Milan Zubrik, Forest Research Institute - Slovakia, Bugwood.org
Eriophyid Rust Mites
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.
