Sacrificing Fake Caterpillars in the Name of Science

Ersatz insects are helping ecologists figure out why bugs are more likely to become meals near the equator

Hong Kong Caterpillar
Thousands of clay caterpillars, like this one glued to a leaf in Hong Kong, were used to measure how often predators are eating insects around the world. Chung Yun Tak / Science

It looks like a bright green caterpillar. It feels, for the most part, like a caterpillar (soft and slightly chewy). It perches on a leaf, poised as if about to inch forward, just like a caterpillar would. But as soon as a bird snaps one up on these munchies, it’s apparent that it’s actually a piece of green plasticine clay, quickly molded by the hands of ecologists.

So why are researchers tricking real animals with fake bugs?

To figure out how often the real bugs are getting eaten, it turns out. "You can't learn that much about nature from staring at individual species," says insect ecologist Tomas Roslin. Five years ago, Roslin found himself studying Arctic insects in remote northern Greenland. But he was thwarted by the fact was difficult to measure how often these bugs got eaten, given how few predators lived at that latitude.

By contrast, a colleague, Oxford University ecologist Eleanor Slade, told Roslin she was having great success in measuring bug-chomping rates in the southern island of Borneo, which straddles the equator. Both Slade and Roslin, who works for the Swedish University of Agricultural Sciences but is based in Finland, were using a creative technique that's gained popularity in the last 15 years: ersatz insects.

To get around the impracticalities of tracking how often actual insects are eaten, ecologists often make use of dummy bugs, made out of modeling clay and placed out in the wild glued to leaves. Birds generally bite down on these but quickly spit them out once they realize they're not food, leaving behind bite marks. Once ecologists re-collect the worms, they can use these marks to see how often birds or other animals attempted to eat them. This can give a general sense of the "predation pressure" on insects in a certain area, Roslin says.

Greenland Caterpillar
A caterpillar glued to a plant in northern Greenland waits to be chewed. Tomas Roslin / Science

Struck by the vast differences between his and Slade's observations, Roslin set out to quantify these predation trends by running experiments on a global scale. In doing so, he hoped to create a framework for other researchers to measure the relationships between predators and their prey more generally. He also hoped that such a framework would help ecologists estimate how climate change and habitat destruction could impact these patterns differently in different areas.

"It's very hard to do it with just a single researcher," Roslin says, but fortunately for him, "there are ecologists all over the world."

Roslin tapped into an informal network of nearly 40 other insect researchers, based in locations ranging from the Smithsonian Tropical Research Institute in Panama to outposts in Africa, Alaska, Australia and Europe. The worldwide team set out nearly 3,000 standardized counterfeit caterpillars for 4 to 18 days to let them get bitten by birds and invertebrates, and then collected them again to measure how chewed they were.

The tropics are some of the most biologically diverse regions in the world—holding about half of Earth’s species despite comprising less than 7 percent of its land—so you’d expect a good amount of predation to be going on. Warm temperatures, abundant moisture and a relatively stable climate allow the environments there to support millions of species feeding off of and supporting each other.

As a result, Roslin expected to find that predation increased significantly closer to the equator and closer to sea level. As he puts it, temperatures are higher and there is generally more available energy in the food chain. In other words, there is more life to be eaten and more life to eat it, especially cold-blooded invertebrates.

"It was one of the rare cases where a great theory proves true," Roslin says. The researchers found that the odds of a dummy caterpillar being bitten increased by 2.7 percent for every degree of latitude one moved closer to the equator, and 6.6 percent for every 100 meters one moved closer to sea level.

At the highest latitudes, caterpillars were just 13 percent as likely to be eaten as those at the equator, while at the high altitudes, they were just 24 percent as likely to be eaten as those at sea level, according to the research published last week in the journal Science. "Now we actually have a prediction for what we should see" when studying organism interactions in different parts of the world, Roslin says.

Chewed Caterpillar
Study author Tomas Roslin holds one of the relatively few caterpillars in northern Greenland were chewed by a predator. Tomas Roslin / Science

"This is a very neat result and it provides clear evidence that the strength of biotic interactions varies with latitude and may be linked to the remarkable diversity of life in the tropics," says Michigan State University ecologist Gary Mittelbach, who has done extensive research on how species diversity differs across large scales, including by latitude.

Mittelbach was especially impressed by the "crowd-sourced" approach of Roslin's study, he says. (By “crowd-sourced” he means not that the study relied on observations from citizen scientists, but that it enlisted scientists from around the world who were already stationed in key locales.) The results, Mittelbach says, mirror those of a study done nearly 40 years ago by University of Wisconsin entomologist Robert Jeanne on how the predation of wasp larvae by ants varies by latitude.

That study required a single-handed "Herculean effort" by Jeanne, however, and Mittelbach hopes that Roslin's research will inspire more simple, worldwide collaborative experiments.

Mittelbach cautions, however, that these results might not translate smoothly to studying real species, because the clay caterpillars don’t move or smell like real worms. (To keep them quick and cost-effective, researchers roll their dummies out in just a few seconds.)

Roslin plans next to take this collaborative approach back to the Arctic, with a more focused study looking at how insect pollination compares in different parts of the region. It would be "prohibitively expensive" for him to travel across the Arctic conducting this research, but if he designs a simple experiment, he can use the scientists already based at research stations around the area to help him out.

"[We] can all be working together solving a big question in a very cheap and very efficient way," Roslin says.

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