As soon as the fungus spore sticks to the mosquito larva, the insect’s life is finished. The fungus eats its way through the exoskeleton and starts to grow, fast. The larva isn’t helping itself: It’s gulping down still more spores, which work their way through its gut and into its body cavity. The fungus grows and grows and grows, tearing up the larva’s insides. The mosquito is donezo.
It almost makes you feel sorry for the baby skeeter–almost. This is the bizarre saga of the fungus Metarhizium brunneum and its victim: the dreaded Aedes aegypti, carrier of yellow fever, dengue, and Zika. New research out today in PLOS Pathogens reveals just how brutal and effective this fungus’ methods are, and how scientists might deploy it to cull an increasingly deadly menace.
The fungus actually attacks mosquitoes in two ways. One variety of the fungus spore, the conidium, is airborne–it attacks adult mosquitoes. The blastospore, though, does better underwater–that’s the one that attacks the larvae, and it’s nasty. But until now, scientists didn’t know why the blastospore is so much more virulent that the conidium.
The secret seems to be the blastospore’s two-pronged attack. “The fungus adheres to and penetrates the cuticle and also germinates and breaches the gut following ingestion of the blastospores,” says biologist Tariq Butt, one of the study’s authors. “So the multiple entry routes facilitate rapid death.” And because the blastospore has drilled a hole in the cuticle, water can seep in. That’s a big problem for an insect that relies on its exoskeleton to shield itself from the big, nasty world.
More remarkable still, the fungus is somehow able to dodge the mosquito’s immune response without shutting it down. The larva continues to fight off other attackers, giving the fungus a monopoly on torturing its host. Then, when the fungus grows large enough, it switches on its own antimicrobials as an extra defense against other marauding parasites. No tasty larvae for them.
Clearly this fungus is bad news for Aedes aegypti. Enough so that scientists are trying to weaponize it in the battle to control the mosquitoes spreading Zika and dengue.
Compared to other mosquito control methods, fungi are a little finicky. You can’t just apply them willy-nilly to ponds; that’s too expensive, since fungus spores have short shelf lives and cultivating them takes a lot of work. “You’ve had issues with producing them, formulations tended to mess them up and things of that nature,” says Joseph Conlon, a technical adviser at the American Mosquito Control Association. Instead, scientists could use oviposition attractants, luring females to lay eggs where less fungus can kill more mosquitoes.
But an imperfect weapon is still a valuable one in the fight against Aedes aegypti. “We’re always looking for new ways of controlling things,” says Conlon, “for no other reason than for potential resistance.” Mosquitoes are now developing resistance to pesticides, but it’s harder to resist predators and parasites that are evolving right along with them. Metarhizium brunneum could be a crucial part of the arsenal–as long as it doesn’t spread so widely that it starts killing more than mosquitoes.
Should the fungus see wide deployment, it’ll be right alongside any number of other mosquito control methods tailored for a given environment–think mosquitoes genetically modified to resist malaria and traps far cleverer than bug zappers. No one method can do all the lifting. “The more we’ve got, the more we like it, and fungi will definitely play a part in it,” says Conlon.
Feel sorry for the blood-suckers yet? Yeah, didn’t think so.