July 1997 - Oak leafminers

 

Oak leafminers

A number of tiny insects in the genera Cameraria and Profenusa, as well as others, mine the leaves of oaks. Last year, oak leafminer activity was observed from Sherburne County north into Cass and Crow Wing Counties on bur and red oaks. This year, most counties in central Minnesota have bur oaks with leafminer activity. Conspicuous infestations occur occasionally, however, they tend to be short-lived and have very little impact on tree health. Leafminer activity causes discoloration of the foliage and, during heavy infestations, may cause premature defoliation .

The caterpillars of the solitary oak leafminer, C. hamadryadella, feed singly in each mine, but tend to merge when a number of mines occur on the same leaf. Caterpillars are flattened, have only rudimentary legs and when full-grown are about five millimeters long. There are two generations each year with the second generation overwintering as caterpillars in the leaf mines on the ground. They change to pupae in the spring. Consequently, they may be controlled by raking and burning the mined leaves in the fall.

graphic drawing of Oak Leafminers

Another closely related species is the gregarious oak leafminer, C. cincinnatiella. As the name suggests, several caterpillars feed together inside a single, large mine. Profenusa species are not caterpillars at all, but are leafmining sawflies. Larvae produce large, blister shaped mines then drop to the ground to overwinter as prepupae. Raking would not be an effective control tactic for these leafminers.