Project Aims To Clean
Holstein heifers bunching in response to feeding stable flies. Bunching damages pasture vegetation, causes heat stress to the cattle, and increases injuries, especially to calves. (K10825-1)
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
tabnearly $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
"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."
Entomologist Philip Scholl
examines an Alsynite sticky
trap used to monitor adult
stable fly populations.
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.
Stable flies caught on an
Alsynite sticky trap.
Every square mile of the site is crisscrossed by a virtual
grid of squares in which the scientists have placed their most important
monitoring toolAlsynite 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.
Entomologist David Taylor
and technician Corinne Kolm
look for stable fly larvae in
residue from a feeding area
near a large round bale of hay.
Residue produced during
winter feeding provides
perfect conditions for stable
fly larval development in
the spring. (K10823-1)
"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.
Entomologist Philip Scholl
prepares to dissect a female
adult stable fly to determine
its physiological age.
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 territoriesan endeavor of particular interest to Hogsette, Scholl's ARS Gainesville, Florida, collaborator.
Settling Into the Suburbs
Hogsette says flies are known to appear from out of nowhere in different
regions of the country. As frontal systems move through, bringing changing
weather conditions, flies and other insects get sucked up into the atmosphere.
In West Florida, for example, the winds deposit the stable flies onto
the beach. What's more, a new generation can arrive and become established
in urban and rural areas overnighteven if there aren't any overwintering
adult flies already present.
Hogsette is examining not only the consequences of flies being attracted
to humans and their actions, but of humans coming to the flies. For
example, as suburban sprawl encroaches on farmland, people have more
contact with both stable flies and house flies (Musca domestica),
creating conflict with livestock producers. House flies don't bite,
but they can transmit many diseases of humans and other animals. They
pick up disease-causing organisms from garbage, sewage, and other sources
of filth and can transfer these organisms from their mouthparts and
other body parts to other animals, including humans.
Because pesticides alone can't control the two fly pests, proper animal
manure management and sanitation are crucial to a successful fly control
program. Hogsette proved that house flies, like stable flies, can reproduce
in small residues of manure trampled into pastures.
But suburban residents can't entirely fault their rural neighbors for
all their fly problems. Hogsette says conditions associated with the
suburbs also lend themselves to fly development. For example, besides
grass clippings from mowed lawns, golf courses, and compost piles, the
flies can also breed in dumpsters behind grocery stores and restaurants.
And instead of pestering cattle in the pasture, they may target the
family dog or even "Henry Homeowner" manning the barbecue
Suszkiw and Jim
Core, Agricultural Research Service Information Staff.
This research is part of Veterinary, Medical, and Urban Entomology,
an ARS National Program (#104) described on the World Wide Web at www.nps.ars.usda.gov.
"Project Aims To Clean House on Filth Flies" was published in the November 2003 issue of Agricultural Research magazine.