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 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; their typical diet is
insects, not leaves or fruit. However, when products such as table
grapes reach a store shelf with a black widow hiding in the cluster,
alarm bells go off everywhere, said Fresno State plant science professor
Dr. Andrew Lawson in explaining recent research on methods 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,” Lawson said.
In 2001, for example, New Zealand banned imports of California table
grapes after four black widows were found in table grape bunches. 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. 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, Lawson reported. Lorsban also provided control of adult
females when the spiders were exposed to treated vine bark.
A key cooperator in the study was Kent Daane, a biological pest control
specialist for the University of California Cooperative Extension (UCCE),
based in Parlier, California. Lawson and Daane collaborated in the
research, and Daane will serve a major role in making control
recommendations through his contacts with the table grape industry.
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,” Lawson stated. “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.
For more information, contact Lawson at
alawson@csufresno.edu or
Daane at daane@uckac.edu.
(Copy by Steve Olson of the California Agricultural Technology Institute
at California State University, Fresno.)
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