The eriophyid mite is a strange and little-studied creature which affects many different plants. Welt-like growths around the edge of aspen leaves, a red felt-like growth along the petiole and veins of maple leaves, finger and bladder galls on leaves and flower-like structures on Junipers are the result of eriophyid mites. Woody galls on cottonwood and aspen where leaves, flowers and stems are supposed to develop are the result of eriophyid mites. Rust on the blossom end of pear fruit and brown rust-like spots on apple and pear leaves are additional problems caused by these mites.

Many gardeners are unable to see the spider mites which infest flowers, vegetables, trees and other plants, yet spider mites are much larger than eriophyid mites. These tiny creatures are about one-fifth of a millimeter in size (there are twenty-five millimeters in an inch). They can be seen with a thirty-power hand lens, but to see details, it is necessary to mount the mites on a slide and observe them using a microscope with at least 400 power magnification. When you do finally see these mites, they will have two pairs of legs near their heads, usually be translucent, and look like tiny walking carrots.

Eriophyid mites, with few exceptions, feed on perennial plants. They are host-specific, with one species of mite usually feeding on a particular plant. The mouth parts of the eriophyid mite are so small, it can penetrate plant cells without causing cell death; the majority of species therefore do not cause any noticeable injury to their hosts. Some species, however, produce leaf, flower and bud distortions by injecting a chemical similar to the weed-killer 2,4-D into the plant tissues as they feed. These species are referred to as gall mites, blister mites, bud mites and rust mites according to the damage they cause.

The eriophyid mite has two nymphal stages (instars), with the second stage developing into the adult stage after a short resting period. Development from the egg to adult stage can occur in ten days to two weeks under optimal conditions. Hundreds of species of these mites are known, but in many cases little information is available on their biologies and habits.

Carbaryl, Kelthane, dicofol, horticultural oils, insecticidal soaps and several other materials are recommended for use on these mites. These materials can be effective on exposed leaf mites. Mites living within galls are much more difficult to control. The application of a dormant oil in mid-March also provides some control. Removing and destroying galls is often necessary to stop the spread of mites on the current host plant and neighboring plants.

Reference:

Cranshaw, W. April 1999. Insect Management Recommendation Guide for Turfgrass, Shade Trees and Shrubs. XCM-38. Colorado State University Cooperative Extension
Dreistadt, S.H. (ed.). 1994. Pests of landscape Trees and Shrubs: An integrated pest management guide. Publication 3339. University of California.
Essig, E.O. 1926. Insects of Western North America. The Macmillan Company.
Furniss, R.L. & Carolin, V.M. 1977. Western Forest Insects. Miscellaneous Publication 1339. U.S. Department of Agriculture, Forest Service.
Johnson, W.T. & Lyon, H.H. 1976. Insects that feed on Trees and Shrubs: An Illustrated Practical Guide. Comstock Publishing Associates, Cornell University Press.
Kiefer, H.H. 1952. The Eriophyid Mites of California (Acarina: Eriophyidae). Bulletin of the California Insect Survey, Volume 2, No. 1. University of California Press, Berkeley and Los Angeles.
Larsen, H.J., Gaus, A.G., Zimmerman, R.J.. Rogoyski, M. 1997. Colorado Tree Fruits: Pest and Crop Management Guide. Bulletin XMC-41. Colorado State University Cooperative Extension. Ohlendorf, B.L.P. (ed.). 1991. Integrated Pest Management forAppless & Pears. Publication 3340. University of California.

Internet sites with additional information include:

Eriophyid Mites - Pests in Saskatchewan
ERIOPHYID MITES ( Eriophyidae ) - Cornell