Filbert Pests in Live Oaks

Oaks ‘n’ Folks – Volume 6, Issue 2 – July 1991


Two of the most common acorn-boring insects found in the San Francisco Bay area are larvae of the filbert weevil, Curculio occidentis, and the filbertworm, Melissopus latiferreanus. Damage to acorn crops by these filbert pests can be as high as 80 percent. In addition, both insects are associated with dripping acorn disease caused by Erwinia quercina. The boring activity of these insects also affects oak propagation by increasing the number of nongerminating acorns and reducing seedling survival of infested acorns that do germinate. Being the ìYear of the Oakî in California, there is renewed interest in better understanding how these insects affect acorn survival and oak regeneration.

Life history and Damage

In late spring and early summer filbert weevil and filbertworm adults emerge from acorns attacked the previous year and the soil underneath the trees. After mating, the filbert weevil female, using her proboscis,drills small holes in immature acorns for the insertion of eggs. The filbertworm female differs in her oviposition behavior in that she lays her eggs on the outside of acorns, leaving the larvae to bore into the nut. Both filbert pests have a single generation per year. However, the filbert weevil can hibernate for several years in the soil as a pupa.

Monitoring and Control

Recent research by the author on filbert pests of coast live oak trees in Marin County revealed that these insects preferentially oviposit on the shady side of trees. Infested acorn counts were three times greater on the shady side compared to the sunny side of trees. Observations also revealed that many of the infested acorns on the sunny side of trees were split open. Once opened, the larvae contained within these acorns are vulnerable to attack by insect predators, pathogens, or parasites. Perhaps filbert pests concentrate their young on the shady side of trees to escape the increased morality factors of the sunnier side.The urban pest manager should consider this behavior of adults for monitoring and protective control by insecticides in the spring. With exponential growth of people and landscapes expected in Californiaís native oak woodlands in the coming decades, it is anticipated that interest in filbert pests will increase.

prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc