Spring is the time of year when we enjoy the beauty of flowering trees and shrubs, the splendor of tulips, daffodils, hyacinths, alliums and all manner of spring blooming bulbs, and the return of European Pine Sawfly larvae, feeding voraciously on Mugho pines and other pine species.
The European pine sawfly is probably the most common insect pest on Mugho pines. The gray-green larvae looks like caterpillars, but are actually the larvae of a primitive wasp-like adult that are also related to bees. Beside Mugho pines, they are known to feed on the needles of Scotch, Red and Jack pines.
When the larvae first hatch, they are only able to feed on the needle surface, leaving behind twisted remnants of the needles, such as what you see in the background of the picture below. As they continue to grow, they will consume all the old growth needles and then move on to the next branch. Generally, they do not feed on the current year’s needles. This feeding habit will produce a “bottle-brush” appearance to the plant, with only the new needles remaining since the older needles have all been eaten away.
The Pine sawfly larvae have an interesting defense mechanism against one of their major predators – birds. The larvae feed in groups and there can be 30 or 40 of them on a single branch. When threatened, they will all rapidly rear up on their hind ends, displaying an imposing site to an easily startled bird.
The larvae will continue feeding for another couple of weeks before dropping to the ground where they will spin a hard brown cocoon in the leaf litter at the base of the plant. They will remain there until the end of August to early September when the adults emerge. After mating, the female will use a saw-like appendage on her abdomen to cut a small slit into a single needle into which she will deposit 6 to 8 eggs. She will repeat this process on 10 to 12 additional needles, located near the tip of the branch. If you look carefully, you may see the yellow eggs on the needles. They are easier to see after the first frost.
The adults often do not travel far from where they emerged from their cocoon, so if you have pine sawfly on your pine tree or shrubs this year, there is a good chance you will have them again next year on the same plant. If you scout for the eggs during the winter or early spring, remove and dispose of the needles that have eggs on them. If they do hatch and the population is small, cut off the branch where they are located and throw it away. You may want to take a paper or plastic bag to wrap around the branch to prevent any from falling off during the removal process.
Any commercially available insect control product will take off these insects. Be sure to read and follow all label directions. Often times, the damage is localized to just a few branches, so it may not be necessary to treat the entire tree. You can contact your local Spring-Green office and they will provide you with an estimate to control pine sawfly, along with other insect and disease pests that can affect your home landscape.