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. Different species are fairly host specific, but as a group they attack a wide array of plants. They attack stems, flowers, buds, leaves and needles, or they feed within needle sheaths. They look like blunt, little slivers and are best viewed with a 20x hand lens or scope. 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. There are also several other less common eriophyid mites that damage plants. See specific hosts for more descriptive information. Despite these symptoms, mite numbers often drop substantially without human intervention. When mite populations are high, the plant creates an abscission layer and drops infested leaves. The mites perish as the leafy tissue dries 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 continuously through the 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 shed. 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 can be dislodged by striking a branch over a dark paper. At other times, it takes perseverance and a good scope to finally locate mites in galls or affected plants. Wrap double-stick tape around twigs where mites are suspected, and where they can be easily inspected for trapped mites.
Damage caused by these mites is generally cosmetic and not a significant problem for the host plant. Tolerance is advised.
Prune out the most infested branches or pick off infested leaves if the 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, cecidomyids, and other predators. Avoid using broad spectrum pesticides that 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:
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...)