Fruit Crops News

Biological Control of Mites in Midwest Apple Orchards
Part 2: Important Natural Enemies

Spider mites, on apples as well as other crops, are attacked by lots of natural enemies. European red mite and twospotted spider mite are both attacked by several kinds of predators and a few types of pathogens; no spider mite parasitoids are known. Although both viral and fungal pathogens of spider mites can cause significant mite mortality, little effort has gone into manipulating these pathogens for biological control on apple. Predators are, by far, the most important group of natural enemies; there are many different kinds of predators and sophisticated mite management programs have been widely adopted based upon knowledge of these predators.

Spiders, predatory mites, and predatory insects are all known to prey on pest mites. Studies in both England and Japan have reported that over 30 different types of small spiders were found to prey on mites on apples; similar studies in the United States and Canada have also found many spiders feeding on mites. There is little evidence, however, that spiders play a major role in either keeping mite populations at a low level or reducing high populations. Therefore, although spiders may be of general benefit, their overall impact is probably minor.

Several families of predatory mites are common predators of spider mites. Two members of the predatory mite family Stigmaeidae are frequently found in Midwest apple orchards. Zetzellia mali and Agistemus fleschneri are similar in appearance and biology. They are about the size of their spider mite prey. The young are yellowish in color but they change to a reddish yellow as they mature. They feed on all stages of spider mites, preferring the eggs and small nymphs; they also feed on rust mites. Because of their small size and relatively slow reproductive rate, these are not the most efficient predators of spider mites, but they are tolerant of several types of orchard insecticides and they do play an important role in mite biological control.

Probably the most important group of spider mite predators overall is the mite family Phytoseiidae. Several species of this group have been found on apple foliage, feeding on pest mites. The most important species in the upper Midwest is Amblyseius fallacis. This is a pear-shaped mite about the same size as its spider mite prey. The eggs and young are translucent to pale white in color; as they feed on prey,their color darkens from yellow to reddish brown. This mite is an active and fast-moving predator capable of finding its prey even when they are scarce. Their generation time is short and they build up in numbers in response to increasing pest populations. Alternate food, including pollen, nectar, and very small insects allow phytoseiid mites to survive in orchards even when pest mite numbers are very low. In many locations A. fallacis has developed resistance to organophosphate insecticides such as phosmet and azinphos-methyl; this allows them to survive even when sprays are needed for other pests such as plum curculio or codling moth. Typhlodromus pyri is a second species found in Midwest apple orchards. It has characteristics similar to A. fallacis.

Several generalist predatory insects feed on orchard mites, including lacewing larvae, lady beetles, and predaceous bugs such as damsel bugs and minute pirate bugs. Predatory thrips are tiny, slender, black insects that can be locally important mite predators. Probably the most important predatory insect is a tiny black lady beetle, Stethorus punctum. The adult is only about 1/8" in size; both adult and larval stages are predaceous. Stethorus has been shown to be one of the most important spider mite predators in Pennsylvania orchards, but is not consistently abundant in orchards throughout much of the Midwest.

In the next issue I will discuss the impact of orchard pesticides on these predators.

- Dan Mahr, University of Wisconsin - Madison