New study finds mosquitoes don't just lay eggs in standing water

For years we've been told that standing water provides ideal conditions for the biggest bummer about summer — mosquitoes — to lay their eggs, but that's not quite the case.

A new study published in the Journal of Medical Entomology changes almost everything we know about mosquitoes, and this new knowledge has huge implications for how we manage the summer's worst enemy.

Why is it important to understand mosquito eggs?

The reality is that we don't have that much to worry about when it comes to mosquitoes all the way up here in Canada. Even if there are 50 to 60 different species of mosquitoes in any given region of Canada that can make our summers miserable, generally, the native species don't carry disease; they're simply a nuisance. 

However, in the rest of the world, mosquitoes are carriers of diseases including of course, malaria and Zika, but also West Nile, dengue and chikungunya. The fact that mosquitoes have such a huge impact on global health means that there is a desperate need to understand as much as possible about their ecology, physiology and behaviours.

What is the new information found in the research paper?

Most people know that wetlands and puddles and any water sources — even a rain bucket in your yard — can be a breeding ground for mosquitoes. That is well known and has been observed for a lot of different species, in particular a subgenus of a common North American variety called the Culex. 

Now, there are almost 800 species of Culex mosquitoes, so when we say it was commonly observed for some Culex species, that doesn't mean all of them, but that is what the so-called mosquito textbooks have always said: that all Culex mosquitoes lay their eggs in little rafts in any standing water they can find. 

Well, the textbook was wrong. Nathan Burkitt-Cadena from the University of Florida is the senior author on the new study and he really, really knows mosquitoes.

"We had collected some other Culex mosquitoes, which are very poorly understood, and we were trying to get them to reproduce in a colony in the laboratory, so we provided them with little dishes of water to lay their eggs in," said Burkitt-Cadena. "Well, they wouldn't lay their eggs. So, we put some other stuff inside the cages and inside the dishes where the water was, and lo and behold all of these mosquitoes lay their eggs out of the water. So we collected as many species of Culex as we could. We let them lay eggs in the laboratory, and we found that across the spectrum, these mosquitoes were behaving contrary to what the textbook tells us."

What does this change about how we manage and understand mosquito populations?

The scientists only looked at a subgenus of this massive family of mosquitoes, the Culex melanoconium. When they really started processing their results, they scoured the literature and now estimate that up to 60 per cent of the Culex family do not lay eggs on the surface of water. Rather, they lay their eggs near boggy, swampy, watery areas, on leaves or just about anything that is just above or just near the water. 

This has big implications for mosquito management. Currently, most cities in Canada occasionally spray to control mosquito populations. This is almost exclusively done in urban centres where mosquitoes don't really have a big role in any large ecosystem, but are a summer-killing annoyance.

Now that we understand more about where and when they lay their eggs, the targeted spraying — and more importantly the timing of the larvicidal spraying — can be done much more accurately.

One of the things this new work discovered was that because the eggs aren't laid directly on the water, the timing of the egg hatching is a bit more asynchronous because each clutch of egg will have slightly different access to water and that will control their hatch timing. So, if a mosquito controlling agent is applied that targets larvae, it must be timed with egg hatching and as early as possible in the larval life cycle to have the greatest effects.

This will change how mosquito control agencies look for and spray for controlling mosquito populations.

Does this work make that mosquito slaughter that much easier?

It turns out the chemicals that are normally used to control mosquito eggs and larva aren't chemicals at all but bacteria. Bacillus thurigiensis loves to infect mosquito larvae but does little other harm to ecosystems.

The thought of finding better ways to try and drive a species on this planet to extinction may worry some, and that's valid, so I asked Burkitt-Cadena what he thinks.

"There are species … which are not native to North America, and these mosquitoes cause the majority of our problems when it comes to nuisance and disease transmission," he said. "The southern house mosquito, for example, Culex quinquefasciatus: that mosquito is not thought to be native to North America. However, it is one of our most common mosquitoes. We create the habitat for that mosquito, with foul and polluted waters. So the idea of going out there and controlling that specific mosquito that doesn't bother me at all."

Contrary to how it may seem, the vast majority of mosquitoes native to Canada don't bother humans one little bit. They are wetland mosquitoes and feed on other animals. Those larvae supply a very important food source for entire food webs, so maybe they're not all bad after all.