How effective are opossums at tick control?—Grace McNally, via email
Not very effective at all. At least, there’s no strong evidence that they are. You’ve probably read the same stat that we have—Virginia opossums, a.k.a. Canada’s only marsupial, eat about 95 per cent of the ticks that feed on them. Hey, good job, opossums! That makes you “ecological traps.” You’re so irritated by the ticks feeding on you that you immediately try to get them off your bodies—by eating them—and the ticks can’t complete their life cycles. Other hosts, such as white-tailed deer, are much more permissive.
But here’s the thing: that conclusion comes from one American 2009 lab study (Keesing et al.). The researchers trapped five opossums and exposed each caged animal to 100 ticks. After four days, they checked the cages. They found that, on average, 3.5 per cent of larval ticks had fed and dropped off the opossums. They concluded that the rest of the ticks had been ingested.
Wild Profile: Meet the opossum
“But that was based on an assumption,” says Cecilia Hennessy, a wildlife biologist in Milwaukee, Wisc. “They didn’t comb their fur. They didn’t check them at all. They just released them into the wild, like, ‘Go with God.’ ”
Hennessy found the study’s conclusions odd. “I’ve handled hundreds of opossums. They’re not fastidious. They’re not like cats.” So she looked at 23 diet analysis studies done on the species; no ticks showed up. She and an undergraduate student at Illinois’ Eureka College then did their own study, analyzing the stomach contents of 32 opossums. They found crickets, worms, fleas…pretty much everything except ticks. Not even a single tick body part. “It was actually quite astonishing,” says Hennessy. “After the 16th stomach, we were like, ‘We should have found something.’ It was as if the opossums were going out of their way to not eat ticks.”
If, in the 2009 study, the opossums didn’t consume the ticks, where did 96.5 per cent of the parasites go? It’s possible that the ticks were still feeding on their hosts after four days. There’s evidence that ticks feed more slowly on animals with a lower metabolism, says Hennessy, and opossums do have a lower body temperature and heart rate compared to other tick hosts. Or maybe the opossums did eat the ticks. They were wild animals, suddenly confined to cages. Maybe that could account for the change in grooming and eating habits. We may never know.
Hennessy understands why the opossum has gained the status of ecological hero. “People want to feel good about these ugly, drooling, rat-tailed marsupials,” she says. “They’re underdogs.” And everyone knows that underdogs make for the best heroes.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the September/October 2024 issue of Cottage Life.
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