Black flies: Everywhere, looking for a meal (that means you)

It's turning out to be quite the year for black flies. Up north, loons are abandoning their eggs. They're fleeing the insects attacking them at their nests. Black fly counts are up too in the metro area. But their population is being carefully controlled in and around the cities.

MPR's Phil Picardi spoke with Mike McLean, who's with the Metropolitan Mosquito Control District, about the matter. What follows is an edited transcript of their conversation.

Phil Picardi: Point of clarity: black flies are not house flies or deer flies or horse flies.

Mike McLean: No, they're not. They're biting insects. They're a close relative of mosquito but they're a little bit hardier, a little bit tougher, they've got some differences, but they do need blood. So they're out to bite you.

Picardi: Well, why has it been such a good year for black flies?

McLean: Well we've had a lot of running water. Black flies, unlike mosquitos that develop in standing water, black flies like to develop in rivers and streams. And we've had an awful lot of high water, an awful lot of water flow, and it's just a lot of really good habitat for a lot of different species of black flies.

Picardi: So the rivers and streams, that's where they lay their eggs?

McLean: That's right, they lay their eggs. The eggs hatch and the larvae attach to things, like twigs and rocks and they filter feed. So they let the food come to them. And then they go through their life cycle and emerge. There's a lot of different species - they kind of come out at different times of the year. They like to bite different sorts of creatures. We've got four species here in the Twin Cities that bite humans. But a lot of black fly species don't bother people at all. Some of them really like to bite birds.

Picardi: So how does this year compare to other years in terms of the amount of larvae you're seeing?

McLean: Well, we're seeing some extra production. The more water you have running, the longer that high water lasts, the more black flies are being produced. We monitor those larvae levels around the metro area. We put out sampling devices in the rivers and in some of the large streams. We're seeing bumper crops of black flies this year. And it's something you kind of expect when you see this much water running off.

Picardi: How do you control black fly populations?

McLean: Well we use a bacteria and it's called BTI in a liquid form. And we pour that into the water, and the black flies are kind of a captive audience, so they will filter that bacteria out and it forms a little crystal that tears them up from the inside out. The nice thing about it is it's very specific to Gnat larvae, so anything else that doesn't share their anatomy is unaffected by it.

Picardi: So no health risks to humans?

McLean: No. In fact we operate under a permit from the Department of Natural Resources, so in order to do a treatment, we actually have to show that the larvae are there in sufficient numbers for the treatment to do its work. So we put out samplers, and when the number of larvae per sampler gets above a certain amount, that's when we do the treatment.

Picardi: Is it true that you could actually eliminate black flies, and if so, why not do that?

McLean: Well, the black flies respond to this treatment. When you put in the stuff, you get all the black flies. And we have a very good success with our program in terms of reducing those numbers. In fact, so successful that you get to a point where you kind of have to ask yourself if you really want to change the ecosystem that much. It's never a good idea to completely eliminate something from a system, even though you're really trying to target those species that bother people. So we do leave gaps in our treatment area in order to make sure that there's a little bit more black flies for next year.

Picardi: And do fish feed on the black fly larvae? Is that why we also keep their numbers at a certain level?

McLean: Well they do some. Over time, we've put out a lot of samplers to show what the effect on the ecosystem is of year after year treatment. And those samplers show that if you eliminate black flies, other species will kind of come in and fill in those niches. And so it hasn't had a big effect over time - and we've been doing this for about 20 years on the overall what we call biomass, or the amount of available food for other organisms.

Picardi: For the uninitiated, including myself, what does the black fly bite feel like?

McLean: Well, it's pretty nasty. I mean if you've ever had a black fly bite, they're not as subtle as a mosquito. They actually take a little chunk right out of your skin, and then they kind of lap up the blood that goes into the little hole they've made. And they put in a bunch of anticoagulants and chemicals in their saliva that keeps that blood flowing. And so, a black fly bite is really pretty nasty. It itches for quite a while - quite a bit longer than an average mosquito bite. Sometimes you'll see a tiny little red dot in the middle of it and a pretty good size bump.