Though several species of the grape mealybug complex have been found to transmit grapevine leafroll virus, it’s the vine mealybug that is most feared. Compared with the other mealybug species, vine mealybug has explosive reproductive capability and can survive on vine roots, making it a potent pest.
As of yet, vine mealybug (Planococcus ficus) has not colonized vineyards in Oregon or Washington State, but other mealybug species are found in the Pacific Northwest. Vine mealybug infestations have been found in South Africa, southern Europe, Israel, Chile, New Zealand, and other parts of the United States. Vine mealybug was discovered in southern California in 1994; within six years, it had spread through the major grape-growing regions, including the San Joaquin Valley and the central and northern coast.
"We know that vine mealybug is an invasive species," said Dr. Vaughn Walton, Oregon State University Extension horticultural entomologist. "That’s why we need to keep a close lookout for it."
Walton, who spoke during the Washington State Grape Society meeting in November, has studied vine mealybug in several countries, including South Africa, France, and California. He received his degrees from South Africa’s University of Stellenbosch, where he was involved in developing integrated control strategies for vine mealybug. Before joining OSU, he worked on alternative options for mealybug control at the University of California, Berkeley.
The most visual difference between species of the grape mealybug complex (grape, obscure, longtailed, and citrus mealybugs) and the vine mealybug is the tail. Species of the grape mealybug complex generally have longer tails (called the caudal wax filament), while waxy filaments of the vine mealybug are all the same length, so there is no tail.
A more important difference is in reproduction. The vine mealybug develops faster than other mealybug species. In California’s San Joaquin Valley, the vine mealybug has five to seven generations per season, compared with two or three generations of the grape mealybug complex.
The vine mealybug also produces much more honeydew than the grape mealybug and, unlike the grape mealybug complex, can feed and survive on roots, making chemical control more difficult. Studies in South Africa showed that in dry, sandy soils, vine mealybug survived on root fragments three years after a vineyard was removed.
Moreover, species of the grape mealybug complex are easier to control with chemicals than the vine mealybug. The developmental stages of the grape mealybug complex are synchronized, which means that most crawlers start moving to the shoots and leaves at the same time and chemical sprays can be effective. With the vine mealybug, developmental stages overlap, making spray application timing more difficult.
All known mealybugs found on grapes can transmit leafroll virus, Walton said, citing UC research by Dr. Deborah Golino that found that grape, citrus, longtailed, and obscure mealybugs can transmit leafroll virus to clean grapevines. Golino’s research did not evaluate vine mealybug transmission rates.
"The main lesson from Golino’s research is that all four mealybug species can transmit virus," he said. "And, you don’t need a lot of mealybugs to transmit the virus. We know that adult females produce large quantities of eggs, from 100 to 350, resulting in several thousand crawlers per square feet of vine. That’s why mealybugs are so efficient in the spread of virus—because they have tremendous reproductive potential and can feed on a vine, acquire the virus, and move on to clean vines."
Walton also shared results from research conducted at various universities that monitored disease-infected and clean vineyards with vectors regularly present to learn how fast leafroll disease can move through a vineyard. In vineyards infected with low levels of leafroll virus at the start of the study, nearly 80 percent of the vines had leafroll disease within eight to nine years. In many vineyards, the virus increased between 9 to 14 percent on a year-to-year basis.
The crawler is the insect stage that moves about and most readily spreads the virus, Walton said, adding that crawlers move by physically walking (crawling), being carried by the wind, and sticking to equipment and clothes of workers that are performing vineyard tasks. "Crawlers are difficult to see but easier to control than other stages," he said. Adult mealybugs produce a thick wax that protects them from chemicals.
Data from a South Africa study showed less than 5 percent mortality when adults that were lying underneath excreted honeydew and covered by waxy secretions were sprayed with insecticides, he reported.
In South Africa, the first vine mealybug was found about 120 years ago, according to Walton. As the South African grape industry expanded, the reach of vine mealybug and leafroll virus has expanded along with it.
"Humans are actually the most efficient spreading agent of mealybug within the vineyard," he commented. "A key issue is the movement of people and equipment through vineyards and from vineyard to vineyard."
Mealybugs can also move by hitchhiking on plant material. They have been found in the graft union of grape plant material. "But it’s the end of the season that’s the real worry," he said. Because mealybug moves onto different parts of the grapevine, the pest moves easily to the inside of the clusters. On the surface, fruit often don’t look infested, but mealybugs can move into the center of bunches, where they are protected from natural enemies.
"The real worry is that growers harvest the infested bunches, and then unknowingly, move fruit and mealybugs to other areas," he said.