Insect on bur oak
Location: Back yard, south Minneapolis, MN
July 14, 2011 8:31 am
Please help me identify this leaf skeletonizer. The bur oak host is very young, planted last fall. Each larva are approximately 1/4” long. If they will cause harm or stress the tree I will remove, as it is only on one leaf. If not, I prefer to let mother nature handle it on her own. Photo was taken on July 13, 2011, cannot find an image in my IPM book from U of Minnesota.
Signature: Julia V

Scarlet Oak Sawfly Larvae

Hi Julia,
We don’t recognize these creatures, and though they look like young caterpillars, we suspect they are more likely larval Sawflies, nonstinging relatives of wasps.  Alas, we haven’t the time to research this at the moment as we are preparing for a long weekend holiday out of town, but we are posting your letter and photo and tagging it as unidentified.  This is our last posting prior to leaving, other than to inform our readers that we will be out of the office until Monday.  You can try searching Bugguide under Sawflies, though these may be Moth Caterpillars.

We couldn’t resist the temptation to provide an identification, so we did some searching and we believe these are the larvae of the Scarlet Oak Sawfly,
Caliroa quercuscoccineae.  Here is the text from the United States Department of Agriculture Pest Alert website:  “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.  Microbial diseases and other natural enemies generally keep the sawfly in check. In outbreak years, insecticides may be needed on high-value trees.”  We would advise you to remove the leaf and closely monitor the tree for additional Scarlet Oak Sawfly Larvae.  There are additional images on the Forestry Images website, and the information at the bottom of that page states:  “Forestry Images is a joint project of the Center for Invasive Species and Ecosystem Health, USDA Forest Service and International Society of Arboriculture” which makes us wonder if this is an introduced species, but we cannot confirm that at the moment.

Location: Minnesota

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