Project aims to clean house on filth flies

Agricultural Research, Nov, 2003 by Jan Suszkiw, Jim Core

Once considered mainly a feedlot pest, the stable fly has extended its reign of terror to the open pasture and rangeland, areas where cattle once grazed virtually unharried by the bloodsucking insect. Its expansion into new territory has added to an already expensive tab--nearly $1 billion in annual production losses to the U.S. dairy and beef industries.

In Mead, Nebraska, ARS and university scientists are collaborating on an areawide project to find out how this problem came about and what can be done to resolve it.

"In the last 10 years, stable flies have become as important a pest of pastured cattle as they once were for confined beef," says entomologist Phil Scholl, who heads the ARS Midwest Livestock Insects Research Unit, Lincoln, Nebraska. "The most logical explanation for this increase is the almost ubiquitous use of round bales as a feed supplement for winter-pastured cattle."

The flies may be breeding in hay bale litter that has mixed with mud, water, and manure. There are other possibilities, too. One is that stable flies are migrating to pasture from breeding sites at nearby farms, such as feedlots and silage piles. It's also possible that midwestern grazing lands are being repopulated each spring by windborne flies from the South.

In May, Scholl and colleagues began a 5-year field project near Mead to monitor the fly's population dynamics, breeding habitat, and dispersal patterns within a 25-square-mile tract of land owned by the University of Nebraska's Agricultural Research and Extension Center, Ithaca, Nebraska. His collaborators are Jack Campbell, with UNL's West Central Agricultural and Research Development Center, North Platte, Nebraska; Alberto Broce, with Kansas State University's Department of Entomology; David Taylor, with ARS's Lincoln unit; and Jerry Hogsette, with the ARS Mosquito and Fly Research Unit, Gainesville, Florida.

Scholl says the university site "offers a unique opportunity" to collect fly data across a broad range of environments. These include 4,500 acres of pasture, an onsite dairy, feedlots, a calf weaning area, a composting site, and a turfgrass area and nearly 5,000 cattle.

Every square mile of the site is crisscrossed by a virtual grid of squares in which the scientists have placed their most important monitoring tool--Alsynite cylinder traps with a sticky outer covering. With these, the researchers can correlate fly numbers with breeding sites and meteorological conditions.

"The objective," says Scholl of the study, "is to better understand the fly's biology, ecology, and breeding habitat so we can devise control strategies that can be used in an integrated approach for managing the pest."

Of special interest is fly activity close to round bale feeding sites. The problem begins when hay from the bales is pulled loose by cattle and falls to the ground. There it is trampled and mixed with urine and manure, creating an ideal habitat that female stable flies can lay eggs in. In late spring, young flies emerge from the sites hungry for blood. Their attacks cause cattle to bunch together, lie down, or wade in water to protect their forelegs.

"Bunching is a big problem," Scholl says, "because if they're doing that, they're not grazing and gaining weight." It also leads to heat stress. By one study's estimation, stable fly attacks on yearling beef steers cut the animal's daily weight gain by nearly half a pound.

Spraying hay bale sites isn't really an option, Scholl notes, because the insecticides now used break down after only a few days, necessitating re-application. The flies' preference for attacking cattle's forelegs can also render ineffective such animal treatments as back rubs and ear tags. The flies' hit-and-run tactics also protect them from lethal exposure to cattle sprays or systemic insecticides.

Most likely, combining insecticides with other measures, such as cultural and biological control, will prove most successful.

ARS entomologist David Taylor, for example, is conducting a trial in which he has treated the soil around a hay bale site with a Steinernema nematode that kills stable fly larvae. He'll assess the nematode's effectiveness by comparing treated and control plots and monitor its longevity in a manure-rich environment. Other control strategies may include moving hay bale locations in pastures and spreading or disking hay litter mixed with manure.

Taylor and Scholl are also investigating use of DNA markers to study genetic variation among stable fly populations from around the world, as well as using rare-earth elements such as selenium to trace long-range fly migrations back to the geographic points of origin. "For example," says Scholl, "if we could find some element that's found only in Arizona and is picked up by flies there, and we captured some specimens in a midwestern pasture that contained that element, we'd have good proof of long-range migration."

Though still conceptual, this approach may prove useful in determining the role that seasonal winds play in carrying stable flies to new territories--an endeavor of particular interest to Hogsette, Scholl's ARS Gainesville, Florida, collaborator.