Since discovery of red blotch grapevine virus, Washington State University entomologists have taken another look at leafhoppers, insects that could be vectors of the new virus.
Just as the connection between mealybugs and grape leafroll associated virus altered grape pest management in Washington, so could linkage between leafhoppers and red blotch.
Red blotch is a serious wine grape disease because it significantly impacts fruit quality by reducing soluble solids (Brix levels) and negatively affecting tannins, acidity, and phenolics.
Leafhoppers and white flies are vectors of viruses in the geminiviridae family, the same family attached to red blotch virus, which is why leafhoppers are candidates for vectoring red blotch.
University of California entomologists are searching for vectors of the new disease. Although leafhoppers are suspected, they have not yet confirmed any findings. The disease is believed to be transmissible through grafting. Preliminary work by UC scientists has found that the virus spreads slowly in infected vineyards, which could implicate some type of insect.
In Washington vineyards, the spotlight has been on Virginia creeper leafhopper due to Dr. Naidu Rayapati’s discovery that the species (Erythroneura ziczac) was capable of transmitting the virus from vine to vine under controlled greenhouse conditions. Rayapati, WSU plant pathologist, specializes in grape diseases.
Washington has two species of leafhoppers that inhabit vineyards, western grape leafhopper and Virginia creeper leafhopper.
Leafhopper populations in the state’s vineyards were generally low last year, says Dr. Doug Walsh, WSU’s integrated pest management specialist. Washington grape growers usually have few pest problems, partly because of cold winters and dry summers and the industry’s move to softer chemicals that preserve biological control agents, like the wasp Anagrus epos, that help keep population numbers low by parasitizing leafhopper eggs.
“But also, leafhoppers are likely collateral kill from Washington growers controlling grape mealybug,” he said. Grape mealybug and other mealybug species are vectors of grapevine leafroll associated virus, a widespread disease in grape growing regions, including Washington. Leafroll virus reduces yield and quality of fruit and shortens the lifespan of the infected vineyard.
In recent years, Washington growers have adopted a zero tolerance mindset for mealybug in their efforts to stop the spread of the leafroll virus.
The new red blotch virus could also change pest management in the state. If a vector of red blotch is identified, growers may need to change their mindset from control to eradication. Monitoring for leafhopper will be more important in the future if leafhoppers are found to be vectors of red blotch disease.
In Washington, both grape leafhopper and Virginia creeper leafhopper usually have two generations per season, although there were three last year because of warm temperatures.
Grape leafhopper moves out of vineyards to overwinter in rosaceous plants, such as stone fruit trees, while Virginia creeper moves out to overwinter in Virginia creeper plants and other closely related weeds. Both migrate back to vineyards in the spring to reestablish populations.
“Years ago, before introduction of neonicotinoid pesticides, clouds of leafhoppers could be seen flying in our vineyards,” said Walsh. “But now, not so much.”
In WSU’s study last year, the only significant population of leafhoppers was in an organic vineyard. Organic growers are limited in their choices for effective methods of controlling leafhoppers. However, some essential oils could be registered this year. Walsh plans to study the efficacy of the organic oil materials in vineyards in 2015.
The last work on leafhoppers in the state was done in 2004. In 2014, Walsh reviewed the recommended presence/absence sampling techniques and sampling thresholds used a decade ago in comparison to current vineyard situations.
Dr. Holly Ferguson, WSU research associate, confirmed that using 18 nymphs or adults per leaf as the trigger to switch from a using a hand lens to using a dissecting microscope was valid today, said Walsh. “If you reach that number of 18 with a hand lens when monitoring for leafhopper, then it’s time to switch to a more precise way of counting, which is a dissecting microscope.”
Finding 18 nymphs per leaf with a hand lens indicates that 100 percent of the leaves in the vineyard likely have at least one leafhopper, he added.
Ferguson also compared monitoring techniques of using a hand lens (as a grower would do) to count leafhopper nymphs and adults on leaves to using a dissecting microscope, which is more precise.
She found that the hand lens is 20 percent less efficient in counting leafhoppers than a dissecting microscope. “So you need to add a 20 percent fudge factor when using a hand lens for counting leafhoppers,” he said. (A count of 18 is actually more like 21 to 22 leafhoppers when 20 percent is factored into the count.)
Previous research shows that the naked eye is ineffective in determining population levels.
WSU found both grape leafhopper and Virginia creeper leafhopper in their survey last year, though both were at levels below economic thresholds. Depending on the vineyard location, there was a mix of both species, Walsh said.
“For the most part, leafhopper populations were well below economic thresholds for treatment,” he said. “Vineyards in Washington are more often than not being treated for mealybug, which is likely helping keep leafhopper populations low.”
DNA bar coding
Walsh and his research team have successfully developed a molecular technique to tell the difference between grape leafhopper and Virginia creeper leafhopper. The technique uses PCR (polymerase chain reaction) to analyze a segment of leafhopper DNA.
“Now, we’ll be able to go out and collect specimens and run them through a PCR machine to know in a very short time what species we have instead of the exhausting task of looking at each insect under a microscope,” he explained. “It’s a nice technique for scientists to truly tell the differences of species.”
Researchers are moving toward the ability to be able to use PCR to determine the proportion of leafhopper species within a sample.
Walsh spoke during the Washington State Grape Society’s annual meeting. •