The ground is littered with corpses, struck dead by a lethal parasite. As the bodies fester with the parasite’s growing spawn, they begin to stink and glow a bright electric blue. Eventually they burst open to reveal the next generation of killers, which crawl from remains to find their next victim. No, this isn’t the next pandemic movie: it’s an everyday scene fueled by a bacteria-worm partnership. Now, scientists are trying to better understand this dynamic duo and turn them into a commercial product to benefit farmers.
The beige, half-a-millimeter-long nematode worm hardly looks like a parasitic threat. These tiny crawlers spend most of their time swimming through the soil, occasionally standing on their tails and poking their little heads through the surface. But really, they’re just biding their time. As soon as they sense that an unsuspecting insect is about to lumber by, they ambush. A nematode can launch like a cannonball out of the soil, projecting itself up to 10 centimeters to infect its unlucky prey.
After landing on a victim, the nematode wriggles into the insect’s innards through any orifice it can find, or hacks its way in using a special tooth. This parasite is so small that its victim doesn’t feel a thing. But the worm carries a deadly weapon in its gut, just waiting to be regurgitated: the bacterium Photorhabdus luminescens. The toxins this bacterium produces are so potent that 10 cells can kill an insect in as little as 24 hours. The bacteria also release chemicals that prevent the corpse from decomposing, so that the nematodes always have a fresh meal to snack on. Think of it as the ultimate bed and breakfast.
The nematodes eat their fill for about two weeks, or until they’ve reached host-bursting capacity. Then they explode through the corpse into the soil, in a live action version of the scene from Tim Burton’s Nightmare Before Christmas when Oogie Boogie unravels at the seams to reveal a wriggling colony of bugs. On their way out of the spent host, each nematode reloads their deadly bacterial arsenal by gobbling up a few Photorhabdus cells. Then they’re back on the prowl to find their next victim.
What the nematodes and their bacterial helpers don’t want is for a predator to snatch up their beloved corpse home before they’ve had a chance to complete their life cycle. And out in your garden, it’s a dog-eat-dog world: Bigger bugs, smaller bugs with big appetites like ants, or even birds on the lookout for a meal are all eager to lug off a helpless insect corpse for sustenance. That’s where those bacterial sidekicks come into play again.
Photorhabdus doesn’t stop at killing the insect host so the nematode can eat and make babies unimpeded. They also turn the corpse into a lurid spectacle that could work as a defense strategy. First, the bacteria secretes chemicals that turn the insect body brick red, a common insect warning that says “Don’t eat me! I’m gross!”
For predators that aren’t as visually discerning, the bacteria also emit an awful stench. How awful? “They smell really bad,” says Rebecca Jones, a lecturer in population genetics at the University of Liverpool who studies this nematode-bacterium duo. “It doesn’t smell like something decaying or rotting.” Instead, she says, they smell like pyrazine, an organic compound that entomologists know rings insect alarm bells. To those who aren’t bug experts, Jones says, “It’s a bit like almonds.”
But isn’t this all a bit overkill? Jones thinks it’s all part of a concerted effort to be as bizarrely unappetizing as possible, allowing the nematodes to stay safe and get busy inside. “Our hypothesis is that they have a range of defenses in order to protect the parasitic colony from predation by a whole host of different predators,” says Jones, who is first author on a recent study published in the journal Animal Behaviour that seeks to tease apart these defense strategies.
“By combining two, or three, or even four signals together, perhaps that creates better protection for the parasitic community,” she says.
To untangle the puzzle, she enlisted help from a few avian predators: wild great tits, which are from a region of Finland where the Heterorhabditis nematode isn’t typically found. This was important, because the birds didn’t have preconceived notions about how these insects should look or smell, making them the ideal test subjects.
For her experiments, Jones separately presented 30 great tits with a selection of eight larval waxworms she had infected with nematodes. The experiment was aimed to determine whether color, scent or a combination of the two was the strongest deterrent. In the scent-only trials she used uninfected, normal white waxworms in a dish, with stinky, infected worms hidden underneath. To test color only, she put red, infected worms in a clear, odor-impermeable container and counted how many times the birds tried to peck them.
“The most surprising thing we saw was that it wasn’t the combination of strategies that worked best,” Jones says. While a red warning hue and a terrible smell both worked independently, using them together wasn’t as effective when it came to avoiding death by great tits. As it turns out, the birds were most averse to eating plain old stinky insects. “Having scent by itself tended to overshadow even the color and scent trials we did,” says Jones.
Scientists surmised that the odorific spectacle put on the bacteria was “like a no-vacancy sign at a motel, saying ‘don’t eat me, I’m horrible’,” says Richard Ffrench-Constant, a professor of molecular natural history at the University of Exeter who was not involved with the study.
These results showcase a powerful example of symbiosis in action. The bacteria can’t survive on their own in the wild; they need the nematode to transport them from one insect to another. Conversely, the nematode needs the bacteria to kill the host and prevent it from being eaten. Ffrench-Constant views this as yet another fascinating example from the burgeoning field of microbiome science. “We’re at the tip of a big iceberg” when it comes to this particular bacterial partnership, he says. “There’s just so much about these bugs we don’t understand.”
While nematodes can attack above the ground, they actually do most of their killing below the surface, moving in packs beneath the surface and tracking their prey by chemical signals. For that reason, Ffrench-Constant says, the bacteria’s defense strategies would have been better demonstrated using ants instead of birds. “I can’t convince myself that in your garden when these things kill your waxworms it’s going to be tits coming down to be the major predators,” he says.
Other mysteries abound. For example, the chemical compounds excreted by these bacteria number in the thousands, and researchers still have no idea what they’re used for. “Well crikey,” Ffrench-Constant says, “If this one chemical repels ants or repels birds, then what do the other 999 do?”
While there’s still much to understand, farmers have already taken advantage of this powerful, diminutive duo. Farmers can purchase vats of nematodes to spray in their fields as a chemical pesticide alternative, thanks to researchers who are figuring out efficient ways to mass produce these bugs in the lab. The nematodes have even been used to save Florida oranges from demise at the hungry mandibles of the citrus root weevil.
Farmers and home gardeners alike are finally starting to catch up to what the plants seem to have known all along. Though Jones hasn’t taken her nematodes home from the lab for any extracurricular experimentation—her flat in Liverpool doesn’t have a garden—she’s still preaching the parasitic nematode gospel. She says, “I’ve told my granddad and he’s been out to buy some and tell all his friends. They’re a little nematode gardening community.”