Bats for Mosquito Control

Bats are, famously, one of the mosquito's natural predators. According to some experts, a single brown bat can gobble up to 1,000 mosquitoes in a single hour. 

So says a recent article in the Miami New Times. Or maybe it's more according to Mother Earth News:

Many bats, and almost all in the United States, thrive on an insect diet. A single bat can eat up to 1,200 mosquito-sized insects every hour, and each bat usually eats 6,000 to 8,000 insects each night.

Really? That's a lot of mosquitos. Just where did this number come from? The Miami New Times article quotes a town supervisor in New York. The Mother Earth News article doesn't provide a reference.

Variations of that quote are everywhere on the internet. The Tampa Bay Times uses it all the way back in 2001. It seems that most articles that use that figure don't really point to where it comes from.

Not on a mosquito diet

So, is it true that bats can eat 1,000 mosquitoes per hour and are bats a good way to control mosquitoes? Questions that are taken as a given in many articles should be questioned.

The only reference to bats eating a large number of mosquitoes that I find comes from an article from 1926 in the Journal of Mammalogy. It quotes a 1920 study where bats were released into a room full of mosquitos and in the first few minutes ate on average 10 per minute. That comes to 600 per hour and was later rounded up to 1,000 (or even 1,200).

So, are bats good at catching mosquitoes? Probably not as good as people hope. To understand that you have to look at the size difference between a bat and a mosquito. The most common bat in Florida is the Brazilian Free-tailed bat with an average weight is about 12.5 grams. The average weight of a mosquito is 2.5 mg or about .02% of the bat's weight.

Mammal and bird predators rarely hunt things that are only .02% of their weight. For example, we wouldn't expect a tiger that weighs 88 kg to hunt wood roach (palmetto bug) with a weight of 2 grams, but it's about the same ratio as a bat hunting mosquitoes. There are a few mammal hunters that specialize in prey that small but they have to get it in large quantities. Baleen whales eat huge mouthfuls krill at once and anteaters tear into ant or termite hills to get many quickly.

I don't eat roaches

Texas Tech says that the Florida's most common bat, the Brazilian Free-tailed Bat's menu consists of:

moths (34%), flying ants (26.2%), June beetles and leaf beetles (16.8%), leafhoppers (15%), and true bugs (6.4%).

Bats are very opportunistic feeders. I'm sure that in other areas and other times of the year the diet would be different. But the size of the prey wouldn't change that much. These insects range in weight from .5 grams - 2 grams, much larger than the .0025 grams of a mosquito. Or to put it another way, a bat can spend his time and energy catching 1,000 mosquitoes or 1 good size moth.

Maybe there are references that lend credence to the quote. If anybody knows of any I'd love to see them.

Escaped the bat again

I write this blog mostly as a way to learn about a subject and to organize my thoughts. In this case I had seen the "bats eat 1000 mosquitoes per hour" quote several times with little backup. When many people use a quote but nobody knows where it came from it's probably not nearly as true as people assume. My conclusion is that this one is probably not true at all.

Bats are an integral part of the ecosystem. They do control some larger insects like corn moths and flying beetles and do eat mosquitoes. But as to mosquito control they're probably not nearly as effective as predators closer to the mosquito's size such as skimmers and dragonflies. And fish eating the larvae are even more effective. Because they're not that effective in controlling mosquitoes shouldn't diminish their worth.

Bats

Bats are so cool. I wrote an earlier article on bats but there's just so much to bats that make it hard to do them justice in a single article. Huge populations living just barely out of our sight that have just so many stories to tell make it hard to do them justice in a single sitting.

The bats in the video below are <a href=http://www.arkive.org/brazilian-free-tailed-bat/tadarida-brasiliensis/>Brazilian free-tailed bats</a>. Also known as Mexican free-tailed bats they are the most common bat in Florida. If you see a bat there's a better than even chance it's a Brazilian free-tailed bat. If you look closely at one you can see the little "mouse tail" trailing as they fly.

Brazilian free-tailed bats are given more credit than deserved for eating mosquitos. Sure, they eat a few but the prefer larger insects like moths and flying beetles. Just not enough meat on mosquito body to go after them. Some surveys have found over 90% of their food to be moths.

I took pieces of the video above at a nearby bridge. I used a very weak light to shine into their crack and went by pretty quickly to get a video of them. Not a very good video but look closely and you can see a couple. Lousy video is the price of minimal disturbance. The last piece is some stock photos but the sound in the background is the bats that I recorded. In the daytime when you hear them it's a very high pitched squeak. As evening approaches they get more excited and start giving lower pitched squeaks that you hear at the end of the video. They know feeding time is near.

My what large ears you have!

The day we went to the bridge to watch them come out a Cooper's hawk joined us. He spend the evening sitting on an unused man-made bat house watching thousands emerge from the bridge, seemingly as fascinated as we were. He never tried to catch any of the emerging bats while we were there, but bats to make up a part of their diet. I think we may have intimidated him from trying on that day.

Bat on the menu?

Right now a vital time of year for the bats. The young were born in June so the mother is feeding the babies. All of the young in a colony are born in a period of 10 days. The males left just after the mating in March so the population of the bridge doubled when the babies arrived. They males fly to the south to give the mothers all of the insects in the neighborhood. As soon as the pups are able to hunt the mothers will fly away and leave the area to the young bats.

In some ways Brazilian free-tailed bats seem to be survivors. They depend on a variety of insects in their large range that extends from the south of Brazil to the south of the US. They can live in a large variety of homes -- caves, mines, bridges, old buildings and trees all fill the bill for habitat.

The bat signal is on

But they're not very resilient to pesticides. Agricultural pesticides are particularly hard on bats. Sprays frequently slow insects without killing them immediately. Bats feeding near a recently sprayed orchard or field can be devastated quickly. Bats eat many insects every night, and their body is very small. They take in a lot of pesticides compared to their body weight.

But I guess the reason I really like the Brazilian free-tailed bat is their ability to hide in plain sight. Bridges have become a favorite haunt but exceedingly few of the cars passing by know that thousands of bats may live in the bridge they just crossed. Nor do they realize the number of night insects that are consumed every night.

Bats and Bridges

So what is it with bats and bridges?

Actually bridges are a prime habitat for bats these days. Once they lived in tree cavities or outside of Florida caves were always a prime roosting spot. But habitat destruction has greatly reduced their trees. Today the prime location to see bats is under a bridge. Small crevasses in the bridge joints can hold hundreds or even thousands of bats.

Brazilian Free-tailed bat

One of the many retirement hobbies I'm pursuing these days is bats. I'm currently volunteering with a group that's trying to map all of the bridges in Florida to tell which ones have bats and which don't.

I've included a couple of pictures of what to check for when looking for bats. First is the odor. A colony of bats puts out a lot of guano in a given day. It has a sour, musky odor. Unmistakable after you've smelled it. Bat guano is dry and dark, very different from a bird. Here's a video of the droppings of the Brazilian Free-tailed bat under a bridge (Just the thing to get everybody into their Friday).

Bat Droppings

Another thing to look for are dead bats. I saw several today.

Brazilian Free-tailed bat, RIP

I don't think the dead bats I saw today was cause for concern. It was just that it was a huge colony. On any given day a few are going to drop dead.

Another way to find the bats is to just listen. Brazilian Free-tailed make an audible chirping noise that you can hear if you stay quiet and they're nearby.

But a better question is why bats? Bats are important predators of about any insect in the air. They're also really cool animals that almost never actually turn into vampires. Yeah, they're creepy and smelly and everybody has heard a story about one that flew into somebody's hair. But they're really harmless.

So, I'll be posting a few updates when I can't think of any other subject for the day look at area bridges for bats. The last statewide survey of bridges was done in 2003 so we'll be looking to update that survey.