Redberry mite—Acalitus essigi
This eriophyid mite (family Eriophyidae) is a pest of blackberry fruit and rarely raspberry. Note that the dryberry mite causes similar damage to both blackberry and raspberry and numerous species of mites can occur on plants.
Because eriophyids are minute, the presence of redberry mite is generally diagnosed by characteristic symptoms on fruit as described below under "Damage." Similar damage to blackberry and raspberry is caused by another eriophyid, the dryberry mite.
The mites are carrot or wedge shaped and pale brown to translucent whitish. Adults are about 1/100 inch (0.25 mm) long. A dissecting binocular microscope or a hand lens of 20× is needed to see them. Deutonymphs (first-stage immatures) are about one-half the length of adults. Protonymphs (older immatures) are about the same length as adults. Adults and nymphs have 4 legs at the head (wider) end of their body. The oval eggs are translucent to white and about one-fourth the length of adults.
Redberry mite develops through four life stages: egg, protonymph, deutonymph, and adult. It overwinters in buds and on bark of axils, where a bud and twig bark touch. In late winter and spring when flower buds swell, the mites become active. They feed in buds and (when present) drupelet bases near the berry core and stem.
Development from an egg to a reproductive adult can be completed in about 2 weeks when temperatures are warm. There are multiple generations per year.
The redberry mite is primarily a pest of blackberry, but also feeds on raspberry. Ripening blackberry drupelets fed upon by redberry mites do not darken in color. They become brighter red, hardened, and sour and remain on old canes through winter. All or only part of the berry may be affected. Dryberry mite causes similar damage to both blackberry and raspberry.
Tolerate or prune out and destroy infested fruit. Where redberry mite has been a perennial problem, consider replacing blackberries with less-susceptible varieties, such as those that mature earlier in the season. Evergreen and Himalaya varieties are most susceptible to redberry mites.
By the time fruit damage becomes apparent, it is too late to control the mite. Where redberry mite was an intolerable problem the previous growing season, spray micronized or wettable sulfur on buds and terminals at the delayed-dormant stage, after buds swell but before they open. Application can be repeated at bloom or when shoots are 2 to 6 inches long. Sulfur application also helps control powdery mildew. Several applications of azadirachtin, narrow-range or horticultural oil, or neem oil at 2 to 3 week intervals can be made at the green fruit stage through when pink fruit appear. Do not apply oil within 1 month before or after a sulfur application and vice versa or plants may be damaged.
For more information see Redberry Mite on Blackberry [Acalitus essigi (Hassan)] from Utah State University and The Eriophyid Mites of California from the University of California.
Adapted from the publications above and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).