Written By: Dr. Ash Sial, Entomologist, University of Georgia
Blueberry bud mite, Acalitus vaccinii (Keifer) is a tiny eriophyid mite. If abundant, bud mites can abort flower buds producing symptoms that resemble cold injury or poor fruit set. Blueberry bud mites are roughly 125th inch long, white to translucent in color, and somewhat carrot-shaped, with the front end or head being a bit larger than the abdomen. Blueberry bud mites spend most of their life cycles inside of blueberry flower and leaf buds.
Unfortunately, blueberry bud mites are usually difficult to confirm because they are so small and they spend most of their life cycles inside of buds. It is very challenging to dissect buds in order to find bud mites, and using heat to drive bud mites out of buds is likewise less than perfect. Confirming infestations in blocks that show bud mite symptoms requires diligence, a dissecting microscope, and some degree of luck as their vagrant stage, the one life stage found outside of the buds, lasts only a few days.
Blocks with damaging infestations of blueberry bud mites typically experience high flower bud mortality, poor fruit set and size, with some fruit reddening and blistering. Excessive bud mortality and substandard fruit set can be attributed to multiple stress factors. Blueberry bud mite-infested blocks, or infested hot spots in an orchard, tend to show symptoms year after year, as opposed to poor fruit set in an area where a ruptured water line might have resulted in poor protection from cold. Growers and IPM practitioners are encouraged to sample suspect blocks during flower bud initiation (August & September) when vagrant bud mites are moving into the next year’s flower buds.
Even in the absence of bud mite confirmation, it is often best to treat suspect blocks because suppression of low levels of bud mites mitigates a potential cause of inadequate fruit set. Management options for blueberry bud mite: post-harvest pruning of bushes, targeted insecticide application if one sees vagrant mites, and dormant oil applications.
1) Post-harvest pruning of bushes, if this is culturally appropriate in a particular block and cultivar, removes existing flower buds and forces bushes to reinitiate flower bud formation. Bud mites that colonized early forming flower buds, and those which migrate before bushes can form new buds will suffer very high mortality.
2) Vagrant stage bud mites, those exposed on leaves and stems, are very easy to control. However, it is important to remember that only the vagrant mites that are actively moving to new buds will be controlled. Control from miticides applied before migration begins are dependent on residual and good fortune, applications made after migration has taken place are of no value. Labeled insecticides/miticides that are active against vagrant stage blueberry bud mites include: Acramite, Portal, Oberon, Magister, Brigade, Danitol, Sevin and horticultural/superior oil (1-2% by volume). When using insecticides, always read and follow the label instructions.
3) High volume (~100 gal/acre), high pressure (200 psi), delayed dormant oil application offers a measure of bud mite suppression. Very thorough coverage and high pressure are needed to facilitate the penetration of oil into flower buds. Good sprayer calibration is imperative because overdosing with horticultural oil thins flower buds, producing the key consequence one is treating to avoid. Growers should also remember that, even when applied in the prescribed rate range, oil applications reduce bud hardiness for several days after application. Avoid oil applications 3- to 5-days before cold events. Further information on management of blueberry bud mites can be found in Blueberry Integrated Management Guide at https://smallfruits.org.