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Kill flies by alternating pesticides, monitoring need - Old-fashioned fly swatters may be the most
foolproof housefly killer, but for dairy farms, insecticides are the
practical choice. Flies spread disease and a host of pathogens that cost
farms hundreds of millions of dollars in annual losses.
Unfortunately,
with the repeated use of the same insecticides, flies develop
resistance through genetic mutations that make these products less
effective.
Cornell entomologist Jeff Scott and
colleagues have analyzed levels of resistance to six insecticides in
flies, and they have identified the mutations that led to resistance in
houseflies from cattle farms in nine states around the country.
They
found high levels of resistance to the most common insecticide,
permethrin, used by farmers around the country. Other treatments varied
by location, and levels of resistance to different compounds varied as
well.
What does Scott recommend? “Only use
insecticides when they are needed,” he said. Some farmers decide, “it’s
Tuesday and I need to spray. We recommend that farmers monitor [fly]
levels and only use an insecticide when they will benefit from
spraying.” In addition, Scott suggests alternating insecticides over a
season or each month and using biological control agents such as tiny
parasitoid wasps.
Genetic mutations are random
and can occur from sunlight radiation or from errors in copying DNA,
Scott said. “They happen by chance,” he said. “When you use an
insecticide and one mutation lets a fly survive, then that mutation is
carried forward in the population.”
Scott and
colleagues published findings last fall in the journal Pesticide
Biochemistry and Physiology and are working to understand three main
mutations that confer pesticide resistance in houseflies.
Unexpectedly,
one of the mutations – which was not the most common – caused the
highest resistance to permethrin, and another mutation that led to the
lowest levels of resistance was the most common in some locations. The
scientists expected the most effective mutation to be the most common.
Future research will solve the dilemma.
“We
think it is due to fitness costs,” where a mutation allows the fly to
survive insecticides, but is not optimal in terms of overall health,
Scott said.
Colleagues in this work include
researchers from the U.S. Department of Agriculture (USDA) Agricultural
Research Service and universities across the United States.
This research was supported by the Cornell University Agricultural Experiment Station's USDA Hatch funds.