The native black walnut (Juglans nigra) produces edible nuts with a distinctive flavor. Black walnut trees generally grow in U.S. Department of Agriculture plant hardiness zones 4 through 8. Black walnut trees suffer from several insect pests, including three that get inside the walnut hulls and ruin the nuts. These pests can occur wherever black walnuts grow.
1. Walnut Curculio
The walnut curculio (Conotrachelus retentus) is 1/4 inch long and has a dark brown snout. This beetle is part of the weevil family. It’s a major pest that can ruin up to one-half the nuts on a tree. Overwintering adults emerge in late April, mate and lay their eggs in May. They chew into the husks and deposit several eggs in each nut. The larvae feed on the nut tissues for about four weeks.
2. Walnut Curculio Effects
Walnuts infested by the walnut curculio drop from the tree in late June. The walnut curculio larvae continue to feed on the dropped nuts for another four weeks, then exit the nut and burrow into the ground to pupate into a second generation that emerges in late July to early August. This generation of adult beetles feeds on walnut tree leaves until the tree sheds its leaves, then drops to the ground and burrows in to spend the winter near the trunk of the tree.
3. Walnut Husk Fly
The walnut husk fly (Rhagoletis completa) feeds on black walnuts and other walnut types. The adult is the size of a housefly. It has greenish eyes, three black bands on each wing and a yellow spot on the thorax just behind the wing attachment points. The adults emerge between May and July and mate. Females lay eggs in groups of 15 in the walnut husks.
4. Walnut Husk Fly Effects
Husk fly eggs hatch after five days and the maggots feed inside the husk, gradually turning the husk mushy and black. After feeding for a month, the maggots drop to the ground and burrow into the soil where they will spend the winter as pupae. The maggots' feeding discolors the nut shells and makes the husks hard to remove. Early summer husk fly infestations also can stunt nut kernel development.
5. Codling Moth
The codling moth (Cydia Pomonella) overwinters in cocoons in the soil around the base of a walnut or fruit tree. The 1/2-inch adults have mottled gray wings. They emerge in early spring. Codling moths prefer fruit trees but will attack walnut trees. After mating, the female lays eggs on developing walnuts and fruits. On walnut trees, the larvae feed on nut husks and the developing kernels, then leave the ruined nut and drop to the ground to pupate into a second generation. In warm climates, codling moths can produce four generations per year.