Managing Two-Spotted Spider Mites In Cherries

Two-spotted spider mites should not be particularly problematic for cherry growers this season, but growers are still faced with the decision of applying a miticide before or after harvest.

Two-spotted spider mites can be a problem on cherries in hot, dry years. Michigan State University Extension does not anticipate this season to be particularly problematic for two-spotted spider mites with our cool temperatures and wet conditions, but we know that cherry growers are often faced with the crucial decision of applying a miticide before or after harvest. If growers intend to spray for mites before cherry harvest, the pre-harvest intervals (PHIs) of most miticides are 14 days or longer, which is an important factor in the decision making process. Another confounding factor of pre-harvest mite control is that often the mite count is low early in the season, and the numbers may not warrant a miticide at that time. Because of these issues, many cherry growers apply a miticide after harvest when the mite numbers are higher and when PHIs are no longer a concern.

Damage and injury

Bronzing is the most common damage caused by high populations of two-spotted spider mites; this phenomenon can cause a reduction in photosynthesis and fruit bud initiation. Bronzing caused by two-spotted spider mites is often more gray in color than bronzing by European red mites. Although bronzing presents an obvious challenge, one of the biggest threats of mites in cherries in a hot, dry year is “firing.” Firing is an immediate result of increased temperatures and droughty conditions, most often in combination with high mite populations. Firing results in a collapse of a portion of the tree; this malformed segment can be a branch, a terminal, or a whole section of the tree. The leaves of a fired part of a tree turn brown very quickly, with no prior wilting, and the overall effect is similar to fire blight in apples. Although firing may occur with low mite populations, it is found most commonly where mite numbers are high. At one time, it was reported that firing only occurred with plum nursery mite infestations, but more recent observations suggest two-spotted spider mites play the major role in firing.

Monitoring

Scouting for mites should begin prior to harvest with enough time to determine if a miticide should be applied before harvest. This pre-harvest monitoring period should begin early enough to take the long miticide PHIs into consideration. One method of monitoring two-spotted spider mite motile populations consists of sampling 25 intermediate-aged leaves at three to five sites within a block. Count the number of leaves that have two or more two-spotted spider mite motiles from each sample and follow the binomial charts to estimate the number of mites per leaf. Treatment thresholds for two-spotted spider mites are eight to 10 mites per leaf from July through post-harvest.

Binomial sampling charts for two-spotted and European red mites

Examine 25 leaves per site at each of three to five sites per block.
Charts are adapted from “Orchard Pest Management” published by “Good Fruit Grower,” 1993

Biological control

Conservation of predator mites in cherry orchards is critical to control two-spotted spider mites. The three most predominant predaceous mites found in Michigan are Neoseiulus fallacis (Phytoseiidae), Agistemus fleschneri (Stigmaeidae) and Zetzellia mali (Stigmaeidae). Predaceous mites are small, but can be detected with a hand lens. Predaceous mites also move very quickly across the leaf surface compared to sedentary two-spotted spider mites. Carbamate and pyrethroid insecticides are highly toxic to mite predators, thus should be avoided if possible. Phytoseiid mites (A. fallacis) respond quicker (reproductively) to increasing populations of two-spotted spider mites, but Stigmaeid mites (Z. mali) can survive and are more effective predators at lower two-spotted spider mite population densities. Herbicide sprays also affect the number of predator mites within a cherry orchard. Clean, weed-free areas under the trees in fall and early spring eliminate optimal overwintering habitat for predaceous mites, and without predator mite presence early in the season, two-spotted spider mite populations can grow unchecked.