Winegrape Pesticide Facing Greater Scrutiny

Imidacloprid ban would take away Pierce's Disease control tool

by Jon Tourney
Imidacloprid grape
Imidacloprid kills glassy winged sharpshooter nymphs as they try to emerge from the egg case. Photo source: UCANR

Sacramento, Calif.—A measure introduced in the California Assembly urging the prohibition of neonicotinoid insecticides to protect honeybees could lead to one of the most effective insecticides for the glassy-winged sharpshooter (GWSS) being taken off the market.

Imidacloprid is used by the California Pierce’s Disease (PD) Control Program to control the GWSS, which is the vector of the disease. The pesticide is a possible cause of honeybee colony collapse and has been targeted for tighter regulations.

Assembly Member Michael Allen, D-Santa Rosa, and by Sen. Mark Leno, D-San Francisco, authored and introduced Assembly Joint Resolution (AJR) 29 in February. The resolution urges “the United States Environmental Protection Agency (USEPA) and the California Department of Pesticide Regulation to act expeditiously to protect pollinators, especially honeybees, by prohibiting the use, marketing, or sale of neonicotinoid products deemed hazardous to pollinators.”The resolution specifically lists neonicotinoid  products containing imidacloprid, clothianidin, and thiamethoxam, as being highly toxic to honey bees. 

California Department of Food and Agriculture (CDFA) deputy secretary Jim Houston briefed the CDFA PD/GWSS Board at its April 23 meeting in Sacramento regarding AJR 29. The measure will have its first hearing in the Assembly Agriculture Committee at the state capitol May 9. A joint resolution can be passed simply by a majority vote of both the Assembly and Senate, it does not require the governor’s signature. A resolution expresses the will, wish or direction of the Legislature, but it does not have the effect of law.

Houston said CDFA has not formally taken a position on the measure but said, “We’re communicating to members of the Ag Committee that imidacloprid is a very important tool we use in the PD program, and it could increasingly become an important tool for the citrus industry in fighting Huanglongbing disease.” (Huanglongbing disease, a major threat to citrus production, was detected for the first time in California in a backyard citrus tree in March in Los Angeles County. It is vectored by the Asian citrus psyllid, now found in Southern California.)

“There seems to be a sufficient amount of angst in the ag industry about this, so the resolution may not make it out of the ag committee,” Houston said.

Imidacloprid History and Use
Neonicotinoids are a nicotine-based class of neuro-active insecticides developed in the 1980s that include several active ingredient chemicals, such as imidacloprid, and these products are now registered for use in more than 100 countries. Imidacloprid was first registered for use in the United States in 1994. Introduced by Bayer CropScience, one of the largest producers of the product, the pesticide is used to control sucking and chewing pests on a wide range of agricultural crops that include lettuce, tomatoes, corn, cotton, potatoes, and fruit and orchard crops. It is used on turf and ornamental plants, for household and structural pests such as termites, and on pets for flea and tick control.

In grapes, it can be used to control leafhoppers/sharpshooters, mealybugs, aphids, white flies, fruit flies, and phylloxera. In vineyards, it can be most effective for longer-term GWSS control with soil applications by chemigation through drip irrigation systems, where it is taken up by the vine as a systemic insecticide. Bayer CropScience’s Admire Pro is commonly used for vineyard soil applications, but imidacloprid is also sold under a number of trade names that include Alias, Merit, Gaucho, Provado, Montana, Nuprid, and Marathon. In certain formulations, it is used as a foliar spray application where it can provide quicker, but shorter-term pest control.

Grape growers in Temecula, Calif., where PD caused vineyard losses of 40 percent in the 1990s, have used imidacloprid to prevent PD losses as vines were replanted over the past decade. Temecula vineyards located near citrus orchards are at particular risk, because citrus can harbor GWSS populations year-round. Studies by University of California, Riverside entomologist Nick Toscano found that Temecula growers who applied imidacloprid in May of each year were able to keep vineyards mostly free of PD, even in the presence of low to moderate levels of GWSS, while untreated vineyards commonly sustained losses.

Imidacloprid  is used regularly in California’s areawide PD control programs in Riverside, Kern, and Tulare Counties where resident populations of GWSS have been trapped, monitored and treated in vineyards and citrus orchards since 2000 to prevent the spread of GWSS farther north. Imidacloprid treatments in these areas include both soil and foliar applications.

Because grapes are a self-pollinating crop, applications of imidacloprid in vineyards are less likely to directly impact honeybees, although pollinators could potentially be affected in treated vineyards with flowering cover crops.

Imidacloprid has also been an effective tool in the PD Control Program’s Rapid Response activities to combat spot infestations of GWSS in Northern and Central California. Soil injections of imidacloprid in urban areas have controlled GWSS outbreaks in landscape trees and plants. The PD Control Program has eradicated GWSS infestations in 15 locations over the past decade.

Alternatives Limited for Now
Pesticides other than imidacloprid have been used for GWSS control, however, some of the most effective alternatives are also neonicotinoids. Organic insecticides are used in vineyards and citrus but they are generally considered less effective and require more applications. An approved organic treatment, Surround, is a kaolin clay product applied to grapevines that repels GWSS from landing and feeding. The PD Control Program also uses biocontrol by breeding and releasing parasitic wasps that parasitize GWSS egg masses, used in organic vineyards and orchards, and in areas where pesticide applications are not practical.

A major emphasis of the PD/GWSS Board has been the investment in research to breed and develop PD-resistant vine materials. Major progress is being made, but it could be a number of years before a sufficient supply of materials are available for commercial use.

Environmental Concerns
Beekeepers and environmental organizations have raised concerns in recent years about the potential toxic effects of imidacloprid on pollinators and some have called for restrictions and bans on the use of neonicotinoids. Honeybee populations in the U.S. have declined significantly since 2006, attributed to “colony collapse disorder,” linked to several possible factors, including pesticides. AJR 29 states that governments in Italy, Germany, and France have taken action to protect pollinators from neonicotinoids.

In February 2009, the California Department of Pesticide Regulation initiated a re-evaluation of certain neonicotinoids based on concerns about residues in pollen and nectar of specific crops and the potential toxicity to pollinators. More recently, a study by the Harvard School of Public Health, scheduled for publication in the June 2012 issue of the Bulletin of Insectology,  presents evidence of a link between imidacloprid and honeybee colony collapse disorder  The Harvard study suggests that bees can be exposed to imidacloprid through nectar from treated plants, or through the high-fructose corn syrup that beekeepers use to feed their bees, given that corn grown in the U.S. to produce high-fructose corn syrup is commonly treated with imidacloprid.

Bayer CropScience recently removed almonds from the pesticide label for its imidacloprid products in California, eliminating its use in almond orchards that rely on honeybees for pollination during the almond bloom every year. Beekeepers from throughout the U.S. bring more than 2 million beehives into California every February, with an estimated 2.6 million total hives needed for almond pollination statewide. As Houston pointed out this week to the PD Board, “Removal of imidacloprid from use in almonds takes it out of one of the primary interfaces California agriculture has with pollinators.”

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