The insects digest more decaying wood when hosting nematodes, potentially benefiting the whole ecosystem.
Leishmaniasis parasites eat human flesh. Cordyceps fungi lead ants to suicide. Toxoplasma gondii eliminate a mouse’s fear of cats. And a barnacle called Sacculina carcini castrates crabs, roots itself into their bodies and brains and transforms them into walking zombie slaves that care for the parasite’s brood as if it were their own.
Parasites are bad — except when they’re not.
In forests across eastern North America, wood-eating beetles chew through fallen logs. This helps break down wood and return nutrients to the soil. But many beetles become infected by the thousands with a common, parasitic worm that makes their insides look like a plate of moving spaghetti.
There’s little evidence the parasites are harmful. Instead, infected beetles seem to be bigger and eat more than uninfected ones, suggests a study published Wednesday in Biology Letters. This increased consumption may help the forest cycle nutrients faster, and benefit the whole ecosystem.
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“Everything is connected,” said Andy Davis, an ecologist at the University of Georgia who led the research. “Here’s a case where there’s a little tiny bug in the forest and it’s actually doing a service within the forest. It’s doing something important. But then there’s another bug that lives inside that bug that’s doing something important.”
The horned passalus, also called the bess beetle, is about the size of your thumb and weighs as much as two raisins. It’s one of the few insects to live with its family, and it communicates by making about a dozen different sounds.
A curved horn protrudes above its monstrous mandibles, which it uses to eat decaying logs from the inside out. It prefers hardwoods — oak or elm — which it chews and excretes as sawdust for microbes to break it down further. The beetle then re-eats this material, spitting out some for its young. About 70 to 90 percent of bess beetles unintentionally also care for Chondronema passali, a parasitic nematode that drains its energy.
Dr. Davis noticed that parasitized beetles had trouble with tasks requiring fast bursts of energy, such as fighting other beetles, closing wounds or evading predators. But they didn’t seem bothered during everyday activities, and didn’t lose as much weight as uninfected beetles when subjected to stress — even with a belly full of worms. That got Dr. Davis thinking: “Maybe they’re somehow compensating for the energy drained by the parasites by eating more.”
To find out, he and Cody Prouty, a student in his lab and co-author, collected more than 100 beetles from the forest and housed them in old ice-cream containers with a moist piece of wood for munching. After three months, the pair determined how much the beetles had eaten by weighing the sawdust left behind. They weighed and measured the beetles, then dissected them to look for nematodes.
About 70 percent of their subjects had parasites, and those beetles were larger and ate about 15 percent more than those without parasites. “That explained how these beetles were existing with all these parasites in them,” Dr. Davis said. “They’re simply eating more.”
But an equally plausible explanation is that bigger beetles eat more, increasing their likelihood of ingesting the parasite.
“From the ecosystem perspective, it doesn’t matter,” said Dr. Davis. “It’s better to have parasitized beetles, because then the logs on the forest floor are being broken down faster.”
The study estimated that 10 adult beetles with parasites could break down a pound more wood in a year than uninfected beetles could.
“Parasites get a bad rap,” said Dr. Davis. “People don’t usually think of them that fondly. But perhaps if we start to look at them this way, maybe that will change.”
More reporting on parasites