These wasp larvae feed on several species of mountain ash throughout the northern tier of states in the U.S. and in southern Canada. The female wasp deposits eggs in short rows along the margins of leaves, leading to brown spots at this point on the leaf. The larvae then feed in clusters on the leave, beginning along the margins and leading to complete defoliation of the branches they feed on. This feeding begins in late spring and multiple generations are likely. The life cycle from egg to adult is about 1 month, with the larvae pupating on the lower stems or in the soil below.
Insecticides are effective but must be directed to the lower surfaces of the foliage. Contact insecticide sprays can be used or a soil-applied systemic can be applied in advance of the presence of the damage on the leaves. The use of horticultural oils or soaps will also be effective but must be applied so that the larvae are thoroughly coated with the spray.