While scouting for pecan nut casebearer damage yesterday, we discovered a newly hatched colony of walnut caterpillars (photo at right). At this point in their life cycle, the caterpillars are so small that they can't get their mouth parts open wide enough to chomp through the entire leaf blade. Instead, these first
instar larvae simply scrape off green tissue from both surfaces of the leaf. Once walnut caterpillars molt into the second instar, they will be large enough to make entire leaflets disappear.
I turned over the leaf and found the egg mass where this colony started (photo at left). Walnut caterpillar moths lay row after row of small white eggs in a large cluster. The eggs that look like empty cups are simply eggs that have already hatched. You can see a few eggs that are solid white indicating that a larvae has not yet emerged. However, if the egg hasn't hatched by this point it probably has been parasitized by a trichogramma wasps.
The walnut caterpillar has two generations per year in Kansas. The first generation hatches in late June to early July while a second generation appears in August. Walnut caterpillars feed together in large colonies. When it comes time to molt the entire colony moves to the underside of a large limb or the trunk and sheds their exoskeletons together in a large clump. You can see photos of later instars in posts I've made during previous growing seasons (
2011 &
2102).
