Sawfly larvae resemble caterpillars at first glance. The young mountain ash sawfly larvae are greenish with black dots down the sides, and with black legs and head. As they reach maturity, the head and legs become distinctly yellow-orange. The larvae feed gregariously. The adult sawfly resembles a stout wasp in appearance, and is yellow with black spots.
Cocoons are tan to brown, oval in shape, and are found on the ground under the tree.
In late May or early June adult sawflies emerge from the overwintering cocoons and the females begin to lay eggs in slits cut into the leaf edges. The tiny larvae begin feeding as soon as they hatch from the eggs and increase gradually in size of the next three to four weeks. When the larvae are fully grown, they drop to the ground and spin cocoons. There is usually one generation per year, but at times, a partial second generation may occur. The larvae feed in groups starting on one or two branches, but soon moving on to others as the food source is depleted.