Global Warming: More Insects, Eating More Crops

Climate change will produce hungrier, more plentiful insects – that destroy more of our food, says new research in Science by UVM’s Scott Merrill and colleagues. (Credit: Keith Ewing / CC BY 2.0)

Burlington, VT (August 30, 2018) - Crop losses for critical food grains will increase substantially as the climate warms, as rising temperatures boost the metabolic rate and population growth of insect pests, new research says.

“Climate change will have a negative impact on crops,” saus Scott Merrill of the University of Vermont, a co-author of the study published in Science. “We are going to see increased pest pressure from climate change.”

The research team looked at how the insect pests that attack three staple crops – rice, maize and wheat – would respond under a variety of climate scenarios. They found that rising global temperatures would lead to an increase in crop losses from insects, especially in temperate regions. Losses are projected to rise by 10 to 25% per degree of warming.

Just a 2-degree rise in global average temperature will result in total crop losses of approximately 213 million tons for the three grains, the researchers say.

Insects like it hotter – up to a point

The losses will come from an increase in insect metabolism, and from faster insect population growth rates. The link with metabolism is straightforward. “When the temperature increases, the insects’ metabolism increases so they have to eat more,” says Merrill, a researcher in UVM’s Dept. of Plant and Soil Science and Gund Institute for Environment. “That's not good for crops.”

The link with population growth, however, is more complex. Insects have an optimal temperature where their population grows best. If the temperature is too cold or too hot, the population will grow more slowly. That is why the losses will be greatest in temperate regions, but less severe in the tropics.

“Temperate regions are not at that optimal temperature, so if the temperature increases there, populations will grow faster,” says Merrill, an ecologist who studies plant-crop interactions. “But insects in the tropics are already close to their optimal temperature, so the populations will actually grow slower. It's just too hot for them.”

Key grain crops to take a hit

According to the study, wheat, which is typically grown in cool climates, will suffer the most, as increased temperatures will lead to greater insect metabolism, as well as increased pest populations and survival rates over the winter. Maize, which is grown in some areas where population rates will increase and others where they will decline, will face a more uneven future.

In rice, which is mostly grown in warm tropical environments, crop losses will actually stabilize if average temperatures rise above 3oC, as population growth drops, counteracting the effect of increased metabolism in the pests. “Rice losses will taper off as the temperature rises above a certain point,” says Merrill.

That means that the most substantial yield declines will happen in some of the world’s most productive agricultural regions. “The overall picture is, if you're growing a lot of food in a temperate region, you're going to be hit hardest,” says Merrill.

"I hope our results demonstrate the importance of collecting more data on how pests will impact crop losses in a warming world — because collectively, our choice now is not whether or not we will allow warming to occur, but how much warming we're willing to tolerate," says Curtis Deutsch of the University of Washington, who co-led the study with Joshua Tewksbury, director of Future Earth at the University of Colorado, Boulder.

France, China and the United States, which produce most of the world’s maize, are among the countries that are expected to experience the largest increases in crop losses from insect pests. France and China, as major producers of wheat and rice, respectively, are also expected to face large increases in losses of those grains as well.

“The areas that produce the most grain, especially wheat and corn – the United States, France, and China – are going to be hit hardest,” says Merrill.

Reduced yields in these three staple crops are a particular concern, because so many people around the world rely on them. Together they account for 42% of direct calories consumed by humans worldwide. Increased crop losses will result in a rise in food insecurity, especially in those parts of the world where it is already rife, and could lead to conflict.

As farmers adapt to a changing climate by shifting planting dates or switching to new cultivars, they will also have to find ways to deal with pests, by introducing new crop rotations, or using more pesticides. But not all of these strategies will be available to all farmers.

“There are a lot of things richer countries can do to reduce the effect, by increasing pesticide use or expanding integrated pest management strategies,” says Merrill. “But poorer countries that rely on these crops as staple grains will have a harder time.”

In addition to Merrill, Deutsch, and Tewksbury, study co-authors include Michelle Tigchelaar, David Battisti, Raymond Huey from University of Washington, and Rosamond Naylor of Stanford University. The research was funded by the National Science Foundation and the Gordon and Betty Moore Foundation.

Image of a European corn borer (Ostrinia nubilalis). (Frank Peairs/Colorado State University)

Climate change projected to boost insect activity and crop loss, researchers say

Scientists have already warned that climate change likely will impact the food we grow. From rising global temperatures to more frequent "extreme" weather events like droughts and floods, climate change is expected to negatively affect our ability to produce food for a growing human population.

