Maple petiole borer
When maples suddenly drop green leaves during late May or early June, it is likely a result of maple petiole borer, Caulocampus acericaulis. Very little, if any damage, is caused by these pests despite the appearance of leaves on the ground. Maple petiole borers attack all maple species, particularly sugar maples.
Introduction and life cycle
A maple petiole borers is a non-stinging wasp commonly called a sawfly. They overwinter as pupae in the soil and emerge as adults in spring. Rarely observed, adults are small, ¼ inch long, with a dark colored head and thorax and a light colored abdomen. They lay eggs in the petioles (stems) of maple leaves in the spring.
After larvae hatch, they tunnel into leaf petioles. Larvae are cream-colored with brown heads, legless, and about 1/3 inch long when fully grown. They feed inside the petioles for 20-30 days. The process of tunneling into the leaf petioles disrupts the connective tissue, causing leaves to fall, often very suddenly. The larvae remain in the portions of the petioles that are still attached to the twigs which drop after another 10 days. Larvae exit the petioles and burrow into the soil to pupate. There is one generation per year.
Charles D. Pless, University of Tennessee, Bugwood.org
Fig. 2 Maple petiole borer larva next to damaged petiole of a sugar maple
Defoliation caused by maple petiole borers is distinctive. Individual leaves with part of the petiole still attached fall to the ground. The ends of the leaf petioles are often black and the leaves are usually still green. Infestations from maple petiole borers occur at low levels. In most cases only 10% of the leaves or fewer are affected by maple petiole borer. Although seeing leaves on the ground in late spring is disconcerting, the actual number of leaves involved is small. Trees easily tolerate this level of defoliation. Maple petiole borers will not impact tree health or detract from its appearance.
Drought, squirrels, heavy aphid or scale infestations can also cause premature leaf drop. However, squirrels chew off whole twigs rather than individual leaves. Premature leaf drop due to drought or aphid or scale infestation generally happens later in the summer and leaves are typically discolored.
Managing maple petiole borer is difficult and unnecessary. Their damage causes no real injury to trees and their activity from year to year is sporadic and unpredictable. Insecticides are impractical to use and should not be applied.
Raking fallen leaves is not effective because the larvae are in the petioles attached to the trees and not the leaves on the ground. Destroying petioles that drop after leaves have fallen can kill larvae but will not make enough of an impact to their populations to make any noticeable difference.