Description and Biology
The butternut woollyworm, Eriocampa juglandis, is actually a sawfly caterpillar (sawfly caterpillars have ten pairs of legs — a pair of legs on every segment — whereas moth and butterfly caterpillars have eight or fewer pairs of legs). Fully grown butternut woollyworm caterpillars are green with blackish spots and a black head. However, the caterpillars are covered with a thick, fluffy secretion that makes them look like nothing ever seen before! They feed in groups, which also contributes to their visual impact. When mature in mid July, the caterpillars crawl to the soil and spin a cocoon that incorporates soil particles. The rest of the summer, winter, and spring is spent as a prepupae inside the cocoon. In April, prepupae molt into pupae that soon develop into dark flylike insects about 7/16 inch long (males are smaller) that emerge from the cocoon to mate. These sawflies have black bodies with white legs. The wings are held flat over the back when at rest. Females lay their very small, creamy-white eggs inside living leaves using their sawlike ovipositors (hence the name sawfly) to cut small slits into the midribs on the upper surface of leaflets in which to deposit her eggs. Eggs hatch in about a week in mid May and tiny butternut woollyworms begin to feed on the underside of the leaves. At first, the small caterpillars chew small holes between veins, but as they grow to 7/8 inch long, they consume entire leaflets leaving behind only the larger veins and midrib. The butternut woollyworm has only one generation of caterpillars each year.
The butternut woollyworm is about 7/8 inch long.
Photo by Bruce Watt, University of Maine, Bugwood.org
Naked after they first molt, butternut woollyworms soon secrete a lovely, fluffy covering.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Butternut woolllyworms spin tough cocoons in which they spend most of their lives.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Adult butternut woollyworms are small, fly-like insects.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Butternut woollyworms mate before females lay their eggs.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Butternut woollyworm eggs are laid along the midrib.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Even very young butternut woollyworms secrete a white fluffy covering.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Host Plants
Although not abundant, butternut woollyworms feed on black walnut, butternut and hickory anywhere these trees grow. Infested trees usually survive these short-term outbreaks without real harm, but the the trees may look bad until new, undamage leaves emerge. The butternut woollyworm is primarily important as a pest of shade trees.
Scars where butternut woollyworm eggs are laid soon turn brown.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Butternut woollyworms usually don't consume the midrib.
Photo by Lacy L. Hyche, Auburn University, Bugwood.org
Residential Recommendations
Most sawflies are very sensitive to pesticides. Sevin, Orthene or one of the other insecticides labeled for landscape use should give more than adequate control.
Other Resources
- Butternut woollyworm: A unique summer pest of walnuts. Bricault, B. 2014. Michigan State University, MSU Extension.
- Seasonal Cycle and Habits of the Butternut Woollyworm. Hyche, L. L. 1989. July 1989 Alabama Agricultural Experiment Station, Auburn University. Bulletin 601.
- Extension Plant Pathology Publications and Factsheets
- Horticultural Science Publications
- North Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local N.C. Cooperative Extension Center.
This Factsheet has not been peer reviewed.
Publication date: May 20, 2019
Revised: March 23, 2024
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