When hornflies swarm around pastured cattle in late summer, producers can almost guarantee those pests are resistant to insecticides.
There are always individuals in the insect population that are resistant to pesticides. They survive and multiply.
“It’s like a selection program. We’re selecting very quickly for those flies that have the ability to resist the insecticide,” said entomologist Doug Colwell at Agriculture Canada’s Lethbridge research centre.
There is nothing new on the market to overcome resistance problems. Some animal health companies are developing new chemistries, but the registration and approval process for pesticides is slow, said Colwell.
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Hornflies in Western Canada and eastern Ontario have shown resistance to pyrethroid and organophosphate for more than a decade.
Producers should alternate in successive years among synthetic pyrethroid, organophosphate, carbamate and organ-ochlorine insecticides when possible.
“By switching the pesticides, you delay for as long as possible the development of resistance,” he said. Once established, resistance persists for years.
When eartags were introduced with a low dosage of insecticide that lasted for the summer, fly problems seemed under control.
Colwell speculates many producers stopped using the tags out of frustration because the flies were no longer being killed. But ignor-ing fly control can be costly.
Affects weight gain
Research shows a 14 percent reduction in weight gains in calves, yearlings and older cows when swarmed by bothersome hornflies who feed along the midline of the animal’s belly. Animals trying to fend off flies don’t graze well.
Fifteen to 20 flies per animal cause some irritation but are not serious.
If there are more than 250 flies per animal, then a combination of things happen. Blood loss from bites, irritation and other behavior changes occur such as head tossing, scratching and ear flapping.
