Pear blister mite Eriophyes pyri is a relatively widespread but minor pest of pear trees and occasionally of apple trees. It is a problem that looks worse than it is as in the majority of cases the damage to the health and cropping of the tree is not very significant.
The microscopic mites which are too small to be easily visible to the naked eye live inside small swellings up to 3-4mm in diameter known as galls on the leaves during the active part of their life cycle. In the spring as the galls develop on the young leaves they have the appearance of small blisters - hence the name blister mites. The galls are usually a lighter slightly yellow colour compared to the green of the rest of the leaf. They may also have a reddish tinge. Later in the summer the galls turn dark brown or black as the leaf tissue dies. The galls are most commonly found along the sides of the main vein of the leaf. Badly affected leaves may drop prematurely. Occasionally fruitlets may be affected resulting in marked or deformed fruit. Light infestations are not very noticeable. Heavy infestations can look bad but the damage to the health of the tree is usually not important.
The mites overwinter under the outer bud scales of pear trees. At bud break in spring they emerge and become active on the underside of the new leaves and gradually penetrate inside the leaf tissue feeding on the cells inside the leaf. They excrete substances which cause the growth of the developing leaf to swell and form galls. They spend the summer feeding and breeding to produce several generations of mites inside the leaves. In early autumn before leaf fall they emerge and migrate to their overwintering sites under the outer scales of buds due to open the following spring. Spread from tree to tree or from one part of a tree to another occurs mainly at this stage when emerged mites may be blown about in the wind.
In the case of light infestations the most seriously affected leaves can be taken off in the spring to limit spread of the pest. But this can be self defeating and care must be taken not to take too many leaves and do more harm than good. At present there are no pesticides available to amateur gardeners for use against pear blister mites. It is therefore a pest that has to be tolerated. In most domestic garden situations predators will usually keep the mite population down.