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Horsefly, Don't Bother Me!


Q: Is there any way to reduce the number of horseflies around the outside of our house? The only thing that seems to help is a contraption called "The Horse Pal". It's an interesting device that really does attract horseflies and ends up killing quite a few. But even with a Horse Pal we still seem to have an endless supply of them. We don't have any livestock, but our neighbors about 1/4 mile away do have a few cows and horses. We live on 10 acres—mostly wooded with a stream. The horseflies are particularly vicious when we're in the pool. I appreciate any ideas you may come up with, and I really do love your show.

----Eric in Wrightstown, PA

A. When I last researched horsefly control, the outlook was go gloomy I decided not to write about it—and I have to be honest and say that extended conversations with our 'common sense pest control' expert, Dr. Bill Quarles of the Bio-Integral Resource Center didn't brighten my mood much. In fact, every expert agrees that these huge, nasty flies might be the most difficult biting insect to control.

But I will fight back my depression and float some ideas that may help Eric focus his thinking.

Now, horseflies are very strong fliers that typically travel miles in search of prey—so having horses next door ups their probability greatly. And that woodland stream provides an ideal nursery; the females lay their eggs on plants near water and the larvae drop down and breed in the muddy areas below.

Because of that mosquito-like activity, I had hoped that BTI would prevent the larvae from developing in the muddy areas around the stream. But Bill says they're probably too big. BTI works best on small larvae like those of mosquitoes and fungus gnats. These monsters are so big they eat baby frogs and toads! Yikes!

But I would still try spreading some BTI granules over the wettest areas of the stream nearest the house this Spring. It could work, it can't hurt, and it will almost certainly keep down the number of mosquitoes and blackflies.

I also suggest that Eric keep the stream clear of debris this fall to keep the mud to a minimum, hang suet feeders near the stream over the winter to attract meat-eating birds, stop feeding the birds in the early Spring and then cultivate the muddy areas to expose at least some of the big larvae to the hungry birds who are now nesting there. Allowing chickens to forage there could also work well, as long as the chickens are protected from predators.

Now, what about this trap—"The Horse Pal"? Bill Quarles says it's a great resource. It's a big structure on metal legs that has a black thing hanging underneath that looks like the rapid bag boxers train on. Horseflies hunt by sight and this trap is designed to make them think they found a live meal—but when they try to bite it, they get trapped in a big glass jar. Unlike many insect traps, The Horse Pal trap does NOT use sex or food lures, because horseflies aren't attracted to the typical lures, just motion and the CO2 we bitable mammals exhale. So one thing Eric might want to try is placing some dry ice underneath the trap right before his family goes outside for a swim.

Dry ice gives off CO2 as it evaporates, imitating the exhalation of a bitable mammal; some researchers use it to lure and trap ticks in the field. And although the traps are expensive—about $300 apiece, I would suggest buying one or two more and experimenting with placement. The website for the trap maker says that just moving the thing by a few feet can sometimes dramatically increase the catch.

I would personally place them between the horses and the pool and in between the stream and the pool. Why the pool? Because that's where the biting is the worst. An excellent article on horseflies from Purdue specifically mentions pools as one of the biggest problem areas. I suppose because there's lots of motion, water and total exposure to the outdoors, which these monsters prefer. Interestingly, they tend not to fly into barns and stables to get at horses; they hunt outside.

Oh, and Bill Quarles adds that you can also catch quite a few horseflies with standard outdoor fly traps. And I'll suggest that if you rig up something that would make the traps move, they might catch more.

But don't even think about pesticides. Even non-organic sources like Purdue specify that because these pests fly in, bite and fly out, insecticides are totally ineffective.

Bill says that when the problem is incredibly bad, people typically set up screened-in areas for outside activity. So that's my final suggestion. It might be impractical to try and screen an entire pool area, but it might be easy to build a kind of safe haven at one end, with the screening supported about a foot above head-in-pool level.

& that's all I got. Good luck, Eric!



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