But new research is showing that climate change is expected to accelerate rates of crop loss due to the activity of another group of hungry creatures — insects. In a paper published Aug. 31 in the journal Science, a team led by scientists at the University of Washington reports that insect activity in today's temperate, crop-growing regions will rise along with temperatures. Researchers project that this activity, in turn, will boost worldwide losses of rice, corn and wheat by 10-25 percent for each degree Celsius that global mean surface temperatures rise. Just a 2-degree Celsius rise in surface temperatures will push the total losses of these three crops each year to approximately 213 million tons.

"We expect to see increasing crop losses due to insect activity for two basic reasons," said co-lead and corresponding author Curtis Deutsch, a UW associate professor of oceanography. "First, warmer temperatures increase insect metabolic rates exponentially. Second, with the exception of the tropics, warmer temperatures will increase the reproductive rates of insects. You have more insects, and they're eating more."

In 2016, the United Nations estimated that at least 815 million people worldwide don't get enough to eat. Corn, rice and wheat are staple crops for about 4 billion people, and account for about two-thirds of the food energy intake, according to the UN Food and Agriculture Organization.

"Global warming impacts on pest infestations will aggravate the problems of food insecurity and environmental damages from agriculture worldwide," said co-author Rosamond Naylor, a professor in the Department of Earth System Science at Stanford University and founding director of the Center on Food Security and the Environment. "Increased pesticide applications, the use of GMOs, and agronomic practices such as crop rotations will help control losses from insects. But it still appears that under virtually all climate change scenarios, pest populations will be the winners, particularly in highly productive temperate regions, causing real food prices to rise and food-insecure families to suffer."

To investigate how insect herbivory on crops might affect our future, the team looked at decades of laboratory experiments of insect metabolic and reproductive rates, as well as ecological studies of insects in the wild. Unlike mammals, insects are ectothermic, which means that their body temperature tracks the temperature of their environment. Thus, the air temperature affects oxygen consumption, caloric requirements and other metabolic rates.

The past experiments that the team studied show conclusively that increases in temperature will accelerate insect metabolism, which boosts their appetites, at a predictable rate. In addition, increasing temperatures boost reproductive rates up to a point, and then those rates level off at temperature levels akin to what exist today in the tropics.

Deutsch and his colleagues found that the effects of temperature on insect metabolism and demographics were fairly consistent across insect species, including pest species such as aphids and corn borers. They folded these metabolic and reproductive effects into a model of insect population dynamics, and looked at how that model changed based on different climate change scenarios. Those scenarios incorporated information based on where corn, rice and wheat — the three largest staple crops in the world — are currently grown.

"Temperate regions are currently cooler than what's optimal for most insects. But if temperatures rise, these insect populations will grow faster," said co-author Scott Merrill, a researcher at the University of Vermont's College of Agriculture and Life Sciences and the Gund Institute for Environment. "They will also need to eat more, because rising temperatures increase insect metabolism. Together, that's not good for crops."

For a 2-degree Celsius rise in global mean surface temperatures, their model predicts that median losses in yield due to insect activity would be 31 percent for corn, 19 percent for rice and 46 percent for wheat. Under those conditions, total annual crop losses would reach 62, 92 and 59 million tons, respectively.

The researchers observed different loss rates due to the crops' different growing regions, Deutsch said. For example, much of the world's rice is grown in the tropics. Temperatures there are already at optimal conditions to maximize insect reproductive and metabolic rates. So, additional increases in temperature in the tropics would not boost insect activity to the same extent that they would in temperate regions – such as the United States' "corn belt."

The team notes that farmers and governments could try to lessen the impact of increased insect metabolism, such as shifting where crops are grown or trying to breed insect-resistant crops. But these alterations will take time and come with their own costs.

"I hope our results demonstrate the importance of collecting more data on how pests will impact crop losses in a warming world — because collectively, our choice now is not whether or not we will allow warming to occur, but how much warming we're willing to tolerate," said Deutsch.

Co-lead author is Joshua Tewksbury, director of Future Earth at the University of Colorado, Boulder. Additional co-authors are Michelle Tigchelaar, a UW research scientist in the Department of Atmospheric Sciences; David Battisti, a UW professor of atmospheric sciences; and Raymond Huey, a UW professor emeritus of biology. The research was funded by the National Science Foundation and the Gordon and Betty Moore Foundation.