Grape bud beetle—Glyptoscelis squamulata
Grape bud beetle (family Chrysomelidae) chews on and kills grape buds. This causes a reduction in the vine's potential production of foliage and fruit. The beetle also feeds on various other plants including bean, corn, desert mallow, mesquite, peach, rose, and tamarisk, but is generally a pest only on grape.
Adult grape bud beetles are oval, 1/4 to 2/5 inch long, and about 1/5 inch wide. Young adults are generally covered with pale gray scales. Older adults are commonly brown or brown and gray with the brown color being where their gray scales rubbed off. Adults are easily overlooked because they hide during the day in crevices of bark, under loose bark, between plant stakes and cross arms, and in crevices or holes in grapevine support structures.
Eggs are pale orange to yellow and about 1/12 inch (2 mm) long and 1/50 inch (0.5 mm) wide. Eggs are laid in bark crevices and under loose bark.
Larvae are oblong and grayish white to pale yellow. The last instar is slightly longer than adults, up to about 1/2 inch long. The larvae and oblong pupae occur in topsoil.
Larval stages feed on grape roots in the soil. After pupating, adults begin emerging from the soil in mid-January. Peak emergence occurs around mid-March. About 1 to 2 weeks after emergence, adults begin to lay eggs.
Eggs are laid in batches that average 1 to 2 dozen eggs. Each mated female lays on average a total of 80 to 160 eggs during her lifespan. Because eggs are inserted under bark or between thin layers of bark, they are difficult to locate.
The hatching larvae drop to the ground and promptly enter topsoil. They can be found feeding on grape roots 2 to 3 feet below the surface but mostly occur within 1-1/2 feet of the surface.
Prepupae (mature larvae) overwinter in soil and pupate in the winter or early spring. Grape bud beetle has one generation per year.
The grape bud beetle is a major pest in California's Coachella Valley. It also occurs in the Central Valley, but is rarely or not a pest there. The adults chew and feed on the center of opening grape buds that are up to 1-1/2 inches long. Although lateral shoots may grow from the fed upon buds, most are sterile and produce no flowers and fruit. This is because the beetle's feeding consumed or killed the flower and leaf primordia, the immature tissue from which the flowers and leafy shoots develop. Although the larvae chew and feed on grape roots, this has not been shown to have noticeable effect on vine health.
Where grape bud beetle has been a problem and warrants control, monitor for the adults from January through March. The easiest monitoring method is to construct emergence traps, a box or cone made of screening that is open at the bottom and placed on soil near the trunk of grapevines. The base can be a 3X3 feet square or a cone 3 feet in diameter at the base. Either type can taper to a peak at the top where the emerged beetles will tend to congregate. Beginning early January, place 1 or more traps near the trunk of grapevines and inspect these cages for adult beetles at least every few days.
Alternatively, beetles can be monitored by inspecting vines at night with a flashlight. The adults come out of hiding places about 1 hour after sundown. A preferred alternative is to inspect vines at night using an ultra-violet (UV) lamp aimed at the grapevines. Grape bud beetles naturally fluoresce a bright silvery blue when under UV light.
Any beetles found can be hand picked off vines and dropped into a bucket of soapy water or otherwise killed and disposed of. When grape shoots reach 1-1/2 inches long, the beetles' feeding damage is not of importance to plant health, and monitoring or control efforts are no longer warranted.
Insecticides available to home gardeners may not be very effective for this pest and no pesticide applications are recommended.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources also available as an eBook and Pest Management Guidelines: Grape, University of California Statewide Integrated Pest Management Program (UC IPM).