Description of acorn weevils
The adult acorn weevil (Cucrulio sp.) is a brown beetle about 3/8 inches in length, and has a very long, thin snout. The larvae is a legless grub which is
curved and fat in the middle, tapering toward both ends. The larva is creamy white colored with a brown head, and
can grow to be 1/4 to 3/8 inches long.
Life cycle of acorn weevils
Adult females lay their eggs inside developing nuts on the trees during mid summer. The egg hatches into a creamy white, grub-like larva that feeds inside the nut until fall. When the acorns fall to the ground in autumn, the larva chews a perfectly round 1/8 inch hole in the nut and emerges in late fall or early winter. The larvae then tunnel into the soil, where they will stay for one to two years before emerging as a new adult weevil to repeat the process.
Damage caused by acorn weevils
The larvae can eat out the entire nut inside an acorn or hickory nut, making it worthless, but they usually do not damage the tree in any way. The reason you often find so many "wormy" or "holey" nuts under the trees is because the squirrels leave them behind. It appears squirrels can select good nuts during their fall frenzy of gathering. If you want to collect the good nuts for yourself, you will have to get up early and beat the squirrels to it.
Management of acorn weevils
Control of nut weevils in backyard oak and hickory trees is not practical. Nut growers use insecticides similar to the apple growers to prevent egg-laying by the female weevils. This is prohibitively costly for the private home owner.
Additional information on acorn weevils
Do you live in Iowa and have an insect you would like identified?
The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them. Please see our website for current forms, fees, and instructions on preserving and mailing insects.
Contact information for each states diagnostic laboratory for U.S. residents. If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.