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Goldenrod Ball Galls Provide Important Source of Winter Food for Downy Woodpeckers

12-11-15 goldenrod ball galls 058A number of insects cause goldenrod plants to form galls – abnormal growths that house and feed larval insects. The Goldenrod Ball Gall is caused by a fly, Eurosta solidaginis. The fly lays an egg on the stem of a Canada Goldenrod (Solidago canadensis) plant in early spring, the egg hatches and the larva burrows its way into the stem; the plant reacts by forming a gall around the larva. The larva overwinters inside the gall, pupates in late winter and emerges in early spring as an adult fly. Prior to pupating, the larva chews an exit tunnel to, but not through, the outermost layer of gall tissue. (As an adult fly it will not have chewing mouthparts so it is necessary to do this work while in the larval stage.)

Downy Woodpeckers (and Black-capped Chickadees) have discovered this abundant source of winter food, and dine on the larva after chiseling a hole into the gall. Downy Woodpeckers tend to make a tidy,narrow, conical hole by pecking, while Black-capped Chickadees tend to make a messy, large, irregular hole by grabbing bits of the gall with their bill and tugging them free. While woodpeckers prefer larger galls that are located high on goldenrod plants growing near wooded areas, these are not the only factors taken into consideration.

A woodpecker extracts the fly larva through the tunnel the larva excavates prior to pupating, as this facilitates rapid removal of the larva. Downy Woodpeckers can determine whether or not a gall has an exit tunnel, and if it doesn’t, they usually abandon the gall without drilling into it. The likelihood of smaller parasitic wasp larvae occupying the gall (and a plump fly larva not being present) is much greater if there is no exit tunnel, and these smaller prey apparently are not always worth the woodpecker’s time or energy.

NB: Correction: this week’s Mystery Photo was of a Gray Birch (Betula populifolia) bract, not a Paper, or White, Birch (Betula papyrifera) bract. While similar, there are differences between these two species of birch that I should have recognized (especially when looking at the leaf!). Thanks to Kathy, an alert blog reader, who caught this error.

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7 responses

  1. I don’t think I’ve ever seen a goldenrod with three galls–so cool!

    December 11, 2015 at 7:44 am

  2. I wondered why my white birch bract looked so different than yours!

    December 11, 2015 at 8:06 am

  3. Al

    I’ve seen lots of these galls on goldenrod. Wondering if they really that abundant that they are a significant food source. Also wonder if they only (or mostly) grow on Canada goldenrod, or are they found on the many other species of Solidago. Wondering what other galls are hit by these birds….

    December 11, 2015 at 8:43 am

  4. Susan Hardy

    Hi Mary,
    We had never noticed holes in the goldenrod galls. Went to check and they all had been pecked into! Looks like it was all down by a downy.
    Thank you!

    December 12, 2015 at 6:42 pm

  5. Kathie Fiveash

    When I was in school at Antioch I did a project on Goldenrod galls to determine whether the galls reduce the seed production of the plants. I counted zillions of seeds from all those tiny flowers that make up the beautiful yellow plume. Turned out that the energy the plant put into making the gall did not reduce the seed production.

    December 12, 2015 at 9:44 pm

    • That is really interesting, Kathie, and very nice to know!

      December 13, 2015 at 12:05 pm

  6. Kathie Fiveash

    However, I think the individuals in a stand of goldenrod are clones, so they may share energy through the roots.

    December 13, 2015 at 4:47 pm

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