Source: Frank Peairs, Colorado State University, Bugwood.org
Description and life cycle
Twospotted spider mite adult males are pale yellow, pale to dark green, brownish, or at times faintly orange. An inconspicuous dark area is generally present on each side, and other dark areas are frequently seen along the middle. Females are oval, about 3/100 inch long and about 2/100 inch wide and vary in color, with gradations ranging from light yellow, straw color, green, brown, and black to various shades of orange. Newly emerged females have two large black spots, one on each side just back of the eyespot. After feeding, black splotches show up in irregular patterns in other parts of the body. Males are readily distinguished from females in this stage by the smaller size of the body, the pointed abdomen, and the small size of the spots.
Eggs are spherical in shape. When first deposited they are clear and watery, becoming opaque and glassy as incubation progresses. Eggs turn a pale straw color just before hatching. The embryo's red eyespots are plainly visible at this time. Newly hatched larvae are round, about the size of the egg, have six legs, and are colorless except for red eyes. Feeding begins at once and the color changes to pale green, brownish-green, or very dark green; two black spots appear, one on each side of the eyespot. Protonymphs, larger and more oval in outline than the larva, have four pairs of legs. They are pale green to dark green, sometimes brownish green. The two spots are larger and more pronounced than in the larvae. Deutonymphs are generally a shade of green, which apparently is influenced by food. The spots are larger and more distinct.
Full-grown females and some immature mites overwinter under bark scales on tree trunks or among fallen leaves and in other protected places on the ground. With the arrival of warm weather in the spring, these mites leave their places of hibernation and start wandering about looking for food plants. Almost all of those on tree trunks crawl down to the ground, where they feed on weeds and grasses.
The first eggs can usually be found about the first week in May. In warm weather, they hatch in 5 to 8 days. A complete generation from egg to adult may require no more than 3 weeks. There are from five to nine generations in the orchard each season, depending on the weather. In mid- or late summer, when drought and other factors, such as herbicide applications, cause poor food conditions among weeds and grasses, mites move from the old host up tree trunks or to low-hanging apple branches in contact with ground vegetation. Low-hanging branches that touch grass or weeds are usually attacked first; then the mites spread upward and into the tree interior.
Once established, the population may become a serious infestation and may cause injury. Injury to leaves resembles that caused by the European red mite, except that a grayish cast is more prevalent. As indicated previously, these mites also spin a fine silken web over many infested leaves. In the fall the adults either leave the trees and hibernate among weeds, leaves, or in the soil, or remain in the tree.
Monitoring and management
Twospotted spider mites should be monitored and managed in much the same way as European red mites. Counts of the two species should be combined in determining whether thresholds are exceeded.