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Posts tagged “Eurosta solidaginis

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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Hairy and Downy Woodpecker Bills

12-31-13 woodpecker billsDistinguishing Hairy Woodpeckers (Picoides villosus) from their smaller relatives, Downy Woodpeckers (Picoides pubescens), can be challenging if you don’t have a chance to view both species at the same time. The easiest way to tell them apart is to note the relative size of their bills. The Hairy Woodpecker’s bill is proportionately much larger than the Downy Woodpecker’s – it’s almost as long as its head — whereas the Downy Woodpecker’s bill is not nearly as impressive. Although a Downy Woodpecker can’t drill or probe as deeply into trees as a Hairy Woodpecker, it does have at least one advantage due to its overall smaller size; it is light enough to balance on the stems of goldenrods, which usually aren’t strong enough to support Hairy Woodpeckers. It is here that Downy Woodpeckers drill for overwintering goldenrod fly larvae inside goldenrod ball galls – a popular winter snack for this species.

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Goldenrod Ball Gall Fly Larva

3-22-13 goldenrod ball gall fly larva IMG_6182The round “ball” that is often present on the stem of goldenrod plants contains the overwintering larva of a fly (Eurosta solidaginis). A year ago an adult female fly laid an egg in the stem of the goldenrod plant. The egg hatched and the larva proceeded to eat the interior of the stem. As it did so, the larva excreted chemicals which caused the plant to grow abnormally, creating a ball-shaped “gall.” If you were to open a goldenrod ball gall today, you would probably find an overwintering larva (if a downy woodpecker or parasitic wasp hadn’t gotten there before you). Within the next few weeks the larva will pupate, and as early as April the adult fly will emerge from the gall, having crawled out the passageway that it chewed last fall. An inflatable “balloon” on its forehead allows the fly to burst through the remaining outermost layer of tissue at the end of the passageway. The adult fly lives about two weeks, just long enough to mate and begin the process all over again.


Larvae-seeking Downy Woodpeckers

12-3-11  Larvae-seeking Downy Woodpeckers

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When cooler days arrive and adult insects become relatively scarce, insect-eating birds are very clever at gleaning the twigs, trunks and buds of trees and shrubs for overwintering eggs, larvae and pupae.  Certain galls (abnormal plant growths that house and provide food for a variety of insects) are sought by specific birds.  Downy woodpeckers seek the larvae of the Goldenrod Gall Fly (Eurosta solidaginis), which overwinter inside Goldenrod Ball Galls (formed on Canada Goldenrod, Solidago canadensis) before emerging as adults in the spring.  A tiny1/4” to 3/8”-wide hole (and an empty gall) is evidence that a downy woodpecker had itself a meal!