Butternut curculio: Walnut leaves have been littering the ground recently

Many walnut tree owners have been concerned and shocked by the hundreds of seemingly perfect walnut leaves found under trees this spring. They have never seen the large number of green leaves dropping daily.

The unwanted dinner guest is called a butternut curculio (Conotrachelus juglandis). The adult is a brown weevil, approximately 0.25 inches in length. Both the larva, which is the juvenile, and the adult do damage to black, English, Carpathian or Persian walnuts, butternut and Japanese heartnut. These trees are all members of the Juglans family and produce nuts.

butternut curculio
Butternut curculio. Photo credit: Tom Murray, bugguide.net

Late this spring, complete green leaves began falling from the trees. A walnut leaf is what is called a compound leaf. It has six paired leaflets opposite each other on each side of a stem-like structure called a rachis. Together, they make up the leaf. When the fallen leaf was examined, the only damage was to the end of the rachis where it was attached to the twig. The rachis end or petiole was brown and broken apart and a hole and tunnel were visible.

The adults spend the winter under leaves and plants around the trees. They feed on new leaf growth and their first eggs are laid on the stems of the new leaves in the early spring. Later, they lay eggs in crescent-shaped scars made on the developing new nuts. When the eggs hatch, the larvae tunnel into the stems and begin to feed. The rachis looks healthy, but is being damaged inside.

In four or five weeks when the larvae grow as big as they are going to get, tunnel out of the rachis and leave an exit hole. They then burrow into the soil under the tree to pupate to become an adult. The new adults emerge from the soil in late summer and feed on leaf petioles, shoots and terminal ends of twigs until cold weather forces them off the tree and into the leaf litter and collapsed plants. The cycle is complete and spring-loaded for the next growing season.

Insect populations are like waves on the ocean; they rise and fall. This was a very big year for butternut curculio. It is often difficult to determine why populations vary, but it usually has to do with winter or summer weather conditions, predators or diseases. The only thing that is certain is that next spring the butternut curculio population will be different.

There is no treatment for butternut curculio. By the time you see the leaves floating down, the damage is done. If you feel particularly sorry for the tree, water it well if the weather is dry, like right now (as of July 3, 2012). This will help the tree to grow new leaves.

Related Articles