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 conditions?"
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 waspEncarsia formosa collected in Egypt. From these base pairs, he developed a DNA probe—a sequence that binds only to a unique portion of DNA; in this case, to DNA specific toE. 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 Research magazine.