Conifers/Sawworms or Sawfly worms
Our mugo pines have been attacked for two years by bright yellow worms which writhe up each branch eating it bare of needles. Two plants have been so damaged that they have mostly died. It happens very quickly. The worms will die when I use a garden pest spray.
How can I prevent the recurrence?
Early spring watch for the European pine sawfly. If branches on short needled pines such as scotch and mugo pine appear to move when you walk past, itís probably a mass of European pine sawfly larva. They will feed on last year's needles, so will disfigure pines but most likely won't kill them.
Usually only one generation occurs and the winter is spent as an egg inserted into slits along the edge of needles. The eggs hatched in April through mid-May and the larvae may feed until mid-June. The caterpillar-like larvae are grayish-green and have a light stripe down the back, a light stripe along each side followed by a dark green stripe. Full grown larvae are about one inch long. The larvae feed in groups or colonies, often with three or four feeding together on a single needle. Distributed larvae raise their heads and tails in a threatening manner. Mature larvae drop to the ground and spin tough, brown cocoons in the duff. A few larvae may pupate on the tree. The adults emerge in late August through September to mate and lay eggs. Each female lays six to eight eggs in a single needle and 10 to 12 needles are used. These eggs can be located after a hard frost turns the egg laying scar yellow.
You can continue to watch in early spring for the caterpillars and spray with Sevin or any garden type insecticide except Bt. OR you can treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soil around the tree and the roots will carry it to the needles and when the caterpillar eats the needles they are killed. This treatment will last for the season. Check with your local garden type store for this product.