Old World parasites stayed behind when the silverleaf whitefly stole into
the United States in 1986 and began an unimpeded feeding frenzy on vegetable,
cotton, and horticultural crops. The pest still costs U.S. growers more than
$500 million annually.
To counter the whitefly and related pests, scientists with the
Agricultural Research Service and other
agencies have collected parasitic wasps from the pests' native habitats. In
quarantine laboratories, researchers with USDA's Animal and Plant Health
Inspection Service (APHIS) are evaluating many of the collected species and
strains for their suitability as biological control agents.
Already, a few of these tiny wasps, which are barely visible lookalikes,
have been released in U.S. environments. "Now," asks geneticist Larry
J. Heilmann, formerly with ARS, "how will one tell the released strains
apart to determine how well each reproduces and disperses under various field
In his research at the ARS Insect Genetics and Biochemistry Research Unit in
Fargo, North Dakota, Heilmann found an oft-repeated genetic sequence of 33 base
pairs in the DNA of the wasp Encarsia formosa
collected in Egypt. From
these base pairs, he developed a DNA probea sequence that binds only to a
unique portion of DNA; in this case, to DNA specific to E. formosa
strains from the eastern Mediterranean region.
The answer: by their unique
genetic material, or DNA.
The test Heilmann developed to ascertain a wasp's eastern Mediterranean
origin involves simply squashing the wasp on filter paper, immersing the paper
in a radioactive DNA probe solution, rinsing, and then testing for any
significant residual telltale radioactivity. For field tests, he says, it may
be possible to replace the radioactive probes with fluorescent ones.
Such tests will distinguish wasps easily and quickly and less expensively
than a laboratory-based polymerase chain reaction assay that APHIS now uses.
With the prototype research done, many DNA probes should soon become
available for identifying exotic and native North American Encarsia and
other wasps. So far, one additional probe identifies strains of
Eretmocerus wasps native to the eastern hemisphere. Another more
specifically pinpoints strains from Pakistan and the United Arab
Emirates.By Ben Hardin,
Agricultural Research Service Information Staff.
For more information, contact Dennis R. Nelson,
USDA-ARS Red River Valley Agricultural
Research Center, P.O. Box 5677, University Station, Fargo, ND 58105; phone
(701) 239-1270, fax (701) 239-1202.
"Beneficial Wasps Get ID Tags" was published in the
April 1999 issue of Agricultural