Apple rust mite

Aculus schlechtendali

Hosts. The apple rust mite attacks cultivated apple and several other species in the genus Malus. It can also survive and reproduce on pear and has been found in mixed populations with the pear rust mite.
Life Stages:
 The egg is about 50 microns in diameter and 30 microns high. It is initially clear but becomes translucent as the embryo matures.
Immatures: The first nymphal stage is white, about 68 microns long, increasing to 104 microns before molting. The quiescent stage is immobile and the cuticle is shiny, later turning to tan. The second instar nymph is a pale tan and 104 to 124 microns long. This stage also goes into a quiescent stage before molting.
Adult: The adult is medium tan in color, becoming darker with age. Protogyne females are 166 to 181 microns long, slightly larger than the males.
Life History. The species overwinters as deutogynes (females) in crevices on twigs and under bud scales. Often large clusters can be found under a single scale. They emerge to feed principally on the undersurfaces of leaves as the buds begin to open in the spring. They lay eggs that hatch into immature mites which rapidly grow through two instars. There are several generations per growing season. Development is more rapid in warm temperatures. In one study, a generation took 39 days to develop at 50 F but only 10 days at 72 F. The lower developmental threshold is between 39 and 45 F. Overwintering forms can be produced as early as mid-July if foliage condition is poor, but by fall only overwintering forms remain. They seek sheltered sites to spend the winter.
In cooler climates, apple rust mite populations peak once in midsummer. In hot, dry climates, populations peak in early summer and again in fall, with the midsummer decline corresponding to temperatures above 95 F (35 C) and relative humidities below 20%.

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