Saturday, 10 February 2018

Microbats eat 1,200 mosquitoes per hour? No they don't.

Microbats in Australia don’t eat up to 1,200 mosquitoes in an hour.  This is a common myth that has been spread for far too long by people who should know better.  Like many myths, this one is well meaning but does more harm than good.

Microbats are great, their habitat is shrinking and I think that more people should build and install bat boxes and other artificial shelters for them, I even found some nice free plans here to build one and would like to encourage you to build a few. 

Far too often people are taken in by this microbats consuming huge numbers of mosquitoes nonsense, they build a bat box, then when it doesn’t reduce the number of mosquitoes at their BBQ they tell people not to bother building bat boxes.  This is where the damage is done.

Let’s look at the reality of microbats and you can decide if you still want to build a bat box.  Build them because microbats are great, not because you have misunderstood these creatures and expect them to do something that is impossible for them to do.  I think if you understand the reality of these lovely little animals that you will be just as likely to build a roosting box for them but will be far less likely to convince others not to build them.

In Australia we have less than 90 species of microbat, there is some controversy over the actual number.  Depending on your location there may be 1 or 2 that are native to your region, or there may be 30.  Australia is a big country, not surprisingly it has a lot of diversity in habitat, vegetation and wildlife.  Not surprisingly, different species of Australian microbats have different diets.

Most species of microbat in Australia don’t eat mosquitoes at all.  Several species of microbat that are native to Australia can and do eat some mosquitoes, they just don’t eat them very often and when they do they don’t eat many of them. 

Very few species of microbats in Australia actually eat mosquitoes, most species only eat larger things such as moths, beetles and spiders.  Of the few species of microbat in Australia that ever eat mosquitoes, on any given night the majority of individuals will not consume any mosquitoes at all.  Of the individual microbats who do eat mosquitoes on a given night, the vast majority of their diet will usually be made of moths or beetles, and mosquitoes will only ever be a very small percentage. 
Eastern bentwing bat - picture from Department of Environment and Heritage
We know what microbats eat from a few studies conducted to figure out what microbats in Australia are eating.  If you are curious how they tell what a bat eats, they capture some from the bush and do a DNA analysis of the bat poo.  This test can detect tiny amounts of insects even in a small sample and completely removes all guess work and conjecture.  This test has also disproven the myth that microbats eat thousands of mosquitoes each and every hour. 

Most studies that I have read indicate that very few species of microbat ever eat mosquitoes, but all species of microbat in Australia eat a lot of moths and beetles.  In one study in a QLD grain growing region it was found that 100% of the diet of microbats was grain weevils (weevils are a type of beetle).  This alone disproves the microbats making a noticeable difference to the local mosquito population myth.  If you have ever been to grain growing regions you will notice that there are a lot of mosquitoes, so microbats could eat them if they wanted.  The microbats just prefer to catch larger, slower , more nutritious meals.  It doesn't mean that you shouldn't still build a nice box for bats to sleep in, just that they don't eat significant amounts of mosquitoes.

So far I am yet to find any research where mosquitoes made up a large percentage of a microbat’s diet anywhere in Australia.  If you are a bat researcher or know of any study that indicates otherwise please let me know as I would love to read it.  This has to be a peer reviewed study, not notes someone wrote after claiming to read a study, or some unverifiable and unrepeated observation written for BATS magazine that has been misunderstood and plagiarised by milkwood.

Ever wonder where the microbats eating 1,200 mosquitoes an hour myth first came from?  The ridiculously high numbers of mosquitoes potentially being consumed was extrapolated from a student in Sweden who reported once observing a single bat successfully capture “up to 20 mosquitoes in a minute” using a stopwatch.  One minute, not an hour, not averaged over an entire night, just one minute with a stopwatch in the field and a best guess.  Have you ever watched microbats feeding?  This method is far from accurate.

