Redberry mite
Acalitus essigi
5 host plants
On blackberry drupelets in the Puget Sound region, watch for uneven ripening where individual segments remain hard, green, or bright red instead of softening and turning black; this is classic redberry disease caused by Acalitus essigi. Mature-season blackberry cultivars like Apache and Triple Crown experience more damage than early-season types. The translucent white mites are roughly 0.1mm long and require 20-30x magnification to see.
Redberry mites overwinter within buds, then move to developing flower buds. Their feeding releases a toxin preventing drupelets from ripening. Scout flower buds in spring and treat before flowering if redberry disease was severe previously. Sulfur applications during bud development are effective.