Glassy-winged sharpshooter

May 12, 2021
How California is protecting its winegrapes

Glassy-winged sharpshooter (GWSS) is a formidable nightmare for grape growers – and growers of other crops, for that matter – that vectors Pierce’s disease, which can decimate grapevines and for which there is no known cure.

While some grape growers may not be familiar with the glassy-winged sharpshooter, California grape growers in 1999-2001 were well aware of the pest as it spread Pierce’s disease in crops in Southern California’s Temecula Valley and destroyed vineyards, forcing their removal, and creating what would be called the California Vineyard Apocalypse. Luckily, GWSS is not established in the northern part of the state.

This event spurred the creation of the Pierce’s Disease Control Program – a unit within the California Department of Food and Agriculture (CDFA) that receives federal funding and works with counties to reduce the impact of Pierce’s disease in California. As a strong flying insect with over 350 host plants in agricultural, urban and residential settings, there’s no real way to eradicate GWSS statewide. The challenge of containment and mitigation is important, along with outreach.

“We are challenged as a program to keep the awareness up for California grape growers,” said Craig Hanes, branch chief and statewide coordinator for the Pierce’s Disease Control Program (PDCP). “The PDCP has been successful in limiting the artificial spread of GWSS, and many grape-growing regions around the state have not had to deal with GWSS directly. Growers are busy with so many issues to manage, and we’ve had success in not allowing GWSS to spread across the state, so continued awareness can be a problem.”

Glassy-winged sharpshooter. Photos: USDA-APHIS

An ultimate vector

California is no stranger to Pierce’s disease. Prior to GWSS, there were other vectors, such as green sharpshooter, and redheaded and blue-green sharpshooters, in various parts of the state, such as the Central Valley and the North Coast. It’s also the reason why grapes are not commercially grown on a wide scale in the Los Angeles basin – there is simply too much disease and too many vectors.

Xylella fastidiosa is the bacteria that causes Pierce’s disease, and is the causal agent for a host of other diseases, such as alfalfa dwarf, almond leaf scorch, phony peach disease, plum leaf scald, citrus variegated chlorosis, elm, oak, and sycamore leaf scorch, oleander leaf scorch and others. When GWSS feeds on plants, the bacteria can be picked up by the insect’s mouthparts, which then carries the bacteria to a new plant when the insect moves to feed on another host. As the bacteria multiples within the host plant, it closes off the water conducting tissues of the plant. For vineyards, if the infection is so bad that vines must be removed, they must be removed completely, as the disease is present in all tissues of the plant.

“It’s in a grape grower’s best interest to rogue out diseased vines before an insect vector can further spread Pierce’s disease,” said Hanes.

Glassy-winged sharpshooter overwinters as an adult, and mating and egg laying begin early in the year in February and March. With each female laying multiple eggs, and 2-3 generations in California depending on location and weather, the state’s growers, researchers and allied organizations have to remain vigilant.

Vigilance is also reflected in the highly cooperative Pierce’s Disease Control Program.

inspector looks for GWSS
A Los Angeles County inspector looks for GWSS egg masses.

A focused program

CDFA, USDA, county departments of agriculture, University of California, industry groups and the public have all been a part of the Pierce’s disease program for two decades, and those cooperative relationships have led to the program’s success in mitigating GWSS. The program receives about $15.5 million a year from the federal government, with state funds sometimes being made available, and by industry organizations, with industry doing the heavy lifting in terms of research funding, outreach and rapid response urban treatments since GWSS isn’t limited to agriculture.

Area-wide programs are key to successfully suppressing GWSS populations in diverse agricultural areas. The program does this by targeting citrus.

“With most of Southern California infested with GWSS, the containment program for nursery stock and bulk citrus has worked very well to prevent the artificial spread of GWSS,” said Hanes. “In locations where there are area-wide treatment programs, grape growers are very familiar with the problem and are very engaged in the issue and working with us, as well as looking out for their own vineyards and protecting them.”

GWSS overwinters in citrus, is easily moved in bulk citrus, and not only is California a major citrus producer, but some crops have windbreak trees that can harbor GWSS. Targeting the pest in one host can have a strong favorable outcome if treatments are done on a large scale and with grower participation, so as not to disrupt an operation.

Glassy-winged sharpshooter adults
Multiple glassy-winged sharpshooter adults on a cane cut.

“Growers in these areas are aware that GWSS overwinters in citrus, which helps with treatment timing,” Hanes said.

This is why traps are set and monitored closely. An example of this is in Kern County, the most challenging county and where the control program started, and where a very large number of traps are set on a quarter-mile grid in a variety of hosts and checked every other week. This monitoring provides GWSS data to the control program, and also helps determine if suppression treatments – since there is no eradication option due to host availability – were effective.

Tulare County is another major citrus area, and with the expansion of organic citrus acreage and the increase in warmer years, GWSS is starting to creep back up despite having a much lower pressure since the early 2000s.

“In the PDCP area-wide programs, several challenges include the lack of effective products to use in organic citrus, recent mild winters and limited funding,” Hanes said.

Ideally, there would be a foliar treatment available to target the overwintering generation and prevent increasing populations, followed by a systemic treatment in the citrus tree to eliminate any pests that were missed with the foliar spray.

In the meantime, growers can help slow the artificial and natural spread of GWSS by contacting their local agricultural commissioner’s office to report any unusual finds as the program looks at organic products as they become available, and continues to work with all cooperators to the betterment of the program.

“We expect we’ll have some continued GWSS population levels in historically challenging suppression areas,” said Hanes, “but we’ll keep working to refine our program with the best science and input from experts and program partners in order to protect the grape industry.”

— Crystal Nay, Western editor; Photo at top: GWSS has been found in northern parts of the state, where it has since been eradicated. Currently, Madera County is the farthest north where the pest has established, and growers, researchers, industry and affiliates of the Pierce’s Disease Control Program are taking steps to prevent any further movement north. Photos: USDA-APHIS




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