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PD/GWSS Board Designates Mealybugs, Fanleaf Virus as Pests for Research Funding: High GWSS Numbers in Kern County Raise Concerns

Designation by itself does not obligate the Board to fund activities related to these pests, but it positions the Board to quickly respond to outbreaks or finds
by Ted Rieger
October 23, 2015

The California Department of Food and Agriculture (CDFA) Pierce’s Disease (PD)/Glassy-Winged Sharpshooter (GWSS) Board approved designation of all mealybug pests of winegrapes, and grapevine fanleaf virus (GFLV), as pests eligible to receive Board research funding at a meeting October 19 in Bakersfield. In addition, members of the PD Board and the PD Advisory Task Force toured vineyards the following day in Kern County to look at and discuss problems associated with significantly higher populations of GWSS this year.

Mealybug and Fanleaf Problems in Vineyards

Different mealybug species present in California tend to be threats in specific grapegrowing regions and the severity of outbreaks and damage differ from year to year. Mealybugs feed on grapes and produce honeydew excretions that allow the growth of sooty molds and result in crop loss and unmarketable grapes, and rejected loads by grape buyers.

Mealybug species in California found on grapes or considered potential pests of winegrapes include: grape mealybug (Pseudococcus maritimus), obscure mealybug (P. viburni), longtailed mealybug (P. longispinus), Gill’s mealybug (Ferrisia gilli), pink hibiscus mealybug (Maconellicoccus hirsutus), in addition to the vine mealybug (Planococcus ficus), designated a winegrape pest by the PD/GWSS Board in 2014. Since the introduction of the vine mealybug, which continues to spread and is difficult to control, growers have been more concerned about all mealybugs now that vine mealybug is known to be a vector of grapevine leafroll viruses.

Some growers treat for mealybugs every year and achieve control with available products such as Imidacloprid (Admire) and Spirotetramat (Movento). If these products were to become unavailable due to regulatory action, the Board will now be ready to look at alternative solutions. By designating all mealybugs that are pests of winegrapes, it will prepare the Board to take action for spread and outbreaks of all species, and be ready if new invasive mealybugs are introduced in California in the future.
Grapevine Fanleaf Virus (GFLV) can be introduced and spread by the exchange of infected plant material and can be vectored between planted grapevines by the parasitic and soil-inhabiting dagger nematode (Xiphinema index). GFLV is well-established in California’s major winegrape regions and is found in grapegrowing regions worldwide. It is controlled by the use of nematode resistant rootstocks and soil fumigation.

The PD/GWSS Board received authorization following 2009 legislation to use industry assessment funds for research and outreach on other serious pests and diseases of winegrapes, as long as such funding does not substantially diminish PD/GWSS research and control efforts. The European grapevine moth was the first new pest to receive designation, and the Board allocated funds for outreach that assisted with successful control efforts. In 2014, the Board designated grapevine red blotch-associated virus (GRBaV), the vine mealybug, and the brown marmorated stink bug as pests of winegrapes. The Board designated grapevine leafroll disease in June 2015.

Designation by itself does not obligate the Board to fund activities related to these pests, but it positions the Board to quickly respond to outbreaks or finds with education and outreach efforts, and to target research projects for control efforts.

GWSS Populations Skyrocket in Kern County

Since 2001, the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) and CDFA have conducted area-wide GWSS trapping and control programs for Kern County overall, and for one hot spot of particular focus--the General Beale Area, a 13,000-acre area of multiple commodities southeast of Bakersfield that includes citrus, vineyards and other crops. The area-wide GWSS management programs involve trapping and monitoring GWSS along with chemical treatments and biological controls with the goal of managing GWSS populations in these infested areas rather than attempting eradication, in order to stop further spread of PD and GWSS , in particular to stop PD spread to vineyards farther north in the San Joaquin Valley.

Nearly 5,600 monitoring traps are in Kern County this year inspected at two-week intervals. Trap count numbers are posted with a map on the CDFA Pierce’s Disease Control Program website. Total county-wide, two-week GWSS counts were generally in the hundreds in the spring months (relatively similar with recent years), but began to skyrocket in July with totals in excess of 19,000 trapped during the period from July 17 through August 1, 2015. Trap counts have remained high, and for the most recently posted two-week trapping period, September 27 through October 10, GWSS numbers exceeded 12,000. GWSS trapped this year have reached numbers not seen since the program began in 2001 when the total number for the year was over 147,000 trapped just in the General Beale area alone. In addition, a higher incidence of PD symptoms have been observed in Kern County vineyards this year.

Weather and higher temperatures are believed to be major factors enabling GWSS populations to multiply. In past years, GWSS would overwinter in citrus groves and cold and foggy conditions tended to keep them in check and reduce numbers until spring. This past year, with warmer winter weather and no fog, GWSS did not just overwinter in citrus, they continued to actively feed and reproduce.

Treatments with Imidacloprid have generally worked well at knocking back GWSS populations since 2001, with a focus on treating citrus groves early in the season when GWSS populations are lower and before they emerge and move to vineyards and other crops. Imidacloprid treatments applied earlier this year did not provide desired effects. PD Board and Task Force members met with USDA experts during this week’s tour to evaluate possible factors contributing to the spike in GWSS populations and to develop strategies for the area-wide programs moving forward.

Based on the 2014 Grape Acreage Report from the USDA National Agricultural Statistical Service, Kern County has a total of more than 67,000 acres of grapes, of which nearly 18,000 acres are winegrapes, with the remainder primarily table grapes. Citrus acreage in Kern County, based on NASS figures for 2014, totals nearly 53,000 acres.

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