Bringing down the mosquito

Nature's humble bloodsuckers could soon be eradicated if some researchers have their way. We must think long and hard about the consequences of this.

 
Not everybody is happy about sharing the planet with mosquitoes. Erik F. Brandsborg/Flickr (CC BY-SA 2.0)

Not everybody is happy about sharing the planet with mosquitoes. Erik F. Brandsborg/Flickr (CC BY-SA 2.0)

 

We’ve all been there before: You’re tucked up in bed, drifting off into a pleasant slumber, when you hear it.

EEEEeeeeEEEEeeeeEEEeee

That dreaded high-pitched whine. A mosquito. You have two choices: Either completely immerse yourself in a blanket fort to avoid the itchifying proboscis, or resign to hauling your sleepy self out of bed to spend a good 15 minutes blearily flailing until you whack the little bloodsucker.

But imagine if this never happened. Imagine a world without itchy bites and sleepless summer nights. Even better, a world without Zika, malaria, dengue fever or the myriad other mosquito-borne diseases that besiege humanity. It’s a tantalising thought. Around 700 million people contract a mosquito-borne disease every year, resulting in millions of deaths­ — and, with climate change potentially expanding the range of disease-carrying mosquitos, these statistics could get even worse. Knocking out killer mozzies would be the greatest boon in human health since the eradication of smallpox.

So, could we eradicate mosquitos if we wanted to? It’s certainly an outcome many scientists are striving for. Aside from the classic approach of spraying large quantities of insecticide, research has delved into alternative methods. Large-scale male sterilisation is one such technique: By genetically modifying males so that they cannot produce offspring, and then releasing them into the wild, we can potentially suppress mosquito populations. A similar approach involves the naturally-occurring Wolbachia bacteria, which affects an insect’s ability to produce offspring. Introducing males infected with Wolbachia into the wild could also control mosquito populations.

Insecticides such as dichlorodiphenyltrichloroethane (DDT) are also used for mosquito control, as seen in the US in 1958 (left) and in a bathroom in Tanzania (right). Public Health Image Library/Wikimedia Commons (public domain); Bjørn Christian Tørrissen/Wikimedia Commons (CC BY-SA 3.0)

Insecticides such as dichlorodiphenyltrichloroethane (DDT) are also used for mosquito control, as seen in the US in 1958 (left) and in a bathroom in Tanzania (right). Public Health Image Library/Wikimedia Commons (public domain); Bjørn Christian Tørrissen/Wikimedia Commons (CC BY-SA 3.0)

But perhaps a more pertinent question is should we eradicate mosquitoes? The benefits to human health would be astounding, for sure — but at what cost? Mosquitoes are as much a part of our ecosystems as any other organism. And as we’ve learnt over and over again, messing with nature’s order can have disastrous repercussions. From releasing cane toads in northern Australia to the ecological consequences of eradicating apex predators like wolves and sharks, our attempts to control ecosystems have inevitably backfired.

But mosquitoes are different, right? In this case, we’d be getting rid of a tiny, insignificant insect — not introducing or eliminating a predator or poisonous pest. Then again, nothing in nature is ever simple; as the classic John Muir adage observes, “When we try to pick out anything by itself, we find it hitched to everything else in the universe”. What about the birds, frogs, bats, reptiles and fish that eat mosquitoes in their larval or adult forms? Or plants that need pollinators, or the mosquito’s role as a waste-processor? A Nature article in 2010 explored this issue and concluded that the ramifications would be slight and temporary. Other itsy-bitsy insects would soon fill the vacant ecological niche as prey, filter-feeders or pollinators. According to entomologist Joe Conlon, quoted in the Nature article, “If we eradicated [mosquitoes] tomorrow, the ecosystems where they are active will hiccup and then get on with life”.

But it’s important to realise that there isn’t just a single mosquito species. There are about 3,500 described species worldwide, and around 300 in Australia alone. Of these, only a few actually bite people — and, even then, it’s only the female who nicks a few drops of your blood to supplement her egg-laying. Despite their much-maligned reputation, the world of mosquitoes is diverse and intriguing.

“Many mozzies are beautiful,” says mosquito expert Cameron Webb from the University of Sydney, “but more so, their fascinating ability to adapt to strange ecological niches always impresses me.” From alpine ranges to sunburnt deserts, and even wave-buffeted littoral rockpools, mosquitoes have found homes right across the diverse Australian landscape. One mosquito species, Aedes nivalis, survives beneath the ice and snow of the Snowy Mountains, emerging in spring when warmer weather creates snowmelt pool homes for them. Meanwhile, the Malaya mosquitoes of northern Australia have a modified mouthpiece that enables them to eat regurgitated gut juice directly from the mouth of an obliging ant.

 
The mosquito Aedes notoscriptus, a common resident of Australian backyards. © Stephen Doggett, NSW Health Pathology.

