Brownheaded ash sawfly larvae are defoliating ash trees in central and northern Illinois. We have received reports from Douglas, Sangamon, Whiteside, and other counties. These larvae are greenish white to yellowish white and look somewhat like caterpillars, except that they have six or more pairs of prolegs. They have green heads with obvious small black eyes. I’m unsure why they are called “brownheaded.”
These larvae feed primarily on red and white ash. There have been reports of trees more than 40 feet tall being totally defoliated. If the tree is healthy, this severe of defoliation should not seriously harm it, and the tree will put out new leaves. Defoliation like this 3 or 4 years in succession could cause the tree to die. Because this sawfly is not common in most years, it is unlikely to be numerous enough next year to cause serious defoliation.
The larvae drop to the soil later in the spring when they are full grown and construct cocoons around themselves in the soil. Wasplike adults emerge the following spring when the leaf buds are turning green and lay eggs in the developing leaflets. The eggs hatch into larvae that feed on the leaves. There is one generation per year.
Because this pest is not a true caterpillar but rather the larval stage of a wasp relative, Dipel or Thuricide (Btk) will not control these insects. Carbaryl (Sevin) should be effective, as well as many other labeled insecticides.