(Acari: Tetranychidae, Eriophyidae, Phytoseiidae, Stigmaeidae)
Most insects have three pairs of legs and three major body parts, whereas mites have two body regions (cephalothorax and abdomen) and can have two, three or four pairs of legs. Many adult insects have wings, but mites never do. Mites are extremely numerous and are found in many kinds of habitats. Their small size makes them difficult to detect, identify, and monitor. The mites that attack fruit trees in the United States fall mainly into two groups: spider mites (Tetranychidae) and rust mites (Eriophyidae).
Life on a Leaf
Eriophyids are also the only important group of mites that transmits plant viruses; the sole tree fruit example is Eriophyes inaequalis, the vector of cherry leaf mottle virus. Mites from all of these groups attack tree fruits.
Like the spider mites, rust mites also suck plant juices, but their feeding causes a silvering or brownish cast on leaves and russeting of fruit. In contrast, the spider mites (which also suck plant juices) cause stippling, a series of white dots on the leaf where chlorophyll has been removed; as the leaves age or damage increases, the leaf takes on an overall brownish cast. On pear, the latter stages of spider mite damage cause the leaf to turn a solida dark brown or black.
Eggs: Rust mite eggs are extremely small, roughly hemispherical, and require 100-power or greater magnification to be seen.
Immatures: Rust mites have two nymphal instars. All stages of immatures and adults have two pairs of legs at the front end of the body, and an elongate abdomen (hystersoma) with many striations appearing as rings. This appearance is referred to as annulate.
Adults: Adult females occur in two forms: deutogynes and protogynes. Deutogynes are a special form of female that overwinters. The differences between these two forms make species identification difficult. Protogynes are the normal females, which occur throughout much of the growing season and reproduced immediately upon becoming adults. These are the forms on which species identifications are usually based. Males also occur in the species that attack tree fruits, but do not overwinter. They are slightly smaller but are similar in appearance to the protogynes.
Rust mites overwinter as inseminated deutogynes. These females do not reproduce in the year in which they are produced. Some chilling is required before they will lay eggs in the spring. As with protogynes, these eggs produce both males and females. The rust mite has two nymphal instars, which resemble the adult but are smaller. Each molt is preceded by a quiescent phase. Male rust mites are produced by unfertilized eggs and females are produced by fertilized eggs (arrhenotokous parthenogenesis). They do not actually mate. The males deposit stalked spermatophores (structures containing a packet of sperm) on the leaves. Females walk over a spermatophore and empty it of the sperm, storing it in a special pouch called a spermatheca. Both sexes will be produced from eggs when females have access to spermatophores. Multiple overlapping generations are produced during the year, the number depending on the climate, location and condition of the host. The hibernating forms (deutogynes) are produced in response to either poor host condition or climatic conditions (cool weather, shorter photoperiod) that occur in the fall.
Relationships between the mite species are complex, and control of one may affect populations of another. Orchard mites have several natural enemies, but some broad-spectrum pesticides used for insect control can reduce or eliminate them. For that matter, many acaricides (miticides) have been as toxic or more toxic to predatory mites than to pest mites. Orchardists can effectively adopt practices (both in terms of chemical choices and cultural practices) that will allow predator populations to thrive. While this might not immediately solve mite problems, it can lead to more stable, long-term mite control and eliminate or slow the development of resistance to miticides.
Comparison of Tree Fruit Mites
|Mite||Crop||Overwintering stage||Description of adult|
|European red mite||Apple, pear, stone fruit||Egg||Red with large bristles on back|
|McDaniel spider mite||Apple, pear||Female||Greenish or yellowish with large spot on each side and smaller spots at rear|
|Twospotted spider mite||Pear, apple, stone fruit||Female||Light green to straw color with large black spot on each side|
|Yellow spider mite||Apple pear||Female||Pale yellow to white with dark markings on each side of abdomen|
|Apple rust mite||Apple||Female (deutogyne)||Very small, yellowish brown, tapered body|
|Pear rust mite||Pear||Female (deutogyne)||Very small, yellowish brown, tapered body|
|Prunus rust mite||Plum, cherry, peach, nectarine||Female||Pale yellow at first, turning brownish yellow or tan|
|Pearleaf (appleleaf) blister mite||Pear (apple)||Female||Very small, white to light red and wedge shaped, usually found inside blisters|