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Killing Off West Nile Virus: Bad For More Than Bugs?


More now on that pesticide called Duet that's being sprayed on Dallas neighborhoods. It's a synthetic version of an insect neurotoxin that occurs naturally in chrysanthemum flowers. We reached out to Bob Peterson, professor of entomology at Montana State University. He specializes in biological risk assessment and he explained how Duet works.

ROBERT PETERSON: What happens is you have this spray with very, very tiny droplets, and these droplets fall on those mosquitoes and knock them down. And those female mosquitoes are actually flying because they're looking for a host to feed on. They're looking for blood so that they can nourish their eggs and continue their generation.

So it's a very specific application of an insecticide that's really designed pretty much only to affect mosquitoes.

BLOCK: Now, why wouldn't it also affect beneficial insects, anything else that's flying around, or get into the food chain?

PETERSON: Well, a couple reasons. One, is the insecticide does not last very long in the environment at all, and typically when you have daylight conditions, the sun actually breaks down that insecticide very rapidly. And also, you're starting with such a very, very low rate of application.

Now, if there are midges, little flies that are about the size of mosquitoes, flying around at night when the spray is occurring, yes, you will knock down, you will kill those insects, as well. But larger insects are much less affected if they even contact the spray, and of course if it's at night, a lot of insects, including honeybees, are not going to be in the path of these sprays.

BLOCK: What do you tell people who say I'm worried about the risks of this chemical, I don't see why it's necessary given the incidence of West Nile disease?

PETERSON: Well, it's - what we say is that the disease is very real. And in this case, with aerial spraying of the Dallas area in particular, we've reached the point where the disease is very, very serious. We know a lot about the disease, we know a lot about the consequences of spraying, and there's just no contest here.

The scientific weight of evidence strongly suggests that there's negligible risk from the spraying, and there's not negligible risk from West Nile virus disease.

BLOCK: When you're talking about negligible risk in terms of exposure to humans, would you tell anybody it's perfectly safe to go outside while they're spraying, go ahead, go out with your kids, go take a walk, absolutely no problem? Or is it really a little bit more complicated than that?

PETERSON: It's not more complicated, but it is more complicated from kind of a psychological perspective. I can talk about the mathematical risks and would still conclude that they're negligible because the risks that we have modeled and looked at for that include people being outside when there are those sprays and all kinds of what we call conservative assumptions.

But I will never say anything is safe as a scientist. That's a value-laden term. What's safe to one person is not safe to another person. So I just talk about the risks in comparison to known levels of harm or regulatory levels of concern, things like that.

So, even with considering the person's outside and they're breathing heavily, and they're rolling around on the grass as the spray, the droplets kind of drift down, there are no issues there from a risk perspective.

BLOCK: I've been talking with Bob Peterson, professor of entomology at Montana State University. He studies mosquito control and the disease mosquitoes carry. Thanks so much for being with us.

PETERSON: Well, thank you for having me. Transcript provided by NPR, Copyright NPR.