Issue 11, July 17, 2017

Oak Skeletonizer

High infestations of oak skeletonizer are being reported from southern Illinois and Missouri. This insect is known to feed on many species of oak and chestnut and is common in low numbers on pin oak and red oak in southern Illinois, but in this case shingle oak is primarily affected.

Oak skeletonizer damage to shingle oak.

Oak skeletonizer caterpillars eat through the lower leaf epidermis and consume the mesophyll, leaving the upper epidermis intact. Initially, damaged leaves appear whitish, but soon the exposed epidermis cells die and turn brown.

Oak skeletonizer leaf damage.

The green to yellow slender larvae pupate on the leaves, forming white, ribbed, slender, silk cocoons about one-eighth inch long. There are two generations per year. The first generation is just ending so another generation will occur within the next few weeks.

Oak seletonizer larva and damage.

There is a high rate of parasitism in this species, so the second generation may not be all that noticeable. It is common to find parasitic wasp cocoons on leaves damaged by oak skeletonizer. They are one-eighth inch long, smooth, oval, white cocoons with indistinct brown to black markings. They are more obvious than oak skeletonizer cocoons and are commonly submitted for identification.

Although the damage is obvious, control efforts are usually not warranted. Severe damage like this in the spring will likely reduce growth this year, but a healthy tree should be able to survive without serious health impact. Aesthetic injury is severe for landscape trees, but infestations are so sporadic and the larvae so small, that scouting is difficult and preventative treatments are not warranted. If necessary, these insects are susceptible to insecticides labeled for foliar caterpillars on oak. (Phil Nixon)

Phil Nixon

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