Commentary: So many mosquitoes! Is a lack of bats to blame for abundance in Minnesota?

A 2009 study by a pair of graduate students in Michigan may help to explain why everyone in a large swath of Minnesota seems to be complaining about the clouds of mosquitoes right now.

Anyone who lives here in the north country, especially those of us who live in the woods, knows that early June is generally the peak of the mosquito season. They’re a fact of life, but I know I’m not the only one who has been surprised the past few years by what seems to be a particularly robust – occasionally shock-and awe-inspiring – mosquito population.

While recently searching this topic online, I came across a study by what were then two graduate students – one pursuing entomology, the other evolutionary biology. They were interested in determining whether bats – in this case, northern long-eared bats – could noticeably impact mosquito populations. They tested this hypothesis in an enclosure in a Michigan forest, in which they housed some bats and some mosquitoes, along with some suitable mosquito breeding habitat.

They also set up a variety of controls, and at the end of the study, they compared the number of mosquito egg clutches laid by mosquitoes within enclosures that included bats, compared to enclosures that did not include bats.

Not surprisingly, the researchers found that the number of mosquito egg clutches in the enclosures that included bats were 32% lower. The researchers readily acknowledged that the study, which concentrated both bats and mosquitoes, might have exaggerated the effects of the bats’ presence, but it definitely provided objective data that bats are a significant factor in controlling existing mosquito populations as well as the number of eggs mosquitoes lay for future generations.

Keep in mind, this was a 2009 study undertaken at a time when northern long-eared bats were still very common in the forests of our region in summer. Indeed, this was the bat species that we were most likely to see out and about in the late evenings, swooping around our cabins and lake houses, munching on mosquitoes and other night-flying insects.

This, of course, was before white-nose syndrome virtually wiped out the northern long-eared bat, which is now on the endangered species list (since November 2022) and possibly headed for extinction.

The loss of this species alone is probably significant in terms of our mosquito population. Every year, we experience the spring mosquito hatch and every year, at least in the past, the bats would start whittling away at them almost immediately. Mosquito numbers would start moderating by mid-July, and in a good year, they were hardly noticeable by August.

That has since changed. In my experience, the spring hatch is now even larger than in the recent past, most likely because more mosquitoes survived to the egg-laying stage the previous year. That means there’s simply more mosquito potential on the landscape, just waiting for the aftermath of the snowmelt to spark a new and even worse explosion of the pests than the year before.

And, as that population continues to build, year after year, without experiencing the controlling influence of bats, the situation continues to worsen.

Unfortunately, there is little reason to expect that our mosquito population won’t continue to worsen in the years to come. Each year, a larger population base is able to lay more eggs and those eggs provide the potential for further population growth the following year.

It’s worth considering that a mosquito population that grows 10% a year doubles every seven years. Imagine twice as many mosquitoes out there by 2030 as we have today.

On second thought, it’s probably better that you don’t.

Helmberger is publisher/managing editor of The Timberjay, of Ely.