Fly nuisance on cattle has been shown to reduce milk yields by up to 0.5 litres/cow/day or growth rates by 0.3kg/head/day. Effective fly control relies on preventing the annual population explosion, otherwise farmers are playing catch-up and usually losing, according to Dr Peter Bates, head of the Veterinary Medical Entomology Consultancy.
... waiting until flies are bothering cattle allows breeding populations to become established and difficult to get on top of ..."
He warns against assuming that pre-Christmas low temperatures could help reduce fly populations this summer. “Although air temperatures were below zero for long periods in most parts of the country,” he says, “a covering of snow acted as insulation. This slows or even prevents the penetration of frost into pasture, where larvae of some species relevant to cattle over-winter typically about 10 centimetres below the surface.
“Larvae of other species—in particular the sheep head fly implicated in transmission of summer mastitis—will over-winter in woodland litter, where frost penetration is also quite rare no matter how cold the weather. As soon as they hatch from pasture or woodland, blood-sucking fly species can migrate several kilometres to find livestock on which to feed.”
While farmers cannot eliminate pasture and woodland breeding sites, Dr Bates says a meaningful impact around farm buildings is possible through good hygiene, for example minimising the presence of dung heaps, unscraped slurry puddles, and old hay and straw stacks. Prompt collection and disposal of carcases, not just livestock casualties but also wild animals such as rabbits, rats and birds, can also help reduce fly breeding sites.
“For maximum control, action must start before the fly breeding season,” he advises. “A good indicator is when average daytime temperatures reach 10°C. Otherwise, waiting until flies are bothering cattle allows breeding populations to become established and difficult to get on top of.”
In conjunction with good farmstead hygiene, Pfizer VPS vet Andrew Montgomery says residual pour-on pyrethroid treatments for cattle such as deltamethrin (e.g. Spot On™) or alphacypermethrin (e.g. Dysect Cattle Pour-On) are licensed to offer control of flies for up to 4 – 8 weeks depending on fly species and level of fly burden.
“Fewer treatments in total can be required, and costs minimised, by minimising local fly breeding grounds, and by starting early with cattle treatments rather than waiting until fly nuisance on cattle is present,” he says.
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