The eggs hatch around late September. The mites will complete one or two generations in the fall, and then when temperatures drop down to regularly freezing, they'll lay overwintering eggs, which hatch around March. The eggs technically hatch into a larva, which looks like a tiny mite, but it only has three pairs of legs, Shetlar notes. When it molts into the first nymphal stage, it gains its final fourth pair of legs that make it a true arachnid. Then there's one more nymphal stage, and finally, the adult stage.
Adults are about half a millimeter, a little larger than a pinhead.
For PMPs, the mites fall into the category of occasional invaders, a group that includes lady beetles, earwigs and other outdoor pests that accidentally get indoors. For this group, Shetlar says it's important to keep in mind that exclusion is the best method of control.