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by Wyoming Livestock Roundup

Control insects for livestock health, comfort

    As warmer temperatures approach, albeit slowly this spring, one summer constant also approaches – insects and their accompanying problems for livestock managers.
    With spring snowmelt, experts across the board say the elimination of standing water will decrease insect habitat and thus the spread of disease. “If you can reduce the standing water in any way, it’s advantageous,” says Research Microbiologist Bill Wilson of the USDA’s Agricultural Research Service (ARS) in Laramie.
    “Keeping the insects off the animals is what we’re most concerned about in terms of controlling diseases,” says Wyoming State Vet Walt Cook.
    According to information released by Spalding Fly Predators, weeds and tall grasses attract flies looking to escape heat, so removing tall weeds from around buildings can decrease pest flies.
    Also, they say to clean stalls, corrals, paddocks and other areas at no more than seven-day intervals because a house fly takes a minimum of eight days to emerge in optimum summertime temperatures.
    Most insect problems for cattle come from pest insects, such as floodwater mosquitoes, horn flies, lice and ticks, rather than from diseases transmitted by insects.
    “A miserable cow is an unhealthy cow, and she won’t gain weight or produce a lot of milk,” says entomologist Will Reeves with ARS. “She’s more likely to be aggressive, and is less likely to be eating and happy.”
    “Those insects cause big economic damages to ranchers, but not by diseases, just by irritating the animals,” says Reeves.
    Reeves says fly predators are not a silver bullet, but they do reduce the number of total pests. “None of the predators eliminate the pest, but most ranchers understand the goal isn’t to eliminate the horn fly or stable fly – which is practically impossible – but rather to reduce them so they’re not as big of a deal.”
    He says the fly predators have a limited use. “They’re somewhat functional for feedlots, but not great for range.”
    In horses, the number one insect-transmitted disease is West Nile virus. “All the evidence implies that vaccinating for West Nile is the solution,” says Reeves. “You can control mosquitoes, but if one infected mosquito gets through to your horse, it’s been useless. Habitat control isn’t practical, considering the cost of the horse compared to the cost of the vaccine.”
    Although reporting of West Nile virus in Wyoming is patchy, Reeves says it seems to be common, with 181 cases reported in people in 2007. Cases in horses aren’t tracked as closely because so many people now vaccinate their horses.
    Although cattle have been found with antibodies to West Nile virus, Reeves says he’s never heard of a problem with it, although llamas have been found susceptible.
    Another horse problem is the black fly, says Reeves. “Black flies are small biting flies that can irritate horses, and they can be controlled through larvicide. Just like in cattle, they bite horses as pests and can sometimes be present in heavy number.”
    Vesicular stomatitis, recently found in several Wyoming horses in two consecutive years, is transmitted by biting midges. Wilson says the disease tends to follow low-lying riverbed areas and that the biting midges don’t exist above an altitude of 6,000 feet.
    “Last year there was a significant outbreak of bluetongue in sheep, which is transmitted by biting midges – a group including all the tiny biting flies,” says Reeves. “Sheep owners need to be aware of the outbreak, and there are problems with controlling bluetongue.”
    Unlike West Nile virus, there are several strains of the bluetongue virus and only a vaccine against one. Further complicating control, there are very few pesticides or repellants specifically for biting midges. “We’re testing a number of new or already-released insecticides to see if they’ll protect sheep against biting midges, but they’re not specifically made for that,” says Reeves.
    Reeves says sheep producers can modify biting midge habitat as a control. “If a producer has a livestock pond with shallow edges and a large mud flat, that’s where the midges breed,” he says. “They can modify that with a sheer-edged pond to reduce the number of midges on the property. Reducing the numbers means there are fewer around to transmit the virus.”
    He says producers should also pay attention to temperature and timing. “In the Big Horn Basin the ranchers had sheep at high elevations and brought them down to the lower elevations at the same time the outbreak was occurring in wildlife. It’s all determined by their contract with the Forest Service or Bureau of Land Management, but if they had known and had been able to hold them up in the high elevations they could have dodged the biting midges.”
    “In part, the unfortunate answer is there isn’t a really good answer,” says Reeves of bluetongue. “What you want to do is take as many prepatory approaches as you can, so that if you know the virus is coming you can restrict access to your sheep by keeping them at high elevation or applying pesticide.”
    “The lab has spent a number of years understanding the environmental conditions that favor an outbreak, and it depends in part of what the spring looks like and what the populations are,” says Wilson. He says bluetongue has been known to overwinter, but its success depends on a number of environmental conditions.
    Cook says Wyoming tends to see cases of vesicular stomatitis and bluetongue later in the summer. “We can see West Nile any time if the mosquitoes are out, but it also tends to be later in the summer, because that’s when mosquitoes switch from birds to horses and they bite more later in the summer.”
    “The cold winter could mean relief from insects this summer, but we’ll also have a lot more water, so it could be worse,” says Reeves of potential insect populations this summer.
    Christy Hemken is assistant editor of the Wyoming Livestock Roundup. Send comments on this article to

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