The mosquito Aedes notoscriptus, a common resident of Australian backyards. © Stephen Doggett, NSW Health Pathology.

 

For all our frenzied research into the human health impacts of mosquitoes, we know surprisingly little about their ecology. “After 20 years in this field, I’m still struck by the lack of ecological research,” says Webb. “There’s a massive gap in our scientific knowledge: We know how to kill them, we just don’t know how important they are”.

There have been just a handful of studies, and they’ve mostly found that Aussie mosquitoes aren’t much more than snack food for birds, bats, fish, frogs and other arthropods. Our knowledge of their pollinating role is even sparser. “Apart from a couple of orchids, not much is known about their role as pollinators,” says Webb. “As of yet, there aren’t any organisms in Australia that we know are completely reliant on mosquitoes”.

Overseas, there is some evidence that losing mosquitoes would have a negative effect — if only on a few esoteric species. The mosquitofish (Gambusia affinis), an American freshwater fish, has such a voracious appetite for mosquitoes that it has been introduced around the world as a form of biocontrol. Without mosquitoes, these fish would likely become extinct. (Admittedly, mosquitofish wouldn’t be missed in Australia, where they are a destructive pest.) Certain pitcher plants may also struggle to survive without the waste-processing services of resident mosquitoes that provide them with essential nutrients.

But Webb has an educated hunch that there’s still more to discover here in Australia. “I suspect one day we’ll learn that mosquitoes may play a subtle yet important role in the life cycle of a plant or animal,” he says. For example, we know that two small bat species, Vespadelus pumilus and Vespadelus vulturnus, eat mosquitoes in small amounts. Mosquitoes are like snack food compared to the bats’ preferred delicacy of big juicy moths, but just as humans binge on snack food, maybe bats do, too.

“Fattening up on junk food may be important at different life stages,” says Webb. “Maybe bats need more mozzie snacks when they’re feeding young or breeding.” It’s this kind of fine-grained complexity that makes pinning down the ecological importance of mosquitoes so tricky.

 
The little forest bat (Vespadelus vulturnus) is one species that could suffer in the absence of mosquitoes. Doug Beckers/Flickr (CC BY-SA 2.0)

The little forest bat (Vespadelus vulturnus) is one species that could suffer in the absence of mosquitoes. Doug Beckers/Flickr (CC BY-SA 2.0)

 

For this reason, Webb stresses that we need a careful and calculated approach to mosquito control. It isn’t just about blindly spraying insecticide or indiscriminately ripping out wetlands. By identifying specific problem species and monitoring them with a well-supported surveillance program, combined with community education, we can learn to live with mosquitoes — minus the disease burden.

Only around 40 of Australia’s mosquito species are known vectors for human disease, including dengue fever, Ross River virus, Barmah Forest virus and an assortment of other less common ailments.

But there’s one species in particular, says Webb, that we could safely eradicate to improve human health, without any environmental repercussions: the yellow fever mosquito, Aedes aegypti. This species is capable of spreading dengue fever, yellow fever, Zika virus and chikungunya, among others, plus its range overlaps substantially with human settlement areas. “Aedes aegypti has the greatest health impact in Australia, it breeds in urban water containers, is not particularly abundant and has low ecological significance,” says Webb. “I feel confident their eradication wouldn’t lead to significant ecological impacts and would improve human health.”

But for Webb, the issue doesn’t end there: “Economic and political sustainability are as important as ecological sustainability,” he explains. Large-scale release of sterile male mosquitoes would be expensive and operationally difficult, and would need to be continually performed as mosquitoes recolonise mosquito-free areas. It seems there’s a reason we still have to deal with mosquitoes and their associated troubles, after all: Eliminating mosquitoes is hard, and our best efforts to be rid of them continue to fall short.

Despite these criticisms, some biotech firms are blazing ahead with field trials. Oxitec, based at Oxford University in the UK, have performed several small-scale releases of sterile males in the Cayman Islands, Brazil and Panama with excellent short-term results. In these urban release areas, A. aegypti declined by up to 90%. However, Oxitec have faced challenges implementing their solution elsewhere: Plans to perform trials in Florida were met with swift and fierce public backlash against the 'Frankenmosquito.' While these initial results may seem promising, handling public perception and assessing long-term effectiveness remain significant hurdles.

The mosquito problem is a complex tangle of ethical, ecological, medical and economic questions that we’re only just beginning to unravel. But if you’ve absorbed anything from this foray into the world of mosquitoes, perhaps it’s a sense that, as a group of organisms, they are not ecologically obsolete. In fact, they’re ecologically fascinating. Mosquitoes are part of the diversity of life on earth — a diversity that we value, where each species has some intrinsic worth. So, while the eradication of certain species — like A. aegypti — would undoubtedly be a good thing health-wise, perhaps we shouldn’t hope for a completely mozzie-free summer night.

Edited by Andrew Katsis and Ellie Michaelides