Warble fly | While insecticides have helped producers manage a pest that once caused millions in damages, grubs could be hiding on organic farms
Entomologist Gary Anweiler brought back a flood of memories for bug experts and cattle producers when he caught a fly in July near Edmonton and posted its picture online asking for identification.
Anweiler had captured a warble fly, which at one time caused millions of dollars in damage to the Canadian cattle industry but has now virtually disappeared.
It turns out Anweiler’s warble fly likes rodents, not cattle, but the picture set in motion a discussion of warbles, government programs, farm kid methods of warble collection and highway signs announcing warble control areas that were once commonplace across the province.
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Warbles were the scourge of cattle producers and packing plants before the introduction of effective insecticides. An Alberta Agriculture warble document estimated that warble flies and their grubs caused $6 million in damage in Alberta during the 1980s from lost cattle weight gain, milk production and hide damage.
The warble fly, often called heelfly, would lay its eggs near cattle’s heels in May and June. The eggs would hatch and the larvae would burrow through the skin and wander through the cattle’s tissue until spring, when they would chew a breathing hole through the hide on the cattle’s back. The larvae would remain on the back for four to 10 weeks and then drop off in the spring and develop into flies.
John Milne, a Fairview rancher and former Alberta district agriculturist, said he remembers as a child heating pop bottles and placing the open end on the back of the cow where the warble had chewed a hole. As the air inside the bottle cooled, it would suck the warble out of the cow’s back and into the bottle.
“We heard people say you could do it and we tried it, but most of the time we just popped them with our fingers,” said Milne, who was in charge of the warble control program at St. Paul and later at Fairview in the 1970s.
He said the warble flies would drive cattle wild.
“When the flies were flying in June and July and laying their eggs, these animals would go nuts. You’d see them racing around the fields with their tails in the air. You don’t see that any more.”
It was the job of provincial employees like Milne to ensure cattle producers complied with the provincial warble control program designed to eradicate the pests.
Doug Colwell, a research scientist with Agriculture Canada in Lethbridge, said government programs, insecticides and packing plant and auction market inspection programs worked together to virtually eliminate warbles by the 1990s.
A 1996 beef quality audit showed that less than .008 percent of carcasses showed evidence of warble grubs.
Colwell estimated that 50 to 60 percent of cattle in Canada were infested with warble grubs in the 1950s and 1960s. The targeted warble programs reduced infection to about five percent, while the introduction of ivermectin in the 1980s virtually eliminated the problem.
Colwell once joked to producers that ivermectin was so effective in controlling warbles that all a cattle producer had to do was show cattle the label on the bottle and grubs would die.
Colwell recently published a report from a three-year study in 2008-10 that showed warble grubs still exist in Western Canada.
“Clinically they have disappeared. I can talk to lots of veterinarians who have graduated in the last 10 years who have never seen one, but serological evidence suggests they are still out there.”
Colwell took blood samples from 1,500 calves from Manitoba to British Columbia that were coming off their first year of pasture. He was looking for the presence of antibodies that could confirm warbles in the calves. While serology testing has its limitations, Colwell said the results were enough to convince him that the warble pest is still around.
The serology showed 30 to 75 percent of the animals tested positive for the warble grub.
Evidence of the grub was higher in the dry year of 2009 than in the wetter years of 2008 or 2010, he said.
The findings reinforced results from similar data collected in 2001.
“It made me relatively comfortable in saying that they’re hiding out there.”
Colwell said he’s not sure where the flies may be hiding, but when a U.S. colleague needs warble infested cattle for a drug trial, he goes looking in herds near organic cattle producers.
“That may be in fact where cattle grubs are hiding out.”
Warbles are still considered a pest under Alberta’s pest act and must be controlled.
Colwell said enforcing warble control may be difficult because so few people run their hands over the backs of the cattle at the right time of the year to search for the grubs.
Camrose cattle producer Bob Prestage said one of his first jobs as a newly minted University of Guelph agriculture graduate was to measure the efficacy of the insecticide Grubex in cattle in Ontario’s Grey and Bruce counties.
In the study, one-third of the cattle were treated with an existing cattle drench, a third were treated with the new Grubex and one third were left as a control. It was part of his job to return in February and count the number of warble grubs protruding from the backs of cattle.
The largest number of grubs he found on the backs of an untreated control animal was 265. The study was supposed to last two years, but Prestage said they had difficulty finding cattle producers willing to leave one third of their cattle untreated a second year because of the success of the product.
Colwell said he is not concerned there will be a resurgence of warbles like in the 1950s, but veterinarians and cattle producers should be aware that warble flies are still out there.
“It is easy to control if there was an outbreak,” he said.
“I do think it needs to be at the back of people’s minds, simply because it is still there. There is a generation of veterinarians who have never seen one and have never put their hand on one. All the veterinarians that remember what they look like are mostly retired.”
