Avian Predators of Eurosta

    The downy woodpecker is an important predator of Eurosta larvae in some areas.  In winter, when the woodpecker's other food becomes scarce, the downy turns to feeding on the larvae overwintering within ball galls. At some sites in central Pennsylvania predation rates sometimes exceed 60%. Research has shown that the woodpecker preferentially chooses larger galls to feed from.  This produces a strong selection for small gall size at sites where woodpecker predation is heavy.  Since, another predator of the Eurosta larva, the parasitoid wasp Eurytoma gigantea, selects galls with small diameter, the selection from the woodpecker and Eurytoma together can create a net selection for intermediate sized galls.  Many researchers have suggested that downy woodpeckers tap on galls to locate the fly's emergence tunnel (which the fly excavates prior to going into diapause in the Fall).  Field experiments have shown that downy woodpeckers use the presence of an emergence tunnel to determine whether a gall might contain an Eurosta larva.  Other researchers have found that when birds attacked galls containing E. obtusiventris, they did not eat the wasp larva, even though it was exposed.  The authors suggest that the wasp may be distasteful due to the presence of plant resin.

    The Black-capped Chickadee is another common avian predator of Eurosta larvae.  At some sites in southern Ontario, chickadees attack up to 50% of galls. Since chickadee's lack the powerful chisel-like beak of woodpeckers, they make large rough holes in the sides of galls by grabbing bits of the gall and pulling them apart.

The drawing to the left shows both the Downy Woodpecker (right) and Black-capped Chickadee (left) with their respective form of gall damage.  You can see a photo of galls showing chickadee and downy damage in the "Key to Gall Contents".

Key to Gall Contents
Solidago Biology
Ecology and Evolution
Insect Parasites and Predators
Eurosta Biology
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