Another new pest has been documented in Western Oregon, the plum bud gall mite. This microscopic mite was found this summer infesting a 15 year old Shiro plum tree in a home garden in the southern Willamette Valley. It was found again a bit later in the north Valley, both times causing multiple swellings of the leaf or fruit buds.

The damage was due to the eriophyid mite Acalitus phloeocoptes, which has not been reported in Oregon previously. This mite normally is found in parts of Europe and the areas around the Mediterranean Sea where Prunus species (which includes plums) are grown. These mites are tiny (see image D) and look like microscopic, cream-colored bananas. They are quite different from spider mites, the pesky creatures that cause yellow stippling and webbing on leaves. Eriophyid mites do not form silk-like webbing, and are much smaller than spider mites. They are difficult to see clearly without magnification.

mite infested bud

Tiny plum bud gall mites highlighted. Photo by Bill Gerth.

Spider mites

For comparison, here is a view of spider mites, which are larger and form webbing. © Ken Gray Insect Image Collection.

Plum Bud Gall

The adult female mites overwinter in the galls formed in the previous season, where she is essentially trapped by the swellings. Photo by Bill Gerth.

The mites cause growth abnormalities as a result of their feeding. They feed by thrusting stylet-like mouth parts into a plant cell and slurping up the contents, leaving behind eriophyid mite saliva. The saliva contains compounds that alter the plant cells on which they are feeding, usually resulting in some type of swelling – in this case, tiny round growths known as galls. The females lay eggs in the buds before they expand, and young mites develop within the buds, emerging three to four weeks after the egg stage. The galled tissues harden with time, so that by winter any remaining females are imprisoned within the galls until the following spring.

Although affected twigs may not be as vigorous as uninfected twigs, there is currently no reason to do anything other than perhaps trim out affected branches and bury, burn, or otherwise destroy them to prevent spread of the mites.

Since this is a new pest for Oregon, if you think you may have these mites affecting your trees, please notify the Oregon Department of Agriculture, your local county Extension Office, or the OSU Plant Clinic to get a positive identification. Sending close-up, in-focus images of the damaged twigs, or sending in the affected twigs themselves, will help us to monitor the distribution of this new pest.

Additional information: