Do bats REALLY eat mosquitoes?

One of the debates a lot of entomologists have is whether bats actually have an impact on mosquito populations. Sure, we hear all the time that bats eat mosquitoes, but there are good reasons to doubt this. I’ve had this discussion with instructors before and there’s really no consensus.

We know bats occasionally eat mosquitoes. They live in the same habitat, are voracious predators of flying insects. We’ve found mosquito parts in bat droppings. There have been caged studies where bats have been observed preying on mosquitoes and observational studies which show them preying on mosquitoes.

Still, some are unconvinced. I personally fall in this camp. Bats generally prey on medium to large insects and mosquitoes definitely fit into the ‘small’ category. Furthermore, there’s the issue of where the bats forage. Mosquitoes stick close to the ground looking for hosts whereas bats generally fly outside of the mosquito’s foraging range. The latter is a relatively weak argument because different bats forage at different heights. Areal ecology has parallels to aquatic ecology in some senses. A study in the Journal of Medical Entomology a few months ago attempted to answer this question. I’ve been wanting to get to it, but have been unfortunately inundated by school, work and general life stuff.

Believe it or not, this question is quite difficult to answer. To fully answer the question of whether mosquito populations are impacted by bat predations, you actually have to measure the populations while ensuring bats are in the area. That’s much more difficult to do than it sounds. The authors of the paper I’m citing might have found a way to do this, though. They constructed enclosures made of mesh and released bats into them. They measured the oviposition rates of mosquitoes inside and outside the enclosures with and without bats in them.

The mosquitoes that were ovipositing in the tents were Culex restuans, a species which primarily feeds on birds. This is convenient not only because this means that it was unlikely that this species was attracted to the bats but also because this mosquito is suspected of being a vector of West Nile. Proving this goes into a whole other blog post…so we’ll just leave it at that.

Anyways, they looked at how many mosquitoes were laying eggs in the enclosure. This essentially quantifies the number of blood-fed gravid females which are laying eggs. This is also important because mosquitoes pick up pathogens after feeding on an infected host…this means that bats are eating potentially infected mosquitoes from the population. It’s an imperfect measure because mosquitoes can easily ignore oviposition sites, which is a chronic problem when sampling for some species of mosquitoes. Culex species also lay their eggs in characteristic rafts that can break apart which can lead to discrepancies in sampling. It’s not exactly the best technique. I’d prefer to see them measure catches based off adult mosquitoes which can be done quite easily.

They found that more mosquitoes laid eggs outside the enclosures and in enclosures without bats than laid eggs inside the enclosures which had bats flying around. If there were bats, fewer mosquitoes laid eggs. The nets didn’t impede mosquito entry and the bats didn’t scare the mosquitoes off. The authors concluded that the bats were eating mosquitoes, which I think is a reasonable conclusion.

However, I’m still skeptical of the assertion that bats have impacts on mosquito populations just because they swipe the occasional mosquito. I say this because of how they conducted the experiment. Here’s what the authors said about the enclosures:

Enclosures. At each site, two 3-m3 enclosures were
built. The frames of the enclosures were made of
1.905-cm (0.75-in.) internal diameter polyvinyl chloride
pipe. The enclosures were screened on all sides
using 0.635-cm (0.25-in.) mesh landscape netting
(Landware Corporation, Reston, VA). To prevent escapes
by bats, seams were sealed with hot glue and
expanding, spray insulating foam (Great Stuff, Dow
Chemical Co., Midland, MI). Access to each enclosure
was via a 1-m2 zippered door made of heavy tarp
material. The two enclosures at each site were posi
tioned 50 m apart. The mesh size used in the enclosures
had been shown to allow mosquitoes (Culex
spp.) and other small insects access to the enclosure
while preventing little brown bats, a similar sizedspecies
to M. septentrionalis, from escaping (M.H.R.
and M.A.W., unpublished data). Each enclosure also
contained a small, shallow pan of fresh water for the
bats to drink. The enclosure that contained bats also
had a small wooden “bat box” to provide a comfortable
roost. The bats used these roosts habitually, which
allowed for ease in recapture by hand after each trial.

They used quarter inch mesh. This might seem like a minor gripe and not something which would throw the results of the experiment into question, but this is a much bigger deal than it seems. Although the mesh didn’t impede the passage of the mosquitoes into the enclosure, this mesh is small enough to prevent some of the medium size to large moths from entering the enclosure. While the mesh will let the small insects in, it still excludes the critters which I’m pretty sure the bats will preferentially feed on.

It’s good evidence that bats will feed on mosquitoes in a natural setting. However, I still doubt that they feed on them heavily enough to deplete their populations. I think that bats are an important part of the environment and that everyone should have a few bat houses on their property. I still think that they benefit farmers more than public health workers.

I should also mention that this paper listed an observational study where bats were observed eating mosquitoes. Unfortunately, I don’t have access to the paper so I can’t tell if they preferentially feeding on mosquitoes, feeding on mosquitoes that just happened to wonder into the barn or whatnot. So this is still an open question in my eyes.

Reiskind MH, & Wund MA (2009). Experimental assessment of the impacts of northern long-eared bats on ovipositing Culex (Diptera: Culicidae) mosquitoes. Journal of medical entomology, 46 (5), 1037-44 PMID: 19769034