Tiny, pesky and deadly, Aedes aegypti mosquitoes are super at spreading disease, including dengue, chikungunya and Zika virus. Yet all over the world, scientists, nonprofits and biotech companies are raising hordes of this species to release into the wild.
Why is that?
For decades people have relied on industrial pesticides to beat back mosquito populations and limit the diseases they spread. But with continued use, some pesticides lose their effectiveness as the bugs build up resistance.
This is a huge problem: The World Health Organization cites mosquitoes as one of the world's deadliest animals. Each year the diseases they disseminate kill millions of people, mostly in the developing world.
Since vaccines aren't readily available for most mosquito-borne illnesses — those developed for chikungunya, dengue and Zika virus are still in the trial stage — we need other tools to fight them.
So biologists are experimenting with new ways to curtail the diseases mosquitoes spread — with mosquitoes.
The idea is to introduce modified Aedes aegypti into wild populations. The lab-grown bugs can be sterile, which could eventually reduce the total number of mosquitoes. Or they can carry bacteria that limits mosquitoes' ability to spread disease.
The British company Oxitec is trying the first approach — a strategy called sterile insect technique. The firm raises mosquitoes en masse, genetically modifies them for sterility (though blasting the bugs with X-rays is another way to accomplish this), then introduces them to wild populations. When the sterile males mate with the wild females, no offspring are produced, diminishing the total number of mosquitoes.
The Australian nonprofit World Mosquito Program is taking a different tack, raising mosquitoes infected with Wolbachia bacteria. These bacteria occur naturally in many insects, but not in Aedes aegypti. Scientists have learned that infecting these mosquitoes with Wolbachia stymies their ability to pick up dengue and other viruses.
When Wolbachia-infected mosquitoes are released into the wild, they infiltrate natural populations and share the bacteria. Over time, this can reduce the number of bugs that carry disease. A recent trial showed that this approach can lower disease-transmission to humans.
Both of these strategies require hundreds of thousands — even millions — of mosquitoes to be bred and then released over months, or even years, depending on the project. But to raise a swarm of Aedes aegypti — also known as yellow fever mosquitoes (for one of the diseases they spread) — you need blood.
Male mosquitoes, which can live on nectar (if it's available), don't bite. But mama mosquitoes require blood to mature their eggs. Hence their desperation to bite us.
Much of the blood that's used for mass mosquito-rearing is drawn intravenously from farm animals — usually sheep, horses and other livestock. But blood doesn't last long, even in the fridge. Once a bottle is opened, it tends to spoil in about a week.
The makeup of blood also varies from batch to batch, with possible down-the-line effects on the eggs of imbibing females.
Biologist Immo Hansen and his team of researchers at New Mexico State University have come up with a solution: a diet for mosquitoes that would mean fewer animals having to be milked for their blood.
To produce a recipe that would satisfy their bloodthirsty subjects, Hansen's team tested different sources of nutrients over several years. The recipe they eventually came up with doesn't eliminate animal products altogether, but it's an improvement over blood-drawing.
The mosquitoes instead feed on protein and iron sources that are byproducts of slaughterhouses, abundantly available because of our own taste for meat.
Using these ingredients can also help research labs at universities that study mosquitoes move away from the practice of feeding them blood drawn from living, anesthetized rats, mice or chickens.
Female mosquitoes need a chemical lure to keep them eating until they've had their fill, because they need to engorge themselves in order to produce healthy eggs. But they tend to stop eating if they don't sense a compound that indicates they've found a good source of nutrition.
Mosquitoes find one of those tasty molecules — adenosine triphosphate, or ATP for short — in our blood. But ATP isn't terribly shelf-stable.
Hansen's team didn't find a substitute for ATP, but they did find that adding a little sugar and leaving the formula freeze-dried until it was used kept it enticing to mosquitoes for more than 80 days.
Hansen likens his team's recipe — called SkitoSnack — to astronaut food for mosquitoes. Using it in lab trials, the New Mexico State researchers were able to raise 20 generations of bugs on the artificial blood.
When they compared their mosquitoes to others raised on real blood, they found that the size, number of eggs produced and hatch rates were similar. The gut bacteria of the mosquitoes raised on SkitoSnack were somewhat different, though it's unclear if that would matter for disease deterrence.
Hansen and his colleagues published their results last month in the journal Scientific Reports.
So far it seems that mosquitoes reared from SkitoSnack females are as healthy as the progeny of blood-suckers. That's important because if the modified bugs are released into the wild as part of an insect-control program, they'll have to compete in order to mate.
Jason Pitts, a biologist who studies mosquitoes at Baylor University and is not affiliated with Hansen's study, says the long-term advantages — in terms of safety and handling issues, expenses and consistency — are clear. And, he adds, they're significant for anyone who's considering raising and releasing mosquitoes en masse.
Hansen agrees. "In order to control mosquitoes," he says, "we need more tools in our toolbox — and sterile insect technique is one of them." It's also "a green one ... you don't use any chemicals. All you do is release sterile male mosquitoes that don't bite anybody. All they do is fly around and look for females and mate with them."
That's the next test for SkitoSnack. Hansen's lab is drawing up a mating competition to see whether females will happily mate with SkitoSnack offspring.
If they do, the formula could eventually help researchers and mass-rearing projects avoid the whole bloody mess.
Carolyn M. Wilke is a freelance science writer based in Chicago. Find her on Twitter @CarolynMWilke.