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Napa County VMB traced to new stock

Following an extensive detection effort in Napa County in 2003, vine mealybug (VMB) has been confirmed in 20 Napa County vineyards.

All infestations appear to be due to planting infested grapevine nursery stock from 1998 through 2003. The infested sites are located throughout Napa County (Carneros to Calistoga, valley floor and mountain vineyards), which is not surprising given that infested nursery stock was the source of the insects.

First found in California in the Coachella Valley in the early 1990s, VMB spread northward (presumably on infested equipment or fruit bins) and was found in the San Joaquin Valley in 1998. At some point, it apparently was introduced into some of the commercial grapevine propagation stream, and was widely spread on infested nursery plants.

In 2002, VMB was discovered in several northern California counties and most of these new finds were associated with new plantings. Spread on nursery stock has raised concerns that the insect could now be widely distributed throughout California and that we are currently seeing only a fraction of the infested vineyards.

To date, 16 counties have confirmed VMB infestations. They are Alameda, El Dorado, Fresno, Kern, Madera, Monterey, Napa, Riverside, Sacramento, San Joaquin, San Luis Obispo, Santa Barbara, Santa Clara, Sonoma, Stanislaus and Yolo.

Complete crop loss

Vine mealybug infestations can result in complete crop loss if allowed to develop without insecticide applications. Control programs are costly and require considerable effort and worker training. During the early stages of an infestation, vine mealybug is difficult to detect. Detection programs rely on the use of pheromone traps to catch the winged males, followed by visual inspection of vineyards to determine where the female infestations exist.

Fifteen infestations were found in 2003 in Napa, on the heels of five finds in 2002.

The large number of finds in Napa County resulted from a coordinated trapping and detection effort spearheaded by the Napa County Agricultural Commissioner's office. This effort was possible with funding provided by the Napa County Board of Supervisors and the local wine grape industry.

No state funding is available to directly support county VMB trapping programs, so detection efforts vary widely from county to county. Some counties have done little or no trapping. While it seems that Napa County may have more than its share of infested vineyards, it is more likely the case that we have simply been looking harder than other counties.

Pheromone traps attract male vine mealybugs, sometimes from over half a mile away. The winged males are tiny and require microscopic examination to properly identify them. In several parts of Napa County, a native ryegrass mealybug is present and males of this species are also attracted to the VMB pheromone. These male mealybugs are similar in appearance to VMB males, which make expert examination of the traps even more critical.

Moved by contact

Only male mealybugs can fly. Females do not have wings and move only by crawling. On their own, infestations would increase in size quite slowly, being limited by the walking range of females. However, females are easily spread throughout vineyards and between vineyards because of the sticky honeydew they secrete. Females will stick to people, equipment and plant material (including fruit), and can then be moved over large distances.

Even birds have been implicated in spread of VMB when feeding on infested ripe fruit close to harvest. These secondary methods of spread account for VMB's rapid spread throughout the state.

It is likely that there are additional VMB sites in Napa County that we have not yet found. Males have been trapped in three areas that are distant from any of the known infestations. Given the limited range over which the traps are effective, it seems likely that these, and possibly even more sites, will eventually be found. In counties with only limited detection efforts, it seems certain that many infestations will turn up once they are large enough to be found by field crews.

All infested sites in Napa County are being aggressively treated with insecticides in an attempt to eradicate vine mealybug. Treatments include Admire, the systemic form of imidacloprid, and the contact insecticide Lorsban. Recent changes were made to the Lorsban label to allow for both fall and late winter applications to control VMB.

Eradication realistic?

One of the vineyards where VMB was discovered in 2002 may have successfully eradicated the infestation. All other sites still have active populations. While eradication is the goal at all of the infested sites, it remains to be seen whether this is a realistic expectation. Vine mealybug may now be with us for the foreseeable future, requiring aggressive treatments each year in order to keep populations at tolerable levels and to limit the potential for further spread.

The movement of fruit and the winery waste generated from infested vineyards are areas of concern for additional spread. Stems from clusters infested with VMB will likely still harbor live insects after passing through a winery crusher-stemmer. If directly spread back into a vineyard, infested stems could lead to new vineyard infestations.

County ag commissioner staff has closely monitored movement of harvested fruit from VMB sites in Napa County in order to minimize the risks associated with infested stems. Stems were composted, solarized or returned to the vineyard of origin. Since VMB is now distributed throughout the state, introductions of VMB on fruit originating from other counties is a significant issue.

Wineries should be aware of the status of incoming loads of fruit and deal with the stems appropriately.

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