Eggs hatch in about one week and larvae begin to feed on the underside of the leaves. The female cuts small slits into the midribs on the upper surface of walnut leaflets and deposits her eggs into the slits. So it is not a new pest of walnut and it is presumed to feed throughout the range where walnuts grow, also feeding on butternut and hickories.įor the butternut woollyworm, adult sawflies emerge from the soil in spring to mate and lay their eggs.

The mystery insect is a butternut woollyworm, Eriocampa juglandis, a type of sawfly that is found across the Northeastern United States and Southern Canada. So following the “looks like a duck, quacks like a duck” principle, I checked my reference books and the Internet for a sawfly that feeds on walnuts and found the culprit. The larvae of sawflies feed in groups in close proximity to each other, stripping foliage as they move through the tree.

Sawflies that are common in Michigan include the pine sawfly and the mountain-ash sawfly. The name sawfly comes from their unique saw-like ovipositor, a tube-like projection which the female uses to lay eggs into plant tissue. The adult sawfly is a small, wasp-like insect that does not sting. The sawflies I have seen feeding on the dogwood leaves are also woolly white larvae that resemble a caterpillar, but sawflies are different since they do not turn into a moth or butterfly. Though I had never seen this pest on walnuts, I had seen a similar insect attacking dogwood leaves, called a dogwood sawfly. Working with Michigan State University Extension is very humbling at times, there is always something new to see and learn. Nature really is amazing! Thirty years working in forestry and horticulture and I had never seen this creature attacking walnuts. Inside the bag feeding on the walnut leaves were strange larvae covered in a thick, white mat that reminded me of knotted-up wool. Nothing unusual about that I often get walnut leaves brought into the office throughout the summer that are damaged by fungal diseases, fall webworms or maybe a petiole borer. In late August 2014, I walked into my office to find a bag full of walnut leaves.

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