The Gypsy Moth and the threshold of extinction

Maggie Koerth-Baker on how protecting endangered species has taught us something about eradicating invasive ones.

In the battle against invasive species, the scientists who are responsible for fighting off and ideally, eliminating the invaders have an unlikely ally — the scientists who try to keep endangered species alive.

For decades, scientists have studied dwindling populations of endangered animals, trying to figure out how to keep them from going extinct. Along the way, they’ve learned a lot about what happens to a species that’s on its last legs, and what kind of situations can push those species over the line from endangered to totally wiped out.

But say that you are instead dealing with an invasive species — an animal that is not native your region and whose presence is upsetting the ecological balance. Turns out, it’s possible to intentionally set up the tipping-point scenarios and use them as a weapon against the animals you don’t want to survive.

Gypsy moths are a great example of this. While this species hasn’t been successfully eradicated in the United States, we have been pretty successful at holding it at bay, and that success is thanks to tricks learned from ecologists.

Entering without a visa

Gypsy moths came to the United States from Eurasia in either 1868 or 1869. That’s a much more specific date than we can give for the landfall of most invasive species, but gypsy moths are a strange case. Unlike the vast majority of invasive, we know exactly who brought them from overseas — Etienne Leopold Trouvelot, an artist from the suburban Boston area who was also an avid amateur entomologist. Among his bug-related interests: Starting an American silkworm industry.

Unfortunately, silkworms have a pretty limited diet, heavily favoring mulberry trees for their sustenance. Trouvelot came up with a great idea — cross the picky silkworm moth with the not-even-remotely-picky gypsy moth and maybe you’d end up with a much more hardy source of silk production. This plan went predictably awry when the gypsy moth larvae escaped his backyard breeding facility. Suddenly, the voraciousness that Trouvelot had prized became a big liability.

By the late 1880s, gypsy moths were devouring enough Massachusetts trees that the state launched the first eradication campaign against them. By 1900, that effort had failed so miserably that Massachusetts effectively gave up.

But if you look at the range of gypsy moths today, they only occupy 1/3 of their potential range in the US, said Andrew Liebhold, a research entomologist with the US Forest Service. In fact, there’s a huge gypsy moth barrier — a sort of insectoid DMZ — that stretches from Minnesota to North Carolina, effectively hemming in gypsy moths and trapping them in the land they already occupy.

Erected in 1998, it’s reduced the rate at which the moths spread by more than 50%. The barrier works by the same mechanism that drives many endangered species to extinction: Lack of ability to find a mate.

On the threshold

An extinction threshold is the minimum number of individuals necessary to maintain a species in the wild. The exact number differs depending on the species, its habitat, and how quickly individuals are dying off — it’s an equation, not an absolute number. But, over decades of studying endangered species, scientists have learned that every population has a threshold. If the number of individuals drops below the threshold, then the population is going to go extinct.

“One of the main drivers is mate finding,” Liebhold said. “At low population levels, for sexually producing animals, it becomes too difficult for males and females to find one another.” Basically, low enough population levels can create a vicious cycle where it becomes impossible to grow the population. The numbers stagnate, they fall, and then the species disappears.

When it comes to endangered species, ecologists spend a lot of time trying to figure out how to avoid letting populations drop below their threshold. With the gypsy moth, different scientists work equally hard at making sure it does.

The gypsy moth barrier zone is set with hundreds of thousands of pheromone-laced traps. The moths use pheromones as a means of communication. The chemicals are like a personal ad saying, “Hey there! I’d like to have sex with you!” As long as the traps remain empty, the scientists know that the barrier zone is gypsy moth-free (or, anyway, free enough). When the moths turn up in the traps, it’s a sign that an invasion is underway.

The scientists react by setting out even more traps and by spraying the mating pheromone over the area with a crop duster. It’s as if somebody released millions of friendly chatbots into one city’s OKCupid sphere. When most of the other profiles on the dating site represent actual humans, it’s reasonably easy to find a date in your neighborhood. But if there are suddenly way more fake accounts than real ones — and if those fake accounts engage you and distract you from seeking out the real people — then your chances of ever successfully getting laid go way down.

The moths make small incursions into new territory, but each time they do, the scientists make sure their numbers stay or fall below the extinction threshold. This way, researchers are able to keep gyspy moths inside a quarantine zone and away from vulnerable trees.

This technique won't work with all invasive insects. Scientists would love to use it against the Emerald Ash Borer, for instance, but can't because the ash borer's sex life isn't sufficiently dependent on pheromone signaling. But it’s a clever way to turn the tool of extinction thresholds into a weapon, and it's a good reminder that ecology isn't always a hippie-friendly, animal-hugging stereotype.