Includes rust mites and gall mites (Family: Eriophyidae)
Pest description and damage The tiny body of the eriophyid mite is translucent and cigar-shaped tapering to the hind end, with only four legs at the front end. Most Eriophyid mite species are host specific, but as a complex, they attack a wide array of plants. Individual species attack stems, flowers, buds, leaves and needles, or feed within needle sheaths. They look like blunt, little slivers and best viewed with a 20x hand lens or scope. Tissue damage varies with the host and includes leafy and woody galls of various shapes and sizes, stunting, erineum (felt-like patches), blisters, leaf curl, rusts, silvering, russeting of fruit, witch's brooms, twisting and chlorosis of needles, big buds on some hosts, stunting and deformities of seedlings and transmission of plant viruses. See specific plant hosts for more descriptive information. These mite numbers often drop or rise without human intervention. Sometimes mite populations are so abundant that the plant forms an abscission layer then drops infested tissues. The mites perish as these tissues dry out.
Biology and life history The fertilized female mites overwinter and emerge as the buds expand in spring. There are only two nymphal instars, the second instar molts into an adult after a brief resting period. The eriophyids reproduce almost through the growing season. They can complete their life cycle in as little as ten days or every two to three weeks. Mites living on leaves, flowers and fruit must migrate back onto the plant before the plant parts are dropped. At that time, they are more exposed and vulnerable to weather and natural enemies. The mites spend winter in permanent tissues on the plant such as needle sheaths, or cracks and crevices on bark or buds and other protected sites.
Pest monitoring Start by looking for silver or bronzed leaves, galls, or other symptoms. Then examine the symptoms for signs of eriophyid mites. Because eriophyid mites are so small, it takes close examination to uncover them in needle sheaths, or beneath bud scales, among the erineum, or within galls. Sometimes, mites are dislodged by striking a branch over a dark paper. At other times, it takes perseverance and a good microscope to locate mites in galls or affected plants. Wrap double-stick tape around twigs where mites can be captured and identified.
Damage caused by these mites is generally cosmetic and not a significant problem for the host plant.
Prune out the most infested branches or pick off infested leaves if this cosmetic damage exceeds your tolerance.
During migration between plant tissues, the eriophyid mites are more exposed and vulnerable to natural controls such as predatory mites, mite destroyer lady beetles, cecidomyid larvae, and other predators. Avoid using broad-spectrum pesticides that may kill predatory mites and other natural enemies.
Eriophyid mite populations often collapse on their own due to natural enemies and possibly plant defense mechanisms (extra hairs on leaves or buds, thicker plant cell walls, or plant chemicals).
See Table 3 in:
Chemical Control of Landscape Pests
For more information
Beers, E., J.F. Brunner, M.J. Willett and G. Warner (Eds.).1999. Indirect Pests Orchard Pest Management On-Line (http://jenny.tfrec.wsu.edu/opm/toc.php?h=3)
Davis, R.S. and T. Beddes. 2011. Eriophyid mites. Utah State University Cooperative Extension Fact Sheet Ent-149-11 (https://extension.usu.edu/files/publications/factsheet/eriophyid-mites20...)