European Red Mite
The European red mite, Panonychus ulmi, a major tree fruit pest that attacks apples, stone fruits, and pears, is considered to be one of the most important indirect apple pests in Pennsylvania. The eight-legged female is 1/20 inch long, bright red, and has four rows of white hairs on her back. The male mite is smaller, lighter in color, and has a pointed abdomen. The overwintering egg is round, bright red, and has a small stalk approximately the length of the diameter of the egg arising from the top. Summer eggs are pale and translucent. Six-legged nymphs hatch from the eggs, molt to eight-legged protonymphs, then become deutonymphs, and finally adults.
The rate at which mites develop is primarily temperature dependent. Hot, dry weather favors development, while cool, wet weather delays mite activities. Overwintering eggs are laid in groups on roughened bark, in crevices and cracks, and around bud scales on twigs and branches. Eggs begin to hatch at prepink bud stages and continue throughout bloom. Young mites move to newly opened leaves where they feed, mature, and reproduce. The first generation requires approximately 3 weeks to develop; summer generations are completed in 10 to 18 days. Each female is capable of laying 35 eggs during her average life span of 18 days. Eight to ten generations occur during the year.
Mites feeding on leaves cause injury to the tree by removing nutrients. The most serious injury occurs in early summer when trees are producing fruit buds for the following season. Moderately to heavily infested trees produce fewer and less vigorous fruit buds. Mites feeding on leaves also reduce the ability of leaves to manufacture enough food for desirable sizing of fruit. A characteristic brown foliage that, in severe cases, becomes bronze results from heavy mite feeding.
In general, on apple trees in the dormant period up to prepink, overwintering mite eggs can be evaluated with a hand lens. Pay particular attention to the bases of twigs and spurs. Look for clusters of tiny (less than 1/50 inch), red spheres. If overwintering eggs are easily visible, especially to the unaided eye, then an early season oil application should be made.
Through June, European red mite populations should be kept low; after July l, mite populations can be allowed to rise. Close watch must be kept on populations at this time of year. If weather conditions are favorable and predator response is low, a miticide should be applied to suppress mites and allow predator populations to increase. In general, an average of one to three mites per leaf is tolerable through May, five to seven mites per leaf in June and July, and seven to twelve mites per leaf for the rest of the growing season. An exception concerns pears, which are badly damaged even by small mite populations. On pears, a miticide should be applied at the first sign of mites.
Several predators can help control mites and reduce the use of pesticides. These include other predatory mites and other insect species. Of these, the most important in Pennsylvania is the black ladybird beetle, Stethorus punctum. This black beetle is small--the size of a pin head--and can be seen walking over the leaves. If disturbed, it quickly falls to the ground. If an average of less than five mites per leaf is present and Stethorus is also present, wait a few days to see if the mite population is rising or falling. If the mite population is rising, a pesticide may be necessary. If the mite population is stabilized or decreasing, the predators might be eating enough mites to control it, and no pesticide should be applied. Also, other mite predators such as Zetzellia mali, Amblyseius fallacis, or Typhlodromus pyri should provide excellent mite control. The applications of pyrethroid insecticides and carbaryl, used for the control of various other pests, are very detrimental to natural mite predators and are known to flare mite populations on fruit trees.