Nematodes: Lords of the Flies?

Biting flies that pester cattle could soon get a taste of their own medicine. Agricultural Research Service (ARS) and University of Nebraska-Lincoln (UN) scientists are testing a way to fight the flies in feedlots where they gather and breed. They're using tiny parasitic roundworms called nematodes that prey on the flies' maggot offspring.

Exploring new, nonchemical ways of protecting cattle is the objective of a 3-year-old project by entomologist David B. Taylor at ARS' Midwestern Insect Livestock Research Unit in Lincoln. In nematodes he sees a biocontrol agent that could be part of an integrated fly-control program at the feedlot along with traps, manure management, sanitation measures, and parasitic wasps.

Since 1999, Taylor and UN associate Thomas Powers have screened 20 species and 50 strains of fly-infecting nematodes for their abilities. Of particular interest were those capable of persisting in cow manure long enough to kill house and stable fly larvae over an entire season.

In Nebraska, where beef cattle are the top agricultural commodity with annual sales of $5.5 billion, stable flies are considered even worse pests than house flies. That's because attacks by swarms of these relentless biting flies cause blood loss, stress, and feed-efficiency problems. The flies may also harbor disease organisms, and they cost the U.S. beef and dairy cattle industry up to $1 billion in annual production losses.

Taylor's and Power's strategy calls for battling the pests in manure around feedlots or in soiled calf pen bedding. That's where 80 percent of the flies' brood hatch and feed. And prolific breeders they are—hundreds of maggot offspring emerge from a single pound of manure. Therein lies the nematodes' appeal, for a mating pair of these roundworms can produce 5,000 to 10,000 offspring in a single maggot in less than 2 weeks. Says Taylor, "The nematodes actually reproduce faster than the flies."

In experiments, up to 99 percent of fly maggots died within 48 hours of infection by the top fly fighter, Steinernema feltiae. In the laboratory, "the nematodes can live in bovine manure for 4 to 6 weeks without hosts," says Taylor. In feedlots, he adds, "We'd like to apply them in May and get season-long fly control." Chemical insecticides, in contrast, must be reapplied, and flies can develop resistance to them.

The researchers are testing ways to apply the nematodes on manure and protect them from drying and ultraviolet light. About 1 million nematodes per square meter are used, but lower rates might suffice, keeping the costs closer to chemical controls. If the approach works, the nematodes could also be used to fight corn rootworms in manure-fertilized fields, says Taylor.—By Jan Suszkiw, Agricultural Research Service Information Staff.

David B. Taylor is with the USDA-ARS Midwestern Livestock Insects Research Laboratory, Plant Industry Bldg., Room 304B, East Campus, University of Nebraska, Lincoln, NE 68583; phone (402) 437-5792, fax (402) 437-5260.

"Nematodes: Lords of the Flies? " was published in the July 2002 issue of Agricultural Research magazine.