Mealybugs have evolved into the biggest pest of California grapes.

There are now five known mealybug species in California — grape, obscure, longtailed, vine, and Gill’s — that feed on grapevines, says Entomologist Kent Daane, University of California Cooperative Extension specialist.

Many excrete honeydew and cause sooty mold, rendering grapes unmarketable, but some growers are now more concerned about mealybugs as vectors of grape leafroll disease. The disease can reduce yields by as much as 30 percent to 40 percent, while lowering quality, and it is showing up more in vineyards.

Daane says controlling mealybugs to limit their direct damage to fruit may be different than controlling them as vectors of leafroll.

“The evidence suggests that small juvenile stages, first instars, may be more effective at acquiring and transmitting the pathogen than the older stages that cause more damage to the fruit cluster,” he says. “The smaller stages are also more mobile and more easily dispersed by wind, birds, machinery and people. This increases their importance as potential vectors.”

Currently, growers can control mealybugs by applying insecticides in late winter or early spring as the population emerges from under the bark, on the trunk, or underground on the roots.

But this may not be the most prevalent period for mealybug crawlers to be produced or to move from vine to vine. Also, mealybug species differ in the seasonal periods when crawlers are produced.

For example, grape mealybugs have one to two generations, with crawler stages present during the winter and in mid- to late summer. In contrast, vine mealybug can have many overlapping generations, producing crawlers from April through November.

Work by Rodrigo Almeida’s laboratory at UC Berkeley has also shown that the population of grape leafroll disease viral particles will change during the season. This may affect the ability of the mealybug vectors to acquire or pick up and transmit the virus to other vines.

Currently, the Daane and Almeida laboratories are working with UC farm advisors in Napa and Sonoma counties to determine changes in both virus and mealybug populations during the season to determine the most effective time for applying insecticides to control mealybugs as vectors of grape leafroll disease.

Because there is still uncertainty, some North Coast growers are adopting a zero tolerance for any mealybugs in order to prevent the spread of grape leafroll disease in their vineyards.