Bat House Forum

[Dave Miller]

This is primarily a question for Mark Kiser, but if anyone else has any info, please chime in.

I was listening to the "Talk of the Nation" show on National Public Radio on July 11, and they were speaking with Stephen Ostroff who is the Deputy director for the National Center for Infectious Diseases at the CDC in Atlanta.

Toward the end of the show a caller asked about putting up bat houses to help increase the local bat population, as a way of reducing the mosquito population and thus the risk of contracting West Nile virus.

Dr. Ostroff quickly dismissed the idea with this reply:

"The reason that we don't necessarily think that that's the the best thing to do is that unfortunately there are other things that can be contracted from bats, and so the more bats that you have around your home environment, the more you have to be concerned about things like rabies. And so in terms of control measures to reduce mosquito populations, putting out more bats in an area is not necessarily one of our recommendations. We would much rather see the application of what we refer to as integrated pest control, which is a combination of what we call source reduction, looking for places where the mosquitoes breed, and doing your best to try to eliminate those breeding sites, application of what's called larvacides, which are quite safe, and keep the mosquitoes from hatching into adults themselves, and then taking a combination of personal measures to reduce your likelihood of being exposed to any mosquitoes that are out there."

You can hear the show here: http://discover.npr.org/features/featur ... Id=1334342
The part with the caller starts about 10 minutes into the program.

I'm guessing that Dr. Ostroff may need a little education on bats, because he twice referred to "putting out bats", as if you could go down to the local garden nursery and buy a colony of bats to release in your yard.

So unfortunately anyone who heard this show would have gotten the message that "the CDC says bat houses are bad - they might cause you to get rabies". I'm sure Mark and the folks at BCI are used to fighting these PR battles.

My real question is: What studies have been (or should be) done on the net result of installing bat houses, in terms of mosquito populations, and cases of rabies in humans? In other words, do the bats REALLY impact the mosquito population, and does having more bats around REALLY increase the chances of people getting rabies?

I did find this page on "Bats and the West Nile Virus" on the BCI site: http://www.batcon.org/media/press006.html However it doesn't reference any studies. And there is a good article by Merlin Tuttle titled "Bats, Man-Made Roosts, and Mosquito Control" here: http://www.batcon.org/bhresearcher/bv8n2-4.html However he doesn't address increased risk of rabies.

Mark - has there been a study like I described, or is one planned? Or has anyone else done any informal studies? I seem to recall Joe offering up some blood in the name of science somewhere in this forum...

If not, what do we say to people who may have heard the radio show, or with whom we are having a discussion about West Nile virus? It is a popular topic these days, and I'm sure it will become even more popular in the coming years.

Thanks,

Dave

[Joe Spencer]

Thank's Dave sure looks like more hysteria has been potentially created out of ignorance. I too am looking forward to Mark's BCI's comments on this one..

[Mark Kiser]

Hi everyone,

The CDC will always object to having bats living near people (big rabies budgets are at stake, despite the astronomically low risk to humans). We put out a bats and rabies brochure with them, and the content was drastically altered before the final version was printed. Let's just say the content turned out less bat-friendly than we would have liked.

There have been no rabies cases associated with bat houses.
Of the 37 or so human fatalities in the US due to bat rabies, only 10 resulted from species that typically use bat houses (free-tails, big browns, etc.) No human rabies cases have been linked to little brown bats, the most numerous US bat species.

There are a few studies that address bats feeding on mosquitoes:

R. F. Long. 1996. Bats for Insect Biocontrol in Agriculture. The IPM Practitioner, vol 18 no. 9, pp 1-6.

Long, R. F., T. Simpson, T.-S. Ding, S. Heydon, and W. Reil. 1998. Bats feed on crop pests in Sacramento Valley. California Agriculture, vol 52, no 1, pp 8-10.

Rydell, Jens, D. P. McNeill, and J. Eklof. 2002. Capture success of little brown bats (Myotis lucifugus) feeding on mosquitoes. Journal of Zoology, London, vol 256, pp. 379-381.

In the first two articles, mosquitoes are lumped together with flies and other Dipterans, so the actual amount of mosquitoes eaten is not given in these two studies.

The third article documents bats feeding on mosquitoes inside an open Quonset hut in Alaska (at night but with bright sun outside at midnight). The authors observed bats for a 2-hr period at a distance of 1 to 3 meters and recorded successful feedings using direct observation and a bat detector. Three bats flying inside caught 92 out of 100 (92%) of the mosquitoes they "attacked." Mosquitoes did not take evasive action to avoid being eaten, unlike some moths that can detect bat echolocation calls.

There's a few others cited in Merlin's article you mentioned.

Here's another:

Anthony, E.L.P and T.H. Kunz. 1977. Feeding strategies of the little brown bat, Myotis lucifugus, in southern New Hampshire. Ecology, 58: 775-786.

Bats that feed on mosquitoes (like little brown bats, Yuma myotis, and southeastern myotis) probably do so when mosquitoes are swarming--this makes feeding a lot more practical in terms of energy conservation. Hunting tiny random individuals is a lot harder and less efficient than feeding on a dense swarm. Mosquitoes are have been found in the stomachs of bats such as Hoary Bats that you would not think of normally feeding on them. Perhaps when swarms present themselves it's too good to pass up.

Anecdotally, the two best cases for bats helping with mosquito reduction are at Yellowstone Lake State Park in Wisconsin--Kent Borcherding's successful 20 + bat houses with 3,000 to 5,000 little brown bats--park staff credit the bats with helping reduce a mosquito problem that existed before the bat houses were installed; and the Chautauqua Institution in southwestern New York---residents credit the numerous little brown bats in buildings (a few in bat houses) with keeping the area relatively free of mosquitoes.

Hope this helps somewhat. Bats' usefulness in mosquito reduction is a controversial topic, and unfortunately there's not much hard evidence in the literature supporting bats.

Cheers,

Mark

[Dave Miller]

Thanks Mark, this helps a lot. It sounds like any anecdotal evidence from other people here would be useful as well.

I found the first two articles on the web:

Bats feed on crop pests in Sacramento Valley http://www.genglab.ucdavis.edu/ding/publication/bat.htm

Bats for Insect Biocontrol in Agriculture - about 2/3 down the page:
http://www.tpwd.state.tx.us/nature/wild ... usters.htm

Reading the articles it sounds like mosquitoes are usually not the first item on the menu for bats, which is not surprising. If I was starving and had to choose between a tasty burrito (moth) vs. a piece of popcorn (mosquito), I'd always take the burrito. However if there is a bowl of popcorn sitting in front of me (mosquito swarm), I'd take a handful.

Dave