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Advisor says pest is spreading leaf roll from site to site.

Lee Allen, Contributing Writer

May 13, 2020

3 Min Read
Vine mealybug is a significant pest in California grapes; it can cause them to be unmarketable, and in wine grapes can reduce the ability of vines to move sugar into the fruit.David Haviland, University of California

Larry Bettiga is urging growers to keep an eye out for vine mealybugs.

As the University of California Cooperative Extension Viticulture Farm Advisor from Monterey County observes, mealybugs are spreading leaf roll virus from site to site along the Central Coast. 

“Growers need to have some pretty stringent programs in place to prevent that virus, to suppress these populations as much as possible to reduce the spread,” Bettiga says.

According to the University of California Statewide Integrated Pest Management Program: “Most life stages of the vine mealybug can be present year-round depending on the growing region. As temperatures increase, populations become more visible as they move from roots or trunk to cordons and canopy. By summer, mealybugs are found on all parts of the vine — hidden under bark or exposed on trunks, cordons, first- and second-year canes, leaves, clusters, and roots.”

The magnitude of the problem is believed to be contingent on the weather. “Early spring here was quite cold,” Bettiga says, “but things are warming now and bugs are going to be more active, moving out onto the canopy and causing problems and growers should be working a rotation of products to try and suppress the baby bug population.”

Acknowledging that he isn’t speaking for the whole state, he adds, “Typically in the areas I work in, the bug was brought in on nursery material planted in the 1990s. In the cooler areas up north, like Salinas Valley, not so much because by the time people figured out that things were being moved on plant material, nursery stock got cleaned up.”

So the battle against mealy bugs continues, much of it weather dependent. 

“I don’t make predictions because every year, every season, is different, but as things warm up we’ll get an indication of how they will play out,” he says. “Cooler weather affects insect movement but as temperatures reach the 80s, we should see rapid development and typical patterns of those populations.”

Mealy movement months

May and June are mealy movement months when the bugs are susceptible to suppressant efforts. “Most growers, if they’re concerned about — especially the virus spread — they’re on a program already, doing rotations of different products dependent on the populations they’re experiencing,” he says. “We have a lot of growers here trying to get on top of the leaf roll virus situation by removing infected source blocks and protecting young plants from getting infected while treating older blocks.

“We’ve seen a big expansion of the virus in the last 10 years or so and now growers are trying to clean things up. But it’s going to take a really stringent program for many years to get a handle on the situation and suppress it.”

The growth and development of mealy bug test populations has shown to be directly relatable to weather conditions. 

“If you put it on a day/degree model, it takes a certain amount of heat to get them to develop and as it gets warmer, they get more active,” Bettiga says. “So if there are insects in the plant, their growth and development is a function of weather, especially early season when it’s more variable year-to-year. Once we get into the summer, weather patterns tend to be more predictable and it’s another year in the life of a grape grower.”

What isn’t so typical is that a lot of previously used insect growth regulating control materials have now had restrictions placed on them. 

“We’re concerned about the development of effective resistance because we need more of them in the toolbox to use on a rotating basis,” he says.

“Our worry is whether or not we can effectively suppress these populations at a level that will minimize the virus spread over the long term,” Bettiga says. “That’s a big issue in the Lodi area, especially involving table grapes that have a low tolerance for any kind of damage.

“Today’s situation is akin to a little bit of a dilemma that will take some persistence to get a handle on and get the vines cleaned up.”

For more news on pests, disease management and other issues affecting vineyards, subscribe to the bi-monthly newsletter The Grape Line.

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