More and more homeowners are becoming alarmed over the “worms” that are devouring the needles of their pine trees, according to the Minnesota Department of Natural Resources (DNR).

“These aren’t worms, but larvae of the European pine sawfly,” said Val Cervenka, DNR forest health program coordinator. “The name is misleading, as the insect is not a fly, but a small, stingless insect that is a relative to the wasp.”

European pine sawfly larvae have black heads and are gray-green with lighter stripes along the body. They prefer red (Norway), Scots, mugo and jack pine, but will go to white and Austrian pine in the absence of the preferred hosts. They feed mostly on last year’s needles, but there have been reports of their feeding on new growth as well.

The larvae have been feeding since late May, but many people, especially in Anoka County, are just now discovering the damage, according to Cervenka. Sawflies rarely kill trees on their own, although severe defoliation may occur. The healthier a tree is, the more damage it can tolerate.

Because the damage to trees has already been done and the larvae are full-grown (3/4 of an inch to 1 inch), treatment with an insecticide now won’t be effective. For treatment against an ongoing sawfly problem next year, a systemic insecticide containing imidacloprid may be applied at the base of the trees later this summer or early fall. It takes several weeks for the tree to “take up” this insecticide through its root system, so if it’s applied in the fall, enough time must be allowed for the tree to thoroughly absorb it before winter conditions prevail.

It will be important to start watching pine trees early next May for young larvae or missing needles when there are more options for control, according to Cervenka. When larvae are less than 1/2 inch in size and damage to the tree is slight, the following treatments are suggested:

• Use a low-impact insecticide such as insecticidal soap or spinosad (e.g., conserve)
• Use a product containing esfenvalerate, bifenthrin, permethrin or carbaryl (Sevin) for a longer lasting, residual insecticide.

“Minnesota sees a relatively small population of European pine sawfly each year, but because of increased winter survival and drawn-out, cool spring weather, we are seeing an abundance of them this year,” said Cervenka.

For more information on European pine sawfly visit the Yard and Garden Newsletter at www.extension.umn.edu.