Spruce spider mites belong to the class Arachnida, like spiders, and the order Acari, along with other mites and ticks. Before reaching adulthood, they pass through the egg, larva and nymph stages.
Eggs: Spherical, reddish and about 0.13 mm in diameter.
Larvae and nymphs: Resemble adults but smaller and paler green. Nymphs have eight legs and larvae six.
Adults: Oval, very dark green or brown body. They have four pairs of legs but no wings or antennae. Females grow to about 0.5 mm and males are slightly smaller.
The adults, nymphs and larvae do not survive in winter. Only the eggs overwinter, hidden between the scales on buds or at the base of needles.
In late April, the overwintered eggs start to hatch. Newly hatched larvae start by attacking the older needles while waiting for the new shoots to develop. The larvae develop into nymphs and the nymphs into adults. After mating, the first-generation females lay their eggs at the base of needles. The rate at which subsequent generations appear depends on the climate, with warmer temperatures shortening the life cycle.
Populations normally reach their maximum in late July and early August. There may be as many as three to eight generations per year.
Starting in late summer (September until the first frost), the females of the latest generations deposit their eggs between the scales on buds or at the base of needles.