It gets worse, the field observations were made in northern Sweden during the summer where the sun only dips below the horizon for 90 minutes per night.  During this time the larger, slower, more nutritious insects were not common.  That means that even if this number was accurate it would not be transferable to Australian microbats.

There is no point having a two dimensional simplistic view of the world.  In Australia we have different species of microbats than they have in Sweden, many larger and easier to catch food insects are present here, and our nights are far longer.  All of this means that even if it were true, this unrealistically high number of mosquitoes being consumed is not applicable in Australia. 

Mosquitoes are small and fast and require a lot of energy to catch.  Due to their tiny size a mosquito provides very little return on this large investment.  It is easier to catch fewer but larger food items and have some time for rest, drinking water, searching for roosting sites, courtship etc.  Catching a mosquito every three seconds for hour after hour when there are larger slower food items around just doesn’t happen, it is illogical to ever think that it would.  So please don't be taken in by it.

In Australia, mosquitoes are more of a convenient treat for some microbats rather than a staple food, microbats much prefer to eat moths or beetles.  Notice how I keep talking about microbats in Australia?  I don’t care what happens in other parts of the world, me building a bat box only effects microbats that I may encounter here in Australia.

Why bother encouraging microbats if they don’t eat many mosquitoes?  To put it bluntly, I attract many animals to my yard that don’t eat mosquitoes as they have other benefits, why wouldn't I do the same for microbats.  Many people buy insect zappers with those blue lights, they don’t attract mosquitoes yet it is a thriving industry.  Lowering the numbers of moths and beetles is fantastic and well worth encouraging microbats into your garden.  The microbats don’t just eat the insects, they also disrupt mating which reduces the number of insect eggs laid which has an even larger effect on pest populations. 

Don’t ever be so gullible that you are fooled into thinking that microbats only eat pests, they also eat many beneficial arthropods.  This is the same as spiders and birds eat pests but they also eat beneficial insects.  This is nature, there is no way around it, no use in pretending it isn’t happening.

Strangely very few studies have been done to see how much of a difference microbats make in protecting large scale crops from pests.  I have heard of a few studies that claim microbats significantly lower damage from coddling moth in walnuts, helicoverpa moth in cotton fields, and grain weevils in grain crops, but would love to read more studies.  Again I stress that microbats are great to have around, they just won't make a noticeable difference to the local mosquito population in your back yard.

I have often wondered if encouraging microbats to live near bee hives would reduce the damage from wax moth.  Would running poultry under the hives during the day and encouraging microbats at night reduce pests significantly?  Unfortunately I can find no research (or even anecdotal evidence) that has tested this.  I guess people are too caught up in thinking microbats only eat thousands of mosquitoes per hour that they can’t think of ways to utilise them to actually reduce pests.

Let me stress that I want to encourage people to build and install bat boxes.  Just remember that microbats are great to have around, but they won’t make a noticeable difference to the mosquito population in your area.

2 comments:

  1. Thanks for that. I'm embarrassed to say I had been taken in my the mythology of microbes eating huge numbers of mosquitoes, but i think I would have blinked at 1200, because I do have some rudimentary knowledge of maths.

    My question is, what do you know about the positioning of a bat box. My one, bought from LaTrobe Uni shop, has been up for years and only spiders and insects live in it.

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    Replies
    1. Hi Parlance,

      The bat box needs to be positioned where the temperature will not vary too much. In Australia keeping on the south or east side tends to help. Keeping it up high tends to help them feel safer and be more likely to use it.

      Microbats won't use the same roost all the time. They tend to use it for a few nights and then move on to another roost within their range. The number of nights and how often they will come back to the same roost varies considerably. Interestingly, the research suggests that if they have access to bat boxes they tend to use it for more nights than they would natural roost. I am not sure why.

      Since writing this I have read some research that suggests that if certain species live in mangroves they will eat more mosquitoes than they would if they lived in woodland. It is assumed that this is due to a lack of larger and slower prey in the mangrove ecosystem.

      Microbats sure are fascinating little creatures.

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