A Virus That Affects Caterpillar Behavior When infected by a virus known as baculovirus, gypsy moth caterpillars mysteriously climb for the treetops. They then die and scatter viral particles, infecting their comrades below. Entomologist Kelli Hoover discusses the insects' behavioral change and the benefits to the virus.

A Virus That Affects Caterpillar Behavior

A Virus That Affects Caterpillar Behavior

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When infected by a virus known as baculovirus, gypsy moth caterpillars mysteriously climb for the treetops. They then die and scatter viral particles, infecting their comrades below. Entomologist Kelli Hoover discusses the insects' behavioral change and the benefits to the virus.


This is SCIENCE FRIDAY. I'm Ira Flatow. Imagine coming down with a bad case of the flu, but instead of feeling exhausted, you want to go climb a tree. Well, that's what happens if you're a gypsy moth caterpillar when you're infected by a baculovirus. The caterpillars leave their hiding places, they climb for the treetops in broad daylight, and what happens next ain't very pretty.

My guest has been studying virus-infected insect interactions for years and has now found a viral gene that controls the behavior in the caterpillar and sends it climbing the tree, and she has just published her findings in the journal Science.

Kelli Hoover is a professor in the Department of Entomology in the Center for Chemical Ecology at Penn State University. Welcome back to SCIENCE FRIDAY, Kelli.

KELLI HOOVER: Oh, it's nice to talk to you again.

FLATOW: What happens at the top of the tree there?

HOOVER: Well, infected caterpillars stay up there rather than climbing down during the day to hide from predators. So the gene that this virus expresses, which is called EGT, causes them to stay in a feeding state by inactivating the hormone that causes them to molt.

So normally when gypsy moths molt, or any insect molts, they stop feeding for quite a long period of time. And in the presence of this gene, the molt is blocked, and so the insect, instead of stopping feeding for a long time, keeps feeding and is therefore found up in the tree in the middle of the day when you normally wouldn't find them.

So if you see a gypsy moth caterpillar out on leaves during the day, they're almost always infected.

FLATOW: But you didn't tell us what happens to them, you know, the final end of the catepillar.


HOOVER: Well, the poor thing ends up dying up there and is converted to a sac of virus that then melts or liquefies and rains virus particles down on the foliage below so that new hosts can be infected by eating the virus on the leaves.

FLATOW: It sounds like something out of "Ghostbusters" or something, you know, the slime that... It slimes the tree.

HOOVER: It's very gross.



FLATOW: And - but that's how the virus then spreads itself around.

HOOVER: Yeah, so it facilitates the virus being able to infect new hosts by having its, you know, its sack of virus higher up in the tree so that when it rains down, it's able to hit the leaves and infect new hosts.

FLATOW: It sounds like it's a very cruel thing for a virus.


HOOVER: It is kind of cruel. It's - I don't know. I can't even think of a virus that infects mammals that does the same kind of thing in terms of turning the host into a bag of goo.



FLATOW: Well, so is it a special kind of gypsy moth, or is it every gypsy moth that...

HOOVER: All gypsy moths are susceptible to this virus. It's a naturally occurring virus. You find it out in the field all the time. It's at low levels in the population until there's a population explosion, and then what can sometimes happen is the population will collapse because tons of insects get infected with this virus.

FLATOW: You know, we really don't like gypsy moths because they come, and they take over the trees and eat the leaves and things, right.

HOOVER: Yeah, and they can defoliate thousands of acres of forest so that it looks like, you know, winter when it's the middle of summer.

FLATOW: Doesn't this seem to - well, I'm thinking about, I'm saying hey, here's a gypsy-moth, anti-gypsy-moth thing. Could we not, you know, learn how to maybe genetically engineer this virus and get rid of the gypsy moths?

HOOVER: Well, we've been looking into that, actually. There - the gypsy moth does have an immune response to this virus, and it can fight it off if it's at the right age, and if the dose isn't high enough, if it doesn't ingest enough virus to kill it.

And so we've been looking into ways to suppress the immune system so that doesn't happen.

FLATOW: And normally gypsy moths would just climb to the top of the tree and molt up there?

HOOVER: Yeah, they'll molt, and then they'll, you know, feed on the leaves, but certainly at night they're out on the leaves, but during the day you'll see them, you know, in the morning they're climbing back down to hide in the bark crevices, or if they're bigger caterpillars, they'll go all the way down to the soil to hide.

FLATOW: You know, this sounds a little bit like rabies. You know, there are rabid raccoons and things that are supposed to be sleeping during the day, and they're out during the day, and you know they may be infected. This sounds something like that.

HOOVER: It certainly does. Because you're right, what happens with rabies infection is that these animals will, as you said they're nocturnal, but they'll be out at the wrong time. They'll be out during the day. And they get more aggressive. They'll approach, you know, another animal and try to bite them.

FLATOW: And do viruses - as you were intimating before, do you know other animals that are affected, the behavior is affected by viruses?

HOOVER: Oh certainly. There are - well, there's quite a few viruses that can - and other parasites and pathogens that can manipulate behavior. But, you know, the thing that comes to mind that I think most people might be familiar with is toxoplasmosis, which is - it's not a virus, it's a protozoan, but you know how they say pregnant women shouldn't clean their litter boxes, they should get somebody else to do it, and that's because they can pick this up from the litter box.

