Brian Bahder transfers a first-instar grape mealybug from a grapevine leafroll-diseased Concord vine to a healthy Concord vine to assess mealybug competency as a vector.
With the insidious spread of grapevine leafroll disease in Washington State vineyards, Washington State University scientists have worked to help growers better detect and control grape mealybug, a known vector of the disease. WSU recently developed trapping density recommendations for a new pheromone-based trap that will alert growers when mealybugs become active in the spring.
Improved monitoring of grape mealybug in vineyards may help growers take action at the proper time to control mealybug populations and potentially stop spread of the disease, says Brian Bahder, who’s working on his doctorate at WSU. By knowing population trends, such as when adult males are flying and looking for females to mate with, growers can better time insecticides to coincide with the first instar or crawler stage, he said.
Grape mealybugs are difficult to control because of a natural defense they have in their waxy, protective coating and where they reside on the vines. Mealybugs overwinter as eggs or crawlers in the egg sacs under the bark, in cracks or crevices of the trunk and cordons. In the spring, crawlers move to new growth to feed. They mature in June, and the adults move back to the wood to lay eggs. A second generation hatches and feeds on fruit and foliage, maturing in July and August. Honeydew produced by the second generation, and the black sooty mold that grows on it, can contaminate fruit.
Crawlers are the most susceptible stage for control procedures because they are the most exposed stage, Bahder said. “Knowing when the first-instar crawlers are moving around can help growers time chemical applications when they are most effective.”
Bahder has worked with the grape mealybug pheromone for several years to determine appropriate trapping densities for Washington. Bahder is using female sex pheromones, synthesized by Dr. Jocelyn Miller of the University of California, Riverside, in his trapping studies. Miller developed pheromones for four different mealybug species. The pheromones are now commercially available through Suterra LLC of Bend, Oregon, and sold under the Scenturian product line.
In his trials, each septum contained 25 micrograms of pheromone placed in the Delta traps supplied by Suterra. The pheromone lasted about three months.
Based on three years of fieldwork in both juice and wine grapes, Bahder found one trap per 30 acres to be as effective as four or eight traps per 30 acres. Differences in the number of male adults caught in the three trapping densities were not statistically significant to warrant the expense and time of additional traps, he said.
UC entomologists recommend one trap per 30 acres as a trapping density for vine mealybug, a more aggressive species. However, confirmation was needed in Washington to determine the appropriate number for grape mealybug.
Bahder believes that the traps will help growers detect when males begin flying in search of females. When they combine trapping with growing degree-day information, growers will be able to estimate when the first-instar crawlers are expected to emerge.
Although mating disruption with pheromones works successfully to control several moth species in other crops, such as codling moth and Oriental fruit moth, the male grape mealybug can mate more than one time, so he doesn’t think mating disruption for grape mealybug would be effective.
Trapping data showed consistent population trends for both 2010 and 2011. He found two distinct flight peaks of grape mealybug. The first peak occurred in mid-June and the second peak in mid-August. Only adult males can fly; females have no wings. Thresholds based on the number of males caught in traps are still being developed.
He recommends that traps be placed in vineyards well before males start flying, in late April to early May.
Bahder also wanted to learn more about the distances that adult males can fly. Previous research done by retired WSU entomologist Dr. Wyatt Cone indicated that males would fly no further than four feet. Not certain that males would even leave the vineyards, Bahder placed traps outside the vineyard at certain distances from the closest host. Finding mealybug in the traps at 50 feet indicated that males could at least fly that far.
He then set transects at other vineyards at different distances, up to 1,000 feet. “In a natural setting, males may not fly more than four feet because they probably don’t have to, but we wanted to see how far they were physically capable of flying,” Bahder said, adding that concentrations of pheromone in the septa in the traps were much higher than pheromone from an unmated female. Each trap was checked weekly for the presence of males.
He found consistent flight distances of around 400 feet, though catches dropped off significantly after 400 feet. “We know they are capable of flying 400 feet, but after that, we cannot definitively say that they cannot fly further because we’re not sure if the mealybugs can’t fly further or if the pheromone was in too low a concentration for them to pick up.” The farthest catch was 600 feet.
His research also involves bioassay work in the lab to assess the efficiency of leafroll-disease transmission by different life stages of the grape mealybug.