Spruce gall midge
The spruce gall midge, Mayetiola piceae, a curious pest, was reported this spring in the North Metro Area, occurring on white spruce. The midge, a tiny fly-like organism, overwinters in numerous swellings that encircle the terminal shoots. The cream to tan swellings make the shoot look a little like miniature corn smut with a few brown spruce needles sticking out between the swellings. Many of the swellings have a slight nipple or protrusion at the end where the pupa has pushed up against (and occasionally through) the outer shell; ready to emerge when they complete their development. In late May, the adults emerge leaving tiny round holes in each swelling.
The two-winged reddish-brown adults mate and lay eggs on newly developing shoots. The bright orange, but tiny, larvae hatch within a couple of weeks and bore into the shoots to feed on plant juices. In response to the feeding, plant tissue swells up around the larvae to form the galls. The larvae continue to feed through out the season.
The damage can kill individual shoots. Repeated attacks can cause brooming, a proliferation of shoots at the ends of the twig and disfigured growth. The damage is rarely severe and does not affect the long-term health of the trees. However, the damage is unsightly and can lower the aesthetic value of ornamental trees.
Fortunately, a tiny parasitic wasp is a common natural enemy. With easy access to the pupae at the end of each gall, the wasps are able to control most populations. Chemical control is not normally advised since the damage is mostly aesthetic and is rarely necessary due to the natural biological controls.
Inspect your trees in the early spring for signs of the galls. If found, prune those shoots out to remove the insects before they can emerge. Prune the shoots back to a side bud or lateral shoot. Be sure to burn or otherwise destroy the shoots you prune out.
If damage is really severe, you miss the time to prune and feel you really must use a chemical product, treat the newly developing shoots during early June prior to egg hatch. Once the eggs have hatched and the larvae have tunneled into the shoots, chemical treatment is not effective. Only use chemicals in severe cases to avoid knocking out local predators and parasites that help control insect pests.