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Blueberry Stem Galls

3-29-17 blueberry stem gall IMG_7405Up to a dozen tiny black wasps (Hemadas nubilipennis) will emerge from this gall in the spring, around the time when blueberry bushes are flowering. After mating, the female wasp lays her eggs under the surface of the blueberry stems. Once she has completed her egg-laying, she climbs to the tip of the shoot and repeatedly stabs it, preventing further growth.

The plant reacts to the wasp’s egg-laying by forming a kidney-shaped gall. The majority of galls (up to 70%) are formed on stems within the leaf litter. These galls can be up to an inch in diameter, and they contain many developing larvae that feed on the walls of the gall and grow during the summer, overwinter as larvae, pupate inside the gall in the spring, and then emerge as adults when the blueberry bushes are in bloom in late May and early June. The adults are almost entirely females.

If a blueberry bush has many galls, it can be problematic.  A branch possessing a blueberry stem gall will not produce flower buds, and no flowers means no blueberries.

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13 responses

  1. Alice Pratt

    Are they on highbush “wild” blueberries and cultivated?

    March 29, 2017 at 8:12 am

  2. Interesting. I just planted several blueberry bushes last summer. If I notice these galls at some point, do you have a recommendation?
    My understanding is that, in many (most?) cases, galls are not significantly harmful to the plant, but it seems that with blueberries, they are problematic… (We’re studying galls in our Four Winds lesson this month. I never realized how many different types of galls there were!)

    March 29, 2017 at 8:40 am

    • Hi Dell, If you find these, pruning and removing them would be a good idea, especially before the exit holes of the wasps appear!

      March 29, 2017 at 8:45 am

      • Thanks, Mary. I’ll be on the lookout! (One more reason to be observant!)

        March 29, 2017 at 8:48 am

  3. Alice Pratt

    Thank you! That was going to be my next question, if the wasps are beneficial or not, as some wasps lay their eggs on tomato hornworms & help “kill them off.”

    March 29, 2017 at 9:21 am

  4. Alice Pratt

    Braconid wasp, and I just read the wasps don’t sting humans 😀

    March 29, 2017 at 9:41 am

  5. Kathie Fiveash

    So the males must be highly polygynous!

    March 29, 2017 at 1:09 pm

  6. How keen of you to capture an adult emerging! I can’t imagine that is easy, even in a ‘lab’ situation.

    March 29, 2017 at 3:27 pm

    • You give me too much credit, Eliza! If you look carefully you’ll see that I am not the photographer. I have tried keeping galls with that in mind, but somehow the insects always manage to escape through the cheesecloth I have over the top of the jar!

      March 29, 2017 at 5:48 pm

  7. Laurie Spry

    What is the theory as to why the female doesn’t want the stem to flower? Would that divert energy from building the gall?

    March 29, 2017 at 7:41 pm

    • I wondered that myself! Your guess would also be mine, but I don’t know for sure.

      March 29, 2017 at 9:19 pm

      • Laurie Spry

        Thanks Mary!

        March 30, 2017 at 7:10 am

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