Fresno State prof seeks control of black widows in grapes

FRESNO
August 11, 2008 11:44am

Reports of black widow spiders found in supermarket table grapes in New Zealand and Massachusetts have prompted a Fresno State researcher to take another look at vineyard control measures for the ominous pest.

Black widows pose no threat to grape production since their typical diet is insects, not leaves or fruit.

But they might prove to be a marketing problem, if not worse, if table grapes reach store shelves with black widows hidden in the clusters.

Fresno State plant science professor Andrew Lawson is researching ways to control black widows in table grape vines.

“The movement of any black widow from the vineyard to the market place presents an immediate concern to the table grape industry,” says Mr. Lawson.

In 2001, for example, New Zealand banned imports of California table grapes after four black widows were found in table grape bunches, he notes.

In the Eastern U.S., three black widow findings in California table grape bunches were reported from Shaw’s supermarkets in the Boston area. Such discoveries can cause consumers to scatter and product demand to evaporate.

Researchers suspected that new, less toxic formulas of miticides and insecticides, used to control common vineyard pests such as mites, leafhoppers and mealybugs, can leave black widows unscathed. Mr. Lawson’s work appears to confirm that.

His project began with three objectives. One was to test the efficacy of chemical control products in the field and laboratory. The second was to scientifically document population densities and specific vine locations of black widows in San Joaquin Valley vineyards. The third was to examine black widow spiderling mortality, dispersion and development throughout the growing season.

Of seven currently-used pesticides tested, only three – Lannate (methomyl), Lorsban (chlorpyrifos) and Danitol (fenpropathrin) – provided 100 percent control of adult male and female black widows by direct exposure, says Mr. Lawson. Lorsban also provided control of adult females when the spiders were exposed to treated vine bark.

Monitoring recommendations include the following: Approximately 1,000 vines per vineyard block should be examined for black widow webs by walking five to 10 rows.

“Before any treatments, we recommend destroying nests to determine the efficacy of treatments,” says Mr. Lawson. “If the treatment is effective, nests should not be rebuilt following treatment.”

This research was sponsored by the California State University Agricultural Research Initiative with additional support from the California Table Grape Commission.