Deadly fungal spores stab holes in Zika larvae

Fungi launch assassin-like spores that attack and kill mosquito larvae – and could help control the spread of mosquito-borne viruses which currently do not have vaccines, such as dengue and Zika. 

Tariq Butt from Swansea University in the UK and colleagues found a particular type of spore produced by the fungus Metarhizium brunneum acts as a natural pesticide by latching onto and killing baby mosquitoes in a matter of hours.

Finding a natural biological pesticide for Aedes mosquitos – the culprits that carry dengue, Zika and Chikungunya viruses – has been on scientists’ agenda for some time. Chemical pesticides, including the mosquito repellent we slap on our skin, are harmful to the environment. Some mosquitos are building up tolerance too.

A family of promising insecticides are pathogenic fungi, such as M. brunneum. Their spores can pick off mosquito adults as well as larvae – the caterpillar-like “wrigglers” found in stagnant water.

M. brunneum produces two types of spores: conidia on solid surfaces and blastospores in liquids. Blastospores, Butt and colleagues knew, were more efficient killers than conidia, but why – no one knew.

So they decided to explore and exploit the blastospores’ lethal attack techniques in the hope they could be harnessed in the fight against Aedes mosquitoes.

They found some blastospores were swallowed by larvae, giving them access to the gut and haemocoel – the larval equivalent of a bloodstream.

But thanks to a gummy, water-insoluble mucous sheath, blastospores could also easily latch on the cuticle, or outer surface, of mosquito larvae. Once firmly attached, the blastospore forced its way through. 

A combination of multiple penetration points and extensive damage to the gut played into the larvae’s rapid demise.

And since blastospores flourish in aquatic environments, they are the perfect assassins in humid areas where mosquitos thrive.

Conidia, on the other hand, lacked the sticky mucous layer so were less able to attach to the larval cuticle. But they could still penetrate, albeit slower, because they used enzymes to break down cuticle proteins rather than a mechanical infiltration, like blastospores.