If you think slavery has been abolished, consider the case of the gypsy moth and the virus. For more than 100 years, people have noticed that some gypsy moth caterpillars climb to the top of trees before they die and decompose, or “melt.”
Melting releases more virus particles and is the normal fate of these caterpillars, but why did only some caterpillars perform this ascending death march?
Gypsy moths are voracious insects that have been spreading across the United States for a more than a century, so nobody is feeling too sorry for them, especially people who have seen them strip forests bare.
Still, it’s nice to read a good explanation for this peculiar “climb, croak, melt” behavior.
All the better to infect you with, my dear!
A study published today identifies a viral gene that blocks one stage of maturation in gypsy moth caterpillars, which normally hide during the day. But when Kelli Hoover, a professor of entomology at Penn State, and her colleagues infected bottled caterpillars with the virus of doom, the caterpillars showed the same climbing ‘n’ dying behavior that appears in the field.
In nature, those caterpillars would melt and then rain virus down to infect other gypsy moths.
The moth misbegotten
Gypsy moths were introduced to Massachusetts in the late 1800s by a bumbler who wanted to raise silk by crossbreeding them with silkworms — a different species, says Hoover. “It was crazy; this guy did not know anything about species, apparently.”
Still, the gypsy moths did bring fecundity and a ferocious appetite to the table — or forest. “They eat so many different kinds of trees and plants … in a bad outbreak, the insect frass dropping down sounds like rain, so you need a hat,” Hoover says.
We had to look it up to be sure, but frass is basically insect poop.
Gypsy moths are such effective defoliators that authorities try to control them with Bt, a bacterial spray that unfortunately kills beneficial insects, not just harmful ones.
Hoover’s study focused on a viral gene called egt, which inactivates a hormone that starts molting – a process that ends each stage, or “instar,” of the caterpillar’s development. “When they stop molting, they keep feeding, and that’s why we looked at egt,” Hoover says.
The study compared the behavioral effects of:
two normal strains of virus;
two strains with a busted egt gene, and
two strains with a restored egt gene.
A dangerous meal
In every case, Hoover says, “if the gene was active, the moth died at the top of the bottle. If the gene was inactivated, it died at the bottom.”
It’s not clear, Hoover says, exactly why the gene changes behavior, but this is the first time it was traced to a single gene.
Because LdMNPV (the Lymantria dispar nucleopolyhedrovirus) infects only gypsy moths, and kill them at a young age, it might work as a biocontrol agent against a disastrous insect invasion. However, Hoover says, “the experiment’s goal was more basic – to understand how the virus enslaves its host.”
Certainly there is evolutionary logic behind changing your host’s behavior for your own benefit, assuming you are a pathogen or parasite, and “body-snatching” is well-known. For example, a fungus forces ants to climb, zombie-like, and die where they can easily spread fungal spores.
And it’s not just insects. The rabies virus, Hoover adds, “causes dogs, raccoons and bats to become more aggressive, to be out during the day, where they approach people and try to bite them,” which spreads the virus even though it endangers the animal.
And toxoplasmosis, a parasite, can make mice less fearful of cats, Hoover says, “so they are more likely to get eaten and infect the cat.”
There is even speculation that toxoplasmosis may cause men to behave with greater jealousy, Hoover says, “but the only thing that’s really been looked at is that mice with toxoplasmosis have a higher level of dopamine,” a feel-good neurotransmitter.
Is slavery therefore not all drudgery?
— David J. Tenenbaum
- A Gene for an Extended Phenotype, Kelli Hoover et al, Science 9 Sept. 2011. ↩
- A guide to the gypsy moth. ↩
- Alien profile (for kids!) ↩
- Gypsy moth fact sheets, regulation and management. ↩
- Zombie viruses. ↩
- Zombie ants. ↩
- Podcast: Toxoplasmosis and rat behavior. ↩
- Toxoplasmosis and human behavior. ↩
- How does Bt kill? ↩
- All about Bt. ↩