Eriophyid Mites

Order: Acari
Family: Eriophyidae

Description and Life History:
These mites are small. Eriophyid mites are about 1/100 inch long. Unless you have a microscope or a powerful hand-lens handy, seeing them is nearly impossible. These mites have sausage-shaped, elongate bodies. Unlike their eight-legged relatives (spiders, scorpions and spidermites), Eriophyid mites have 2 pairs of legs.

Eriophyid mites overwinter as adult females, called a deutogyne. This is a special kind of female mite that is able to go dormant and wait out bad weather and conditions, like winter or extreme heat. Females seek protection in cracks and crevices in the bark, buds or down in leaf litter. Once spring comes and foliage starts to develop, females begin feeding and laying eggs. A female can lay about 80 eggs over a month’s time producing both male and female mites. Mites hatch and need to pass through two developmental stages. The time of development from egg to adult can take one to two weeks depending on conditions. Mites do not mate with each other; sacs that the male leaves lying around on the leaf surface fertilize the females as she walks around. No wining, dining or song in an Eriophyid’s lifestyle.

While Eriophyid mites are small, they are mobile and can move around on a plant just fine. However if you are 1/100 of an inch long, how the heck do you get from plant to plant or tree to tree? Eriophyid mites rely on wind, birds, and flying insects to disperse. Eriophyids, along with other arthropods, have been collected blowing in air currents hundreds and hundreds of feet above the ground. For the home landscape, people are also an important mode of transportation for mites. Many introductions into your landscapes are from movement of infected plant material.

Damage:
All Eriophyid mites are plant parasites. They are considered parasites because they rarely kill plants, much like animal parasites. Eriophyid mites penetrate plant cells and suck up the cellular contents. This feeding causes plants to deform their tissue. This type of deformation is usually called a ‘gall.’ Galls are specific responses to plant parasite feeding. Plants respond by forming a tissue barrier around the feeding animal. This contains the animal from spreading and the animal in turn gets a custom-made food supply. The plant’s response is specific to the species of mite feeding on it, causing predictable symptoms. Instead of having to identify the mite species, we usually identify the mite by the plant’s response to the feeding damage.

Bud mites are specific to infesting the developing buds and fruits of certain plants. Common bud mites found in Whatcom County are the redberry and dryberry mites. Both these mites cause the developing fruit of black berries to be deformed and stunted. Next time you pop a Himalayan blackberry with an undeveloped druplet in your mouth, you probably just ate a world of mites!

Gall mites cause abnormal tissue growth of the plant’s hairs and leaf cells. The gall forms a pocket that provides a protective area for the small mites to feed and reproduce. These galls can be quite apparent like we see in the Maple bladder gall mite. Galls can also appear as hairy mats called “erinea” like we see on walnuts infested by the walnut blister mite.

True blister mites cause plant deformations very similar to those of gall mites. The difference is that the pocket is formed in the mesophyll (internal leaf tissue) instead of the outer surfaces of the leaf. Pears in Whatcom County can have infestations of the pear blister mite.

Rust mites are common on apples and pear leaves. Rust mites generally do not cause any extreme deformation of the leaf surface like the other Eriophyid mites. Rust mites feed on the cellular contents of the leaf, which results in a bronzing or silvering effect. Very high populations can cause early defoliation.

Monitoring & Management:
Since these mites are so small, most detection of Eriophyid mites is through diagnosing plant symptoms. Look for leaf symptoms such as galls and blisters or leaf bronzing. Plants can tolerate large populations of mites; we find that it is the gardener that can’t tolerate the damage. With a pimply, blistery maple tree, the damage looks much worse than it actually is. Based on your perception, these deformations can cause very pleasing, unusual colors and shapes.

If you don’t feel this way, prune off infected leaves and use a dormant oil to smother the overwintering females.

It is only under rare circumstances that a pesticide application is recommended. Eriophyid mites do very little damage to plants and most plants can tolerate huge populations. Apple growers don’t even get a little trigger-happy when rust mites build up to over 300 per leaf. In fact, Washington State apple growers enjoy having established populations of rust mites. WSU’s own Dr. Stan Hoyt was one of the world’s pioneers in Integrated Pest Management during the 1960’s. He found that if growers could tolerate populations of rust mites, spidermites out-breaks happened less frequently. Why? Because rust mites offer a great alternative food source for our friends, the predatory mites. Broad-spectrum insecticides used for controlling codling moth, killed off the predatory mites. This, along with pesticide resistance, caused uncontrollable spidermite out-breaks. Learning this, Dr. Hoyt developed a program to conserve predatory mites by using selective pesticides for codling moth and tolerating rust mite populations. This program allowed predatory mites to build up enough to control spidermite populations as they appeared. Dr. Hoyt’s efforts paved the way to changing Washington State grower’s attitudes and contributed to the worldwide adoption of Integrated Pest Management.

So don’t be ashamed of your red, pimply maple; this type of tolerance was key to changing pest management practices.