Filbert weevils and filbertworm
Oak acorns are commonly infested with larvae of filbert weevils (beetles such as Curculio aurivestis, C. occidentis, C. pardus) or the filbertworm (the moth Cydia latiferreana =Melissopus latiferreanus, Tortricidae). Less commonly in California, acorns are infested by the acorn moth [Blastobasis (=Valentinia) glandulella, Blastobasidae]. Larvae of some of these insects can also infest chestnut, hazelnut (filbert), other nut trees, and tanoak. Filbertworm is also an occasional pest of pomegranate.
Identification
Compared to the slow-moving larvae of the filbert weevils, larvae of acorn moth and filbertworm are generally active when removed from an acorn. Larvae of acorn moth and filbertworm have three pairs of short legs; filbert weevil larvae have no apparent legs. Nuts infested with larvae of acorn moths or filbertworms can contain fine, silken threads, and filbertworm larvae commonly drop on a silken strand when disturbed; filbert weevils do not produce silk.
Filbert weevils. Adults are brown to orangish snout beetles (Curculionidae) with long, curved, thin mouthparts. The body of filbert weevil adults is about 1/4 inch long.
Curculio species larvae are legless with a brown head and a cream-colored or whitish body. Larvae grow up to 1/3 inch long and commonly curl into a C-shape when disturbed or exposed.
Filbertworm. Adults (moths) have brown or copper-colored forewings with markings of black, gray, or both. A thin, brown, lengthwise band occurs along the top of forewings in some individuals. The moth's wingspan is about 3/4 inch.
Eggs of filbertworm are scalelike, flattened, and oval. They occur on nuts and resemble eggs of codling moth. Eggs are white when newly laid, then darken as hatching approaches.
Larvae grow to 1/2 inch long. They have a dark brown to black head and beige to pink body. Filbertworm pupae occur in a silken cocoon in litter and topsoil.
Life cycle
Filbert weevils. Adult females chew a small hole in the seed coat of acorns or other nuts on the tree and insert one or more eggs during summer and fall. Hatching larvae tunnel and feed on the nutmeat. Heavily damaged nuts may drop prematurely during summer.
Larvae mature in fall to early winter. Mature larvae (prepupae) chew an exit hole about 1/16 inch in diameter and enter topsoil to overwinter. Pupation occurs in spring or early summer, then adults emerge. Filbert weevils have one generation per year.
Filbertworm. The adult female oviposits on nuts. Eggs are laid singly and hatch in about 8 to 11 days. The emerging larva chews inside and develops through several increasingly larger instars. After acorns drop, larvae chew an exit hole and move into organic litter or topsoil where they overwinter as prepupae.
Filbertworms pupate in a silken cocoon in spring and emerge as adults in late spring or early summer. In California there is one or (in warmer locations) two generations per year. Second-generation adults, if any, appear in the fall.
Damage
Although harmless to tree health, larval feeding causes acorns to fail to sprout. Insect-damaged acorns can also become colonized by bacteria that cause annoying, sticky liquid to drip from oaks (drippy acorn, or drippy oak disease). When hazelnut, other nut trees, or (by filbertworm) pomegranate are infested, the nuts or fruit become inedible.
Solutions
No methods have been demonstrated as effective for controlling these insects in gardens and landscapes. Insect abundance and the proportion of infested acorns or fruit vary greatly by host species, location, and year.
Collect and dispose of dropped fruit and nuts in covered containers to potentially reduce the abundance of these insects' next generation. When collecting acorns for planting, inspect them and dispose of those that deform easily when squeezed, feel unusually light-weight, or have holes; generally these are not viable. Another method is to place acorns in a tub of water and discard those that float; larval feeding creates air pockets that cause infested acorns to float.
Where filbertworm infests pomegranate, you can place out a sticky trap baited with the filbertworm commercial pheromone (sex attractant) to monitor adults. When filbertworm moths are caught in abundance, Bacillus thuringiensis or spinosad can be sprayed. Bacillus thuringiensis must be applied when larvae are small and weather is expected to be dry and warm for several days. A second or third treatment at 7 to 10 day intervals may be warranted because of its short persistence. Spinosad is highly toxic to bees and certain natural enemies; do not apply it to plants that are flowering.
For more information see Filbert Pests in Live Oak, A Field Guide to Insects and Diseases of California Oaks, and Hazelnut Pest and Beneficial Insects - An Identification Guide (PDF).
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
