Viruses can kill wasp larvae that grow inside infected caterpillars

When parasitic wasps come calling, some caterpillars have a surprising ally: a viral infection. 

Insects called parasitoid wasps lay their eggs inside young moth larvae, turning the caterpillars into unwitting, destined-to-die incubators for possibly hundreds of wasp offspring. That’s bad news for viruses trying to use the caterpillars as replication factories. For the caterpillars, viral infections can be lethal, but their chances of survival are probably higher than if wasps choose them as a living nursery.

Now, a study shows how certain viruses can help caterpillars stymie parasitoid wasps. A group of proteins dubbed parasitoid killing factor, or PKF, that are found in some insect viruses are incredibly toxic to young parasitoid wasps, researchers report in the July 30 Science.

The new finding shows that viruses and caterpillars can come together to fight off a common wasp enemy, says study coauthor Madoka Nakai, an insect virologist at Tokyo University of Agriculture and Technology. A parasitoid wasp would kill a host that the virus needs to survive, so the virus fights for its home. “It’s very clever,” Nakai says.

What’s more, some moth caterpillars make the wasp-killing proteins themselves, the team found. It’s possible that in the distant past, a few moths survived a viral infection and “got some presents” in the form of genetic instructions for how to make the proteins, says study coauthor Salvador Herrero, an insect pathologist and geneticist at the University of Valencia in Spain. Those insects could have then passed the ability down to offspring. In this case, “what doesn’t kill you makes you stronger,” Herrero says.

Previous studies had shown that viruses and insects, including moths, can swap genes with each other. The new finding is one of the latest examples of this activity, says Michael Strand, an entomologist at the University of Georgia in Athens who was not involved in the work.

“Parasite-host relationships are very specialized,” he says. “Factors like [PKF] are probably important in defining which hosts can be used by which parasites.” But whether caterpillars stole the genetic instructions for the proteins from viruses or if viruses originally stole the instructions from another host remains unclear, Strand says.  

Researchers discovered in the 1970s that virus-infected caterpillars could kill parasitoid wasp larvae using an unknown viral protein. In the new study, Herrero and colleagues identified PKF as wasp-killing proteins. The team infected moth caterpillars with one of three insect viruses that carry the genetic blueprints to make the proteins. Then the researchers either allowed wasps to lay their eggs in the caterpillars or exposed wasp larvae to hemolymph — the insect equivalent of blood — from infected caterpillars.  

Virus-infected caterpillars were poor hosts of the parasitoid wasp Cotesia kariyai; most young wasps died before they had the chance to emerge from the caterpillars into the world. Hemolymph from infected caterpillars was also an efficient killer of wasp larvae, typically destroying more than 90 percent of offspring.

C. kariyai wasp larvae also didn’t survive in caterpillars, including the beet armyworm (Spodoptera exigua), that make their own PKF. When the researchers blocked the genes for the proteins in these caterpillars, the wasps lived, a sign that the proteins are key for the caterpillars’ defenses.

Some parasitoid wasps, including Meteorus pulchricornis, weren’t affected by PKF from the viruses and also beet armyworms, allowing the wasp offspring to thrive inside caterpillars. That finding suggests that the wasp-fighting ability is species-specific, says Elisabeth Herniou, an insect virologist at CNRS and the University of Tours in France who was not involved in the work. Pinpointing why some wasps aren’t susceptible could reveal the details of a long-held evolutionary battle between all three types of organisms.  

The study highlights that “single genes can interfere with the outcome of [these] interactions,” Herniou says. “One virus may have this gene and the other virus doesn’t have it,” and that can change what happens when virus, caterpillar and parasitoid all collide.