Walnut leaves falling in quantity

Butternut curculio may be causing an unwanted surprise for walnut tree owners.

Damage to the end of the petiole caused by butternut curculio. Photo credit: Patrick Voyle
Damage to the end of the petiole caused by butternut curculio. Photo credit: Patrick Voyle

In some areas of Michigan, people who have walnut trees in their yards have walked outside to a scary sight. They are finding hundreds of green leaves littering the lawn that appear to be green and healthy. When the wind blows, leaves fall from the trees and cover to the ground. At first glance, the leaves look perfect, but on close inspection, the end of the petiole that attaches to the tree is dark brown and broken instead of cupped and green. A small hole may be seen close to the end of the petiole.

Walnut trees have leaves that are described as compound leaves. Each leaf is made up of 15 to 23 leaflets with seven to 11 pairs of leaves on each side of a central stem called a rachis and one single leaf at the end. The other end of the rachis is the petiole that attaches to the tree. Often, the end single leaf is missing, so when leaves fall, there is more material on the ground than just single leaves. Fifteen leaves attached to the rachis make a big statement.

The damage is the handiwork of a small insect 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 edible nuts.

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 early in the spring. Later, they lay eggs in crescent-shaped scars that they make 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 from inside. In four or five weeks when the larvae grow as big as they are going to get, they tunnel out of the rachis and leave an exit hole. After the leaf falls, 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 leaf petioles, shoots and the terminal ends of twigs until cold weather forces them off the tree and into the leaf litter and collapsed plants. Their lifecycle is complete and ready for the next growing season.

There is no treatment for butternut curculio. By the time the tree owner sees the leaves floating down, the damage is finished. Smart gardeners will water the trees if there has been little rain. If the lawn is fertilized, it should provide plenty of nutrients for the tree. This will help the tree to grow new leaves. Michigan State University Extension horticulture educators and Master Gardener hotline staff will be passing this information along to alarmed tree owners in the next several weeks. Then, the leaves stop falling and butternut curculios continue preparation for next year’s leaf surprise.

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