Blueberry Gall

Shiny blueberry (Vaccinium myrsinites) grows in sunny spots along the trails at the Oslo Riverfront Conservation Area (ORCA).  Its tiny fruits, shown above, are enjoyed by birds and other wildlife.

This low growing evergreen shrub with small alternate leaves flowers, often profusely, in the spring, and its whitish-pink urceolate (urn-shaped) flowers attract a variety of pollinators.

When I was checking for blueberries on an orchid stroll on June 27, 2020, I was surprised to see .a profusion of somewhat spherical growths on the stems of the shiny blueberry plants …

Blueberry stem galls are what they turned out to be.  They are the handiwork of a tiny shiny black wasp, Hemadas nubliipennis.  This wasp, which does not sting people, is .078 to .098″ in size, smaller than a mosquito and easily overlooked.

Blueberry stem galls wasps lay their eggs in soft green stems of many blueberry species including “commercial” blueberries.  Twelve to fourteen days later the larvae, creamy white legless grubs, emerge and begin to feed.  The plant responds with bizarre growth, and a gall is formed.

The galls vary in size from .19 to 1.25″ and in from from round to kidney-shaped.  Initially the galls are greenish and soft.  With time, the galls harden and turn a brownish color.

Each gall is said to contain an average of 12 larvae.  They spend the winter inside the gall protected from predation.  They pupate in the spring, and adults emerge during or after the plant blooms.

The blueberry stem gall most often prevents the stem from flowering and fruiting.  Blueberry growers cut off the galls since insecticides are ineffective against the well-protected larvae.

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