The worms in your blueberry fruits are likely the larvae of either the cranberry fruitworm or the cherry fruitworm, both of which can be serious pests on blueberries. Control measures are similar, so you don't really need to identify which is the major source of the problem. Adults of these pests are tiny moths that become active in spring shortly after the blueberry bushes begin to bloom. They lay their eggs on the developing fruits, the eggs hatch, and the larvae tunnel into the fruit. Because these pests are moths, the organic pesticide B.t. can be used to control them. Spray during the egg-laying period, from bloom through June. Other steps you can take to reduce the problem include clean cultivation around the shrubs. (Adults of the cranberry fruit worm overwinter in the soil near the bush. Tilling the soil disrupts them, and kills many of them.) Infested fruits usually color earlier than non-infested ones, so removing and destroying fruits that begin to color early can break the life cycle of the pests, too.
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