Question
We have one inch white worms that are severely eating the leaves of our oak trees.  We live in north western PA, rural area.  What is this worm and is there anything we can do to eliminate it.  Thanks.

Answer
Sounds like Scarlet Oak sawfly. The scarlet oak sawfly, Caliroa quercuscoccineae (Dyar) skeletonizes leaves of scarlet, black, pin, and white oaks in eastern North America. It is also called the oak slug sawfly because of the fact that the larvae are covered with a coat of slime that helps them adhere to foliage.

Larvae feed on the lower surface of the leaves, leaving only a fine network of veins which gives the leaf a transparent appearance. Defoliation starts in the upper crown in early summer and progresses downward. By late summer, heavily infested trees may be completely skeletonized.

Larvae overwinter in cocoons in the litter layer, and adults emerge in the spring. The adults, which resemble small fly-like insects, are about 6-8 mm long and are black with light yellowish legs. Females lay eggs in rows in the lower leaf surface along the sides of the midribs and larger veins. Eggs hatch within 1-2 weeks, depending on the temperature. Several larvae feed on the same leaf. Full-grown larvae are slug-like, yellowish-green, and about 12 mm long. There may be two to three generations per year.  

Insecticides can be sprayed on the foliage and these will kill the sawfly larvae. But on large trees this is almost impossible to get the spray to the foliage. Conventional insecticides include acephate (Orthene), carbaryl (Sevin), chlorpyrifos (Dursban), malathion, and diazinon.

The defoliation will not kill the tree but can reduce the growth of the tree. Hardwoods can withstand complete defoliation without much damage. Usually the population of sawflys is kept in check by natural predators and the following year the population is very low or non existent. Natural enemies normally control insect numbers, plus leaf damage from second and third generation larvae occurs late in the season, minimizing impact on the host. Repeated defoliation, however, may decrease growth, reduce vigor, or kill trees.



Here are some pictures of this insect.  http://www.forestpests.org/subject.html?SUB=107