The maple petiole borer is an introduced species that has extended its range from the northeastern states to the Great Lakes area arid the Midwest. It is commonly found in southern Wisconsin, although it generally occurs at low levels and is usually overlooked. Occasional out breaks can appear, though, and leaf drop can be substantial. However, leaf drop rarely exceeds 25—30% of the leaves on a tree, even during peak years.

Symptoms and effects
The maple petiole borer [Caulocampus acericaulis (MacGilivray)] causes large numbers of leaves to drop to the ground in June, due to larval tunneling. The leaf stems (petioles) usually break off near the leaf blade, and the breaking point is often darkened in color.

This disorder usually affects only sugar maples and often occurs very suddenly. However, only a few of the tree’s leaves are actually involved, so this disorder has little effect on the health of the tree.

You can usually accurately identify an infestation of maple petiole borer in the field. The time of year that it occurs and the green coloration of the leaf blade distinguish this disorder from drought, aphid or scale infestations, or other problems that cause some leaf drop. However, squirrels may occasionally cause leaf and twig defoliation in early or mid summer. Thus, to identify maple petiole borer damage, cut a petiole near the leaf blade and examine the interior carefully for larval tunneling.

Life cycle
Adult petiole borers are sawflies that appear in May and deposit their eggs in the maple petiole near the leaf blade. After hatching, larvae tunnel into the petiole for 20—30 days. Because of this tunneling, the stem breaks off near the leaf blade and the leaf drops. Larvae remain in the portion of the petiole still on the tree for about 10 days more—larvae are about 1/3 inch long at this time. This portion then drops to the ground and the larvae move into the soil to pupate. They overwinter in the soil until the following spring.

Control
Chemical treatment is not recommended because attack by the maple petiole borer occurs infrequently and is unpredictable, and because this disorder does not significantly affect tree health and appearance. Also, damage occurs over a short period of time, and it probably is too late for effective chemical control by the time you notice the leaf drop.

However, you can reduce maple petiole borer populations the following year if you pick up and destroy infested leaf stems (especially the short sections without leaves) about 7—10 days after first leaf drop. Continue this stem cleanup throughout the leaf drop period. For such cultural controls to be effective, they must be practiced on all infested maples in the vicinity.

1997 University of Wisconsin-System Board of Regents and University of Wisconsin Extension, Cooperative Extension

Author: P.J. Pellitteri is distinguished outreach specialist of entomology and DL. Mahr is professor of entomology, College of Agricultural and Life Sciences, University of Wisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension. Produced by Cooperative Extension Publications, University of Wisconsin-Extension.