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The blueberry stem gall
is caused by a small chalcid wasp, Hemadas nubilipennis, which belongs to the family Pteromalidae. In recent years, these galls have become a concern because they occasionally contaminate the finished blueberry product.
Blueberry stem galls
are small, kidney-shaped to irregular spherical growths on the stems of blueberry plants. They range in size from .19 inch to .98 inch. The adult wasps are tiny; being only .078 inch to .098 inch in length. The head, thorax and abdomen are black. The eggs and scape of the antenna are light amber. The antennal club is black. The wings are infuscated with black. The larvae are creamy white, legless grubs. Facts:
- The adult wasps emerge from galls during or after bloom and lay several eggs in young stems.
- The adults are almost entirely females and emerge from the gall before the buds break.
- Adult wasps seek out a developing blueberry shoot and lay several eggs in the stem. The majority of galls (up to 70 percent) are formed on stems within the leaf litter.
- After laying her eggs, the female climbs to the tip of the shoot and stabs the tissue at the tip of the stem several times, causing severe damage.
- Egg laying damages the plant cells near the eggs, which causes abnormal tissue growth.
- A chamber is formed around each egg.
- The eggs typically hatch in 12 to 14 days, and the larvae feed on tissue of the wall of the chamber. During larval feeding, the plant cells divide and multiply into large masses of tissue, which eventually form the gall. On average each gall will contain about 12 larvae. As the gall continues to grow, the outer covering is at first soft but becomes hard and woody by maturity. The larvae spend the winter in the gall and pupate within the gall in the spring
- Stem gall wasps cause kidney-shaped or spherical growths (.78 inch to 1.57 inches in diameter) on blueberry stems.
- Galls develop around the stems during the year, turning from green to brown.
- Pruning the galls out of the fields is the most effective control for this insect.
- There are no chemical controls registered for this insect.
- Burning as a pruning method may have some effect.
- Crozier, Lorne. The Blueberry Stem Gall. Updated 16 January 2001. Retrieved 06 July 2010.
- Isaacs, Rufus, Annemiek Schilder, Eric Hanson, Bill Cline. A Pocket Guide to IPM Scouting in Highbush Blueberries. (December 2004.) Type "2928" in the Inventory Number box on the ordering page.