Do you have a pine, especially a mugo, Austrian, jack, or red pine in your yard?
Now is a good time to check it for European pine sawfly activity. These insects are caterpillar-like with black heads and gray-green bodies and dark green stripes (when they are young, the dark green stripes may not be visible). While they are relatively small now, they eventually will grow to be about 1 inch in length.
Look closely for these insects as they are hard to see because they blend in so well with the needles. An advantage when inspecting for them is that they are gregarious, meaning that they occur in nonsocial groups, so there can be many feeding on a given branch (which is easier to find than individual sawflies).
Your first clue that European pine sawflies are present could also be finding defoliation on branches. As feeding becomes more severe, it is usually easier to find the damage than the insects themselves. European pine sawflies feed from about mid-May through June on last year's needles. Fortunately, large trees are typically not injured, although it is possible for small trees or shrubs to be severely defoliated. There is only one generation per year.
If you find European pine sawflies, first determine whether they are worth managing. If they are, there are a few options available to deal with them:
First, consider physically removing them. This can be done by wearing rubber gloves and running your hands up the branches, crushing the sawflies. There are several low impact insecticides that can be sprayed, especially insecticidal soap and spinosad. Keep in mind that Bacillus thuringiensis, which is effective against caterpillars, does not kill sawflies.
Most residual insecticides will work against sawflies. Because sawflies feed in groups, it is possible to spot treat infestations instead of treating entire trees. (Caution: Read the label directions of the specific product you intend to apply to ensure it is used correctly).
These insecticides are hazardous to bees, so don't apply them to flowering plants. Treat plants in late evening to minimize exposure to bees.
Jeffrey Hahn, University of Minnesota Extension Entomologist