemicals had been applied to control spider mites. The implementation of integrated mite control in the late 1960s reduced the need for applying chemical controls for spider mites. For the last 30 years only about 10 percent of Washington apple orchards have been treated annually with pesticides specifically for control of spider mite.

Three important components have led to this widespread biological control:

Tetranychidae - Spider mites

Tetranychus urticae The twospotted spider mite is an economic pest of many crops.

Overall, the spider mites are one of the largest, most important and most destructive groups of pests in agriculture. Spider mites have several natural enemies. However, some broad-spectrum pesticides used for insect control can reduce or eliminate them from orchards. For that matter, many miticides (chemicals that specifically kill spider mites) are as toxic or more toxic to predatory mites than to spider mites.

Panonychus ulmi Release from biological control (through the use of pesticides) allows European red mite levels to build.

Apple growers can effectively adopt practices that will allow predatory mites to survive in their orchards. While such practices might not immediately solve spider mite problems, it can lead to more stable, long-term mite control and have the added benefit of eliminating or slowing the development of spider mite populations that are resistant to miticides.

Eriophydae - Rust Mites

Rust mites can be an alternative food source during periods of low spider mite densities.

Rust mites play a critical role in integrated mite control because they are an alternate food source for predatory mites. The presence of apple rust mites in orchards allows predatory mites to survive, especially when spider mite densities are low. If predatory mite densities are too low, spider mite densities can rapidly increase and cause damage to apple trees before predatory mite densities can increase to levels providing control. When predatory mites can find apple rust mites they maintain higher densities in apple trees and are then able to attack spider mites before their densities reach damaging levels.

Phytoseiidae - Predator Mites

The Western predatory mite feeds on spider mites that attack deciduous tree fruits.

The predatory mites are the most important biological control agents in the Pacific Northwest. The western predatory mite, Galandromus occidentalis, is by far the most important predator in most orchards.

Three factors limit the number of western predatory mites in an orchard:

Number of prey
Since apple rust mites usually become numerous during May and June, numbers of predatory mites can increase rapidly during that period as they feed on the rust mites, reproduce and disperse over the tree. Predatory mites usually do not completely eliminate rust mites but keep their numbers below levels that cause leaf damage. Because of their importance in sustaining predatory mites in orchards it is critical to follow practices that keep apple rust mites in orchards.

Winter mortality
Cold, dry winter weather can drastically reduce the numbers of overwintering predatory mites. However, if there are sufficient numbers of rust mites as food in spring, predatory mite populations can rebound in time to control spider mites.

Toxic pesticides
Applications of pesticides that are highly toxic to apple rust mites or predatory mites have the most devastating effect on biological control of spider mites. Because spider mites are usually not affected by pesticides toxic to predatory mites, their densities can develop during the time it takes for predatory mite populations to recover from the pesticide application.