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  • Writer's pictureDavid Jefferson

Invasion!

From the book, Maine Horse Doctor, On the Road with Dr J by David A Jefferson, DVM


I am writing this article on a warm Sunday afternoon early in September. Everything is peaceful on my farm right now, except for the bang! bang! bang! coming from the barn. I know the sound well. I have heard it every day for the last few weeks. It’s Shamus and Shiloh, my two mini donkeys slamming their feet on the wooden floor of their stall. They have free access to the outside, but these warm days they prefer the stall where the stable flies aren’t quite as bad. I’ve been a New Englander for 45 years, a Mainer for 42, and I have never seen a summer like this for stable flies! I know it’s not just on my place. As an equine vet I spend my days on other farms, and it seems like everyone’s horses (and cows) are being attacked. More than annoying, these flies hurt! I have learned not to wear shorts in the barn. They prefer horses, but if there are enough of them, and my legs are available, I get bitten. Where do these flies come from, and what makes them bite? First of all, I love their scientific name: Stomoxys calcitrans. Translated from the Latin the first word means “sharp mouth” and the last word, “kicking.” They do have a sharp mouth, and they sure cause kicking. They are in the same family as the house fly, but are 1⁄2 to 3⁄4 their size. Unlike house flies they have very sharp teeth and a specialized mouth part that acts like a hypodermic needle to draw in blood. Both male and female flies bite and take blood for their nutrition. The male dies soon after breeding while the female takes off on her mission. She lands on animal legs, bites, and takes in their blood. I don’t know who does research on things like this, but she has to have at least 3 separate blood meals before she will lay any eggs. Breaks are taken by landing on a fence, a stall wall, or anything nearby. She does some digesting and returns for more blood. Now, engorged with blood, she flies sluggishly to a pile of rotting vegetation or manure and lays bunches of eggs. Without the blood meal the eggs

will not mature. 10 or 11 egg laying periods occur during this last part of the life of the female. As with many insects, the eggs hatch in a day or two and become larvae (maggots) who eat the rotting vegetation or manure around them. During the next 15 days or so the larvae go through some further stages and then become pupae (like a cocoon). This is the stage that is susceptible to fly predators (parasitoids) that many of us buy to stop the development from pupa to adult fly. Unimpeded, the fly will hatch out from the pupa in about 15 days and then head directly for a warm blooded animal. Some adult flies live a few days, others as long as 4-6 weeks. You might be asking, doesn’t this vet have the sense to spray some insecticide on his equines. I have, and nothing that I have tried lasts more than an hour. I hung a 3 foot diameter fan in the stall which the donkeys can stand in front of, but those determined flies still zoom in and bite. Mosquitoes can’t fly against that wind, but it doesn’t deter the tiny stable flies. Why the stable fly invasion this particular year? I’m not sure, and neither is an entomologist at the Maine Department of Agriculture, Conservation and Forestry. She told me that the stable fly can travel long distances from their breeding ground. That means that if you are using the fly predators in your compost pile, they may be ineffective against the stable fly that can leave your neighbor’s compost and fly right over yours to reach your horse. I think that the 80 degree days we have had for most of the summer must play a part as well. In Maine this was the hottest August ever. The heat makes compost break down quicker. The adult fly is also more active on hot days. I have noticed that when the temperature drops below 65 degrees my donkeys stop kicking, and their legs and bellies are free of flies. A few years ago Damariscotta Lake State park was shut down for the afternoon and everyone’s admission price refunded because the beach goers were so viciously attacked by stable flies. The massive fly attack was the result of piles of pond weeds that had washed up onto the beach. The rotting vegetation

was the perfect place for the fly to lay its eggs. After they hatched, they headed for all the beach goers. So, what can we do about these pests? Your first line of defense is good housekeeping. Keep your stable, paddocks and pastures clean and dry. Mow grass low around the perimeter of these areas. Clean up feed spills promptly. Create good drainage and keep composting bedding, manure, and other potential fly breeding habitats as far away from animals as possible. If you get your manure picked up regularly, you are ahead of the game. Spreading manure so that it dries makes it unsuitable for fly breeding. You can keep the female from laying eggs by covering the pile with a tarp. Keep its edges well secured with something heavy such as rocks, boards, or the side walls of old tires. (If you use whole tires, drill holes in them so they don’t collect water where mosquitoes can breed.) Fewer eggs hatched mean fewer flies. Here are some additional measures. Put up bird boxes. Cultivate some types of flowering plants to attract nature’s hit squad. That would include natural enemies such as beneficial insects, birds, spiders, and frogs. If you are considering the tiny predatory wasps do your homework and choose a reputable supplier. Ask the company’s technical staff which species mix is best for your situation. The final line of defense would be the use of stable fly traps to capture the adults. Some that get good reviews are the “Starbar Bite Free”, the “Olson Biting Fly Trap”, and the “Knight Stick by Bugjammer”. Each has a sticky coating which must be cleaned and replaced periodically. They have to be set in the right locations and at the correct height to work effectively. A downside is that occasionally small birds are trapped. Hanging the simple fly strips can help. I hung six in my barn when the stable flies first appeared this summer, and they have trapped hundreds, but it didn’t get them all. Dozens more are still landing on my donkeys’ legs. Bug zappers that attract and electrocute are not a good idea. They tend to kill more beneficial insects than pests. I’ve been thinking that leg wraps might help when the fly population is intense. Perhaps then the flies would all go for the belly. There are fly sheets

that have a broad belly band, but the stable fly is so small and determined that I’m not sure how well they would work. Besides being an extreme annoyance any biting insect that draws blood can carry disease. Of special concern is Equine Infectious Anemia (EIA) that is usually fatal. We monitor for this disease by using the Coggins test. Any biting insect including the stable fly has the potential of carrying the EIA virus. We will always have stable flies, just like we will always have worms in horses and fleas on dogs. Awareness, good management, and wise choices in their control should make the situation livable, even in extreme years like this one.

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