The brownish-red body of the adult female is about 0.4-0.7 mm (0.016-0.027 inch) in length, whereas males are much smaller and of a lighter color. Winter eggs are almost spherical, about 0.15 mm (0.01 inch) in diameter and brick-red; the color of summer eggs varies
P. ulmi mites feed by puncturing cells of the leaf parenchyma with their stylets and sucking out their contents. This causes physiological changes in the leaves, and photosynthesis, transpiration and nitrogen accumulation can be severely constrained.
At first, pale spots appear in tissues around the mite colonies. Later, entire leaves assume a rusty brown („bronze“) color; if infestation then continues, they may drop prematurely. In spring, large congregations of larvae on young shoots can lead to their deformation; infested flower primordia will fail to bear fruit.
Both fruit quality (e.g. color, sugar content, size) and yield are reduced. High population densities can even result in diminished flowering and fruit load in the following season.
Mites are found mainly on the undersides of leaves, along the veins. As the name “spider mite” suggests, they spin silken threads that can be seen connecting adjacent plant parts if there is heavy infestation. The threads help to protect the mites from being carried off by the wind.
P. ulmi overwinters in the egg stage. In April/May, when apple buds begin to open, the six-legged larvae start to hatch and immediately move to the leaves, where they start feeding. The other two immature stages (protonymph and deuteronymph) each have eight legs, like the adult mite. There is an inactive period before each molt.
During the summer, the life cycle of mites can be completed in as little as 2-4 weeks, depending on temperature, so several generations can appear, with the number varying from 3-5 in cool temperate regions to up to eight in warm areas.
The highest population densities are normally reached in July/August.
Females live about 2-3 weeks and produce ca. 20-50 eggs each. Summer eggs are laid on leaves and begin to develop immediately. In August/September, the females start laying batches of the diapausing winter eggs on rough bark, e.g. on old wood, in scars, or around buds. These are often numerous enough to be easily visible as red spots.