Heads up! Is the Glassy-Winged Sharpshooter on the Rise Again?
May 18, 2021 
The glassy-winged sharpshooter is large for a leafhopper — about one-half inch long — and has the ability to fly a quarter mile or more.
Photo courtesy of CDFA
Glassy-winged sharpshooter (GWSS), a pest superseded in the headlines in recent years by the many other invasive insects such as spotted wing drosophila and brown marmorated sting bug that have assaulted California, made quite a comeback in 2020. Warmer weather than usual in the winter that continued into the spring surged populations in the Southern San Joaquin Valley, where the insect has largely been found in the recent past.
In Kern County, located at the south end of the valley, the number trapped shot up from below 50,000 to more than 150,000 last year, according to Beth Stone-Smith, California Assistant State Plant Health Director, USDA Animal and Plant Health Service, Plant Protection and Quarantine.
In Tulare County, immediately to the north, the number of finds through the years have rarely exceeded 10,000. But last year the number caught jumped to nearly 30,000. Continuing north up the valley, Fresno County saw little activity in the past, but after catching just over 200 in 2019, officials trapped 1,800 in 2020. Even Madera County saw 34 GWSS, which isn’t many, says Stone-Smith, but it’s many more than have been found trapped before.
“We’re concerned. We don’t want this to move farther north,” she says. “This thing can be a real beast for us to contain and control.”
BAD REPUTATION
Anyone involved in the California wine grape industry for more than a couple of decades can relate to Stone-Smith and may well find themselves nodding at her assessment. Glassy-winged sharpshooter can spread the bacterium Xylella fastidiosa, which causes Pierce’s Disease (PD) in grapes, an incurable malady that hammered one small Southern California wine region at the turn of the millennium.
PD wiped out nearly 60% of the vineyards in Temecula in just two years and threatened to do the same to vineyards across California. The experience was so jarring that it led to an overwhelming number of the state’s growers voting in favor of taxing themselves to fund research on PD, other pests, and diseases that threaten California’s wine grape industry.
GWSS was inadvertently introduced to Southern California in the late 1980s or early 1990s, likely arriving in an egg mass on agricultural or ornamental plant foliage from its native habitat is in the Southeastern U.S. and Northern Mexico.
It can live in many habitats, including agricultural crops, urban landscapes, native woodland, and riparian vegetation. Its host list is long. As Stone-Smith puts it: “What isn’t a host?”
GWSS adults are large compared to other leafhoppers, about one-half inch long. It’s also an impressive flier, having the ability to soar a quarter mile or more without stopping, making it a highly mobile threat. It’s also active in winter and is particularly a menace in Southern California and the Southern San Joaquin Valley because there it has at least two generations per year — and has even had three — substantially increasing the threat of PD spreading there. It’s also aggressive, actually feeding on the grapevine’s wood.
The pest overwinters as an adult, feeding on citrus and other non-deciduous plants, moving to deciduous plants in January and February when adults feed on the sap from the leafless twigs before returning to the non-deciduous plants during cooler evening hours. These overwintering adults begin laying eggs in February but lay most of their eggs in late March and April.
Eggs hatch in 10 to 14 days, and the nymphs feed on the leaf petioles or young succulent stems while they progress through five immature stages. In the summer, first-generation adults begin to appear in May through July. Egg laying for the second generation occurs between mid-June and October. The nymphs emerging from these egg masses typically develop into overwintering adults.
TARGETING CITRUS
As noted above, one of glassy-winged sharpshooter’s favorite hosts is citrus. And because it’s green in the winter, that is the crop the Pierce’s Disease Control Program is targeting for its suppression program, says Stone-Smith. “We’re not going to eradicate GWSS; it’s about suppression,” she says. “We’re really targeting citrus (for suppression), even though it’s a grape pest, because citrus is its favorite overwintering host.”
The PD Control Program is a cooperative effort by USDA, the California Department of Food and Agriculture, the University of California, the affected counties, industry groups, growers, and the public that gets about $15.5 million each year for surveying, containing, public outreach, research, biological control, area-wide programs, as well as rapid response in urban areas. The effort is to slow both the natural spread and the artificial spread, the latter often caused by moving on citrus or nursery plants.
Yellow sticky traps are generally placed on a quarter-mile grid in the citrus groves. In treating, a common approach is a foliar spray and a systemic imidacloprid. Stone-Smith says the PD Control Program has an approved list of foliar products that are labeled for GWSS that have been proven effective and provide flexibility for growers’ specific situations. But because each county has unique circumstances, contact your respective county Department of Agriculture for assistance to see what’s available in your area.
Citrus growers face a pest of their own that causes an incurable disease, as Asian Citrus Psyllid (ACP) vectors Huanglongbing, also known as citrus greening. However, Stone-Smith says not only are the materials used often different for ACP than for GWSS, but treatment also generally comes at different times.
Priority areas for treatment are major glassy-winged sharpshooter finds near grapes, finds that are outside of areas already deemed infested, and areas with extraordinarily high levels of GWSS. In a sign of the times, one problem they are wrestling with is the increasing amount of organic citrus acreage being planted in the San Joaquin Valley, says Stone-Smith.
“We need longevity of product,” she says, “and organic treatments don’t have longevity.”
Like San Joaquin Valley citrus growers, the valley’s grape growers face an invasive insect pest that causes an incurable disease, and they use similar language, “buying time,” being the operative phrase. But at least they have that option, says Stone-Smith.
“Never did I think when I started on this program 20 years ago that we’d be buying time 20 years later,” she says, “or that we’d be able to buy time 20 years later.”
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