Walking Dead: zombie beetle edition
Beetles infected with a fungus behave oddly, all in the service of spreading the fungus to healthy beetles.
The fungus leads the dying beetle to struggle up to the top of a flowerhead where the bug dies.
And as Dr. Donald Steinkraus from the University of Arkansas tells Bob, at night, the zombie beetle spreads its wings and takes on what looks like a mating position, ready for some heavy petting.
This interview has been edited for length and clarity.
Bob McDonald: Paint me a picture of what you saw when these beetles are infected with the fungus.
Dr. Donald Steinkraus: As an entomologist and a naturalist, I'm always just sort of aware of what insects are doing. So if you walk by a patch of flowers in the fall where are these beetles are active, you'll sometimes see these ones that are very dramatically spread. Their wings are wide open. Their bodies are enlarged with this fungus. And they're holding on extremely tightly just by their mandibles.
BM: So you're saying that the fungus makes the beetles climb the plant and hold onto the flower at the top. And by their mandibles, you mean their mouthparts, right?
DS: Yeah. The fungus actually does make the beetles go up to the flowers and die on the flowers. So they're not just dying randomly all over the landscape. They're actually being forced to go up to where the other beetles are and die there.
BM: So the corpse is hanging on the flower, in other words….
DS: Correct. Yes. And that's kind of unusual. A lot of these fungi and other groups of fungi do make the hosts do things, but this is very unique, but not totally unique. But it's very unusual that just the mandibles what holds it on. And they are really forcefully holding on. It's like rigor mortis. I mean those mandibles are in the plant and those beetles are not going to fall off, so they're hanging down from these flower heads.
BM: Wow. A death grip on the flower.
DS: Exactly. That's what we call a death grip. It's really quite impressive.
BM: So what happens from that point?
DS: Well that's another one of the unusual things about it. So in the morning of the first day, after the beetle has died and gripped the flower with its mandibles, the fungus is developing inside the body still and maturing. And then about 20 hours later, in the middle of the night - like around maybe 2:00 a.m., the fungus starts growing. Basically it's filled up the beetle at this point. And it presses against the membranes. When it does that, somehow it's forcing the wings open so that they're wide spread.
My analogy is, if you had a morgue and human bodies were in it. Then 20 hours after they died, you went into the morgue and all of them sat bolt upright and opened up their arms and legs widely, you know, you'd be really freaked out.
BM: Why would the fungus do this - first get the beetle to climb up to the top of a flower, grip it with its mandible, die there, and then 20 hours later, open its wings?
DS: Well because it maximizes the chance that the spores of the fungus will contact future hosts. So if a beetle dies far away from other beetles, it's a dead end. And that fungus will not survive and reproduce. So it's evolved somehow to affect the host to cause it to die in exactly the right situation for more dissemination of the spores.
BM: How does the fungus get to other beetles?
DS: So there is a dead beetle on a flower head. And here's a whole bunch of healthy beetles running around on the flower head and eating pollen and mating. They definitely do not seem to avoid the dead beetles. And there may actually be an interaction — a mating type of interaction. We don't know that for sure, but they they definitely contact the dead hosts and when they do the spores get on their body. So in human terms, it would be as if you went to this pub, and somebody had died in the pub, and everybody was still interacting with that dead person who's covered with an effective material, as if they were normal — and touching him and patting him on the back. If you did that, it'd be very bad for the host — you, but very good for the pathogen.
BM: Boy, that's a very clever strategy on the part of the fungus…
DS: You know, a lot of these pathogens do this. There's a whole area of study interested in how pathogens, microorganisms, affect their host. And more and more people are learning these pathogens altering behaviours of individuals that are infected. It's an up and coming area of research.
BM: Why do you think the beetles haven't developed a strategy to avoid this?
DS: That's a very good question. I don't think they can. They don't seem to notice it. Animals do a lot of things that aren't necessarily beneficial to the animal. The mating urge is extremely strong. These beetles have to feed. They have to find a mate in order for them to reproduce. And so by putting the dead ones right up there where other beetles are, it benefits the fungus.
BM: So the beetles just don't notice that the person at the pub with their teeth buried into the bar and not moving is dead.
DS: Right. That's exactly correct. And so not only do they not avoid him. They may actually be attracted to him.