Cats get infected with this, and they often will get it from mice that they eat. So mice infected with this lose their innate fear of cats, and they're more likely to get eaten. So their behavior does change. And it's even been speculated that people infected with toxoplasmosis that their behavior can be altered as well.

That's sort of, I would say, controversial, but it's certainly been suggested.

FLATOW: Are there any other viruses that might affect human behavior?

HOOVER: There might be, but I - you know, I can't think of one that has similar types of affects on its host.

FLATOW: And so is that what you do, you study viral genes?

HOOVER: Yup, I do. I've been working on looking at different genes in this particular virus and how it interacts with the gypsy moth for quite a few years. And it's a pretty interesting system.

FLATOW: Let's get a question or two from our listeners. Derrick(ph) in Grand Rapids, hi Derrick.

DERRICK: Hi, Ira, how are you?

FLATOW: Hi there, how are you?

DERRICK: Good. I'm just finishing up "The Origin of the Species," and this evolutionary link kind of made me wonder how the virus evolved to manipulate the behavior of the insect. And isn't there a - there's one that affects the ant, too, where it climbs to the top of the grass.

HOOVER: Right, zombie ants infected with fungus, yeah.

DERRICK: Okay, right, right, so..

HOOVER: Go ahead.

DERRICK: Yeah, just wondering how it actually evolved to get the behavior it wants.

HOOVER: Well, it's thought that this gene probably originated in an insect because there are genes that are similar to this that are found in insects that have kind of a similar function, but they don't necessarily influence behavior. They do similar types of chemistry.

But I would think what would happen is, you know, if you've got different strains of the same virus species, let's say, and one of them acquires this gene, and it gives them an advantage because it keeps your host from molting, it keeps it feeding and also causes it to die in a position that allows you to enhance transmission to new victims, then you have a selective advantage. So that would be selected on and might then spread not only to within your own strain, but it could end up being a gene that you end up then seeing in other viruses, as well.

In fact, most baculoviruses have this gene, at least the ones that are in this group.

DERRICK: So it perpetuates the virus?

HOOVER: Yeah, it enhances the amount of virus you can get out of a given caterpillar because by blocking it from molting and keeping it feeding, you get a bigger caterpillar, so you get more virus out of a given insect.

FLATOW: Derrick mentioned the zombie ant. I can't let that pass.


FLATOW: Without telling us about it.

HOOVER: Well, in fact, one of the co-authors on this paper, David Hughes, is the guy who works on zombie ants. And his work has shown that when ants are infected with this cordyceps fungus that the fungus induces them to climb up the tree just to the right location so the humidity and temperature is perfect for when the ant dies, it - the fungus not only starts growing out of the ant's head, which is really strange in time-lapse photography, but then it sporulates and rains spores down on unsuspecting ants below to infect them.

The really cool thing, as well, is the fungus causes the ant jaws to clamp down on the vein of a leaf. It's very hard to pry them loose. And that way, you keep the ant in that location for a long time because it takes the fungus several days, you know, to grow out and sporulate. It's pretty gross.


FLATOW: There's a screenwriter here listening, I'm sure.

HOOVER: Well, you know, I think it's really creepy for people to think about a parasite or pathogen could manipulate your behavior. It's a little creepy.

FLATOW: You didn't see "The Tingler" back years - in the '50s, where...

HOOVER: No, I guess I didn't.

FLATOW: "The Invasion of the Body Snatchers."

HOOVER: Oh, I've seen that, yeah, yeah.


HOOVER: But that's not real.


FLATOW: Oh, I forgot.


HOOVER: Yeah, but you could turn this in - maybe that's how they got the idea, you know, looking at science.

FLATOW: So as you say, you don't know of a whole lot of other viruses that might influence behavior or microbes, but they might be, and we may not know it, there might be other ones? You've got to think we don't know all of them, right?

HOOVER: Right, and there are actually quite a few that are - different parasites and pathogens that do this. You know, there's this organism that - a pathogen or I guess you'd call it a parasite that it infects snails and causes them to stay not only up on the leaves, but it changes their - it causes their tentacles to pulsate and, like, advertise their presence to birds so they can then get eaten by a bird and infect the bird. It's really a trip.

FLATOW: Wow, well, if a bird eats this caterpillar with the virus, will it spread it that way?

HOOVER: Yes it will because the virus remains viable when it passes through predators like birds. So if they're out on the leaves in the day, and they're advertising that they're there, the bird will eat them, and as they drop their droppings around on plants, that will spread the virus, as well.

FLATOW: Well, got to get them before they turn to slime.

HOOVER: Exactly.

FLATOW: Kelli, this has been delightful. Thank you for - and, you know, it gives us something else to think about this weekend.

HOOVER: Well, you're very welcome.

FLATOW: Kelli Hoover is a professor in the Department of Entomology at the Center for Chemical Ecology at Penn State University. We'll see you again. Have a good weekend.

HOOVER: You, too. Bye now.

FLATOW: We're going to take a break, and when we come back, we're going to change gears and talk about a renewed finding of fossils, very interesting finding of fossilized bones that might force us to re-evaluate human evolution. Stay with us. We'll be right back.

I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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