The Very Hot, Very Hungry Caterpillar

Climate change will mean more insects, and less food for humans.

A locust
Tengku Bahar / Getty

Since the dawn of agriculture, humans have been unwillingly nourishing insects by growing plants that they then devour. Their mandibles consume somewhere between 10 and 20 percent of crops produced around the world. And these losses are likely to grow as the world slowly warms.

By looking at how insects will respond to rising temperatures, a team of researchers led by Curtis Deutsch and Joshua Tewksbury have calculated how rice, maize, and wheat—which provide 42 percent of humanity’s calories—will fare as the globe heats up. The results aren’t pretty.

They estimate that the portion of these grains that’s lost to insects will increase by 10 to 25 percent for every extra degree Celsius of warming. Some predictions say that we can almost certainly expect 2 degrees of warming by the end of the century. If that happens, and the team’s calculations are accurate, then every year, the burgeoning legions of insects will deprive the world of a further 19 million metric tons of wheat, 14 million metric tons of rice, and 14 million metric tons of maize. “We’re not talking about the collapse of agriculture, but we’re talking about significant losses,” says Deutsch, who works at the University of Washington.

Even if insects do nothing, warming temperatures are already likely to affect plants directly and reduce crop yields by around five percentage points per degree of warming, according to Deutsch. The losses triggered by insects will likely add on top of those, reducing yields by another two to four percentage points. Put it this way: If a farmer is currently growing 100 tons of grain per hectare, she’d likely get just 84 tons in a world that’s 2 degrees warmer.

If anything, May Berenbaum, an entomologist at the University of Illinois at Urbana-Champaign, thinks that those figures are probably underestimates. “Three decades of studies have shown that elevated carbon dioxide and temperatures can lower the nutritional quality of certain plants, and decrease the ability of plants to produce toxins to protect themselves,” she says. Both factors might drive insects to devour crops even more readily than they otherwise would.

Still, she commends the team’s attempt to put some figures on the future impact of crop pests. So far, scientists have tried to estimate how crops might react to climate change by focusing entirely on the plants themselves. Very rarely have they turned their attention to the pests that eat those plants, and never have they done so at a global scale. “It’s remarkable that this aspect of climate change has received so little attention,” Berenbaum says.

In thinking about insects, Deutsch and his team focused on two big patterns. First, insects burn calories at a faster rate when their environment heats up, which forces them to eat more food. This is a straightforward trend that’s been documented in a number of earlier experiments, with a wide range of pest species. As the future gets hotter, the very hungry caterpillar is going to be even hungrier.

The second pattern is more complicated: Insect populations grow fastest at certain optimal temperatures—neither too hot nor too cold. Those that live in the tropics tend to live at those temperatures already. Their numbers will probably grow more slowly as the climate heats up, which will partly mitigate their expanding appetites. That’s good news for rice, which is mostly grown in tropical environments. Losses will still rise with temperatures, but to a lesser degree than for the other crops.

But in temperate parts of the world, insects mostly live in cooler-than-ideal conditions. “Some of them go through a dormant state in the winter, and more of them will make it through a winter that’s warmer,” explains Deutsch. “And when they emerge, they develop and reproduce faster because their metabolic rates are higher.” This is especially bad news for wheat, which is mostly grown in cool climates. Wheat growers will face more insects, which will also want to eat more to support their metabolisms. “It’s a double whammy,” Deutsch says.

By simulating these changes across the globe, the team calculated that the most productive agricultural regions will also be the most severely affected, with France, China, and the United States suffering the brunt of the incipient wave of pests. But the consequences of a global decline in grain will likely have the biggest effects on the poorest nations and households—those that are already living on a nutritional knife-edge.

Many factors, for which there is little data, could complicate and change the team’s estimates. For example, “the team don’t seem to have considered the influence of warming on the predators of these pests,” says Navin Ramankutty, who studies food security at the University of British Columbia. Along with hunters like spiders, every insect is targeted by at least one species of body-snatching parasitic wasp, which helps to keep their numbers in check. “If they also respond positively to warming, that might somewhat mute the impact of the crop pests themselves,” Ramankutty says.

Many pest insects also rely on bacterial symbionts that help them to digest their meals or provide them with essential nutrients. There’s a smattering of evidence that some of these microbes are very sensitive to high temperatures, which might again limit their hosts’ ability to thrive in the heat. And of course, extreme weather, from heat waves to droughts, might kill insect pests directly.

Finally, insects can obviously move. Besides changing their behavior to avoid the hottest part of the day, over time, entire populations could also move to new locations, tracking temperate climates northward as many animals are already known to do. These changes could have unknown consequences for the overlaps between pests and crops.

These intricacies might change the degree of the losses that Deutsch predicts, but it’s hard to imagine that they might wipe away those losses entirely. This, he says, is the time to start preparing, whether through agricultural practices like crop rotations, or by applying pesticides, or by planting genetically modified crops. “But preventing climate change is the thing I would first turn to,” he adds.

“Humanity faces this food-security chal­lenge at a time when training and job op­portunities for expert entomologists are shrinking,” writes Markus Riegler from Western Sydney University in an accompanying editorial. “These experts are urgently needed” to deal not just with the problem of pests, but also with threats to insect biodiversity.

Most of the millions of species of insects are not pests. They are the linchpins of many ecosystems. They’re sources of food for birds, and carriers of pollen for plants. If insects decline, ecosystems everywhere will too. And insects are declining, even in protected areas worldwide. As I wrote in The Atlantic last year, “Insects are the most diverse and numerous group of animals on the planet. If they’re in trouble, we’re all in trouble.”