Small greenish caterpillars are eating the needles on my evergreen trees. What are these insects and how can I control them?
Sawflies are the immature, larval stage of tiny, non-stinging wasps. They are a common pest of evergreen trees and can seriously damage conifer trees through defoliation. There are many different species of sawflies, each with a preferred host. Several species of sawflies are present in Nebraska, including the European pine sawfly, yellowheaded spruce sawfly and larch sawfly. Most commonly damage is seen on Scotch, Austrian, Ponderosa, Jack and Mugo pine, along with spruce and larch. White pine is rarely damaged.
Lifecycle
Adults resemble small non-stinging, wasps or flying ants. Females cut slits in evergreen needles with a saw-like apparatus and insert eggs. A new generation of immature insects begins to hatch out from late April through May, and will feed on evergreen foliage until mid-June. The larvae resemble a caterpillar and are light to olive green with a reddish-brown to black head. Larvae usually have several darker green stripes longitudinally down their bodies. There is only one generation of this insect per year.
After hatching out in spring, the larvae feed in large groups on the branches of affected trees. At first the larvae eat only the outer portion of the pine needles, leaving behind the stringy, tough, central needle vein, which dies and turns brown. As the insects mature, they begin consuming entire needles. Due to the coloration of the insects, which blends in well with the color of the needles, the insects are often not noticed until considerable defoliation has already occurred. When disturbed, larvae rear back into a defensive "S" position.
Control
If you're working with a small tree, or only a few larvae are present, then simply knock them off the tree either by hand or with a garden hose. They seldom find their way back up onto the tree and are very susceptible to predators such as birds, rodents and predatory insects.
Large populations should be treated as soon as possible. Several chemical controls can be used, such as insecticidal soaps, horticultural oils, permethrin, bifenthrin, malathion or orthene, but all controls are most effective when the larvae are small. Dipel, or Bacillus thurengiensis, is NOT effective against sawflies because they are in the Hymenoptera family (ants & wasps), not the Lepidoptera family (butterflies & moths).
For more information:
Insects of Evergreen Trees, Nebraska Forest Service
Sawflies of Trees and Shrubs, University of Minnesota Extension