Extreme Heat Poses an Emerging Threat to Food Crops

As the American West grapples with another dangerous heat wave in the midst of a megadrought, official advisories rightly focus on short-term measures to keep people cool and hydrated. 

Yet as record-breaking heat waves become more common in a warming world, they pose a longer term threat to human well-being. Excessive heat interferes with pollinator interactions with plants that produce about a third of the world’s food crops. Scientists are scrambling to understand the complex ways spiking temperatures are disrupting those relationships.

Extreme heat can have multiple knock-on effects that disrupt the intricate interplay between bees and the flowering crops they feed on, researchers at Michigan State University warned in a review of heat’s effects on the pollinators and their host plants  published in Insect Science last month. 

“Extreme heat can indirectly limit plant reproduction by disrupting the pollination services of bees through reduced access to floral nutrition,” the researchers noted. And the bee’s reduced supply of food could exacerbate yield loss from heat-stressed crops. 

These indirect effects urgently require research attention, they argued, yet scientists have focused mostly on direct effects of heat on crops and their pollinators.

Bees support about 100 commercial nut, fruit and vegetable crops, from almonds and blueberries to tangerines and zucchini. As bees collect nectar and pollen for larvae back in their nests, they fertilize crops by distributing pollen from flower to flower.

Extreme heat can directly reduce both pollinators and plants’ ability to reproduce, develop and survive. Heat stress can hinder photosynthesis in crop plants and diminish the nutritional value of their flowers. If flowers produce less nectar and pollen, bees will have less food to support the development, survival and reproductive success of their colonies. 

Fewer bees means less pollination, and lower crop yields.

“We know that heat is directly impacting the quantity and the quality of floral resources,” said Jenna Walters, a doctoral candidate at Michigan State University’s Pollination Ecology Lab who led the review. “How that indirectly impacts specifically understudied bees, like solitary specialist bees, is one of the biggest neglected problems in our field and one of the biggest things we need to focus on.”

Most studies focus on honeybees and bumblebees. But most of the world’s roughly 20,000 bee species are solitary, some of which depend on just one plant. Squash bees, as their name implies, feed exclusively on pollen from squash and other gourds. If larvae are fed other types of pollen in the lab, they don’t develop. Undernourished bees get smaller over generations, which means they can’t fly as far. And that means they’ll pollinate fewer crops. 

“We simply don’t know how heat is impacting the nutrition of bees and how that is impacting development of bees and populations and communities of bees in our landscapes,” Walters said. 

Making matters worse, the megadrought that’s gripped the West is likely compounding the negative effects heat has on plants. Droughts reduce nectar production for one thing, and an extreme heat event is likely to reduce it further, Walters said.

Extreme heat and drought are likely to become more common as climate change accelerates. Scientists across disciplines have to start working together to figure out how these extreme conditions may alter pollinator-plant interactions, Walters said, because the future of pollinator-dependent crops hangs in the balance.

An ‘Insane’ Development

Walters, 25, came of age when the dire consequences of climate change were becoming harder to ignore. It was always in the back of her mind as she pursued undergraduate work on pollinators and ecology at Michigan State. 

Then, she saw an unprecedented event devastate Michigan’s blueberry crop.

Blueberries, native perennials in temperate regions like Michigan, bloom in the spring when temperatures typically hover in the mid-70s. But during the 2018 blueberry bloom temperatures soared past 95 degrees. 

Everyone worried about how the temperature spike would affect the harvest, Walters said. “But literally no research had been done at that point on how heat impacts blueberries, because it’s just so uncommon for that crop to be exposed to heat when they’re flowering.”

Researchers and growers alike had to wait for the harvest to gauge the impacts. What they saw shocked them. Some growers lost 50 percent of their crop. Across Michigan, the crop yielded about 30 million pounds less than normal. 

That’s a “massive” deficit for a specialty crop, Walters said. “Thirty million pounds in just one state is insane.”

When Walters and her colleagues looked at all the other conditions needed for pollination—not too much wind or rain, plenty of bees on the wing—everything pointed to a “great year” for yields, she said. Heat was the only thing that could explain the loss. 

Cascading Effects

Excessive heat can interfere with photosynthesis and diminish the nutritional value of flowers, which in turn can impair the survival, development and reproductive success of pollinators that feed on them, including bees. It can also change the size of bees’ legs, wings and proboscis and even hinder their memory, all of which support efficient pollination. 

“How extreme heat is affecting plant-insect interactions is super important,” said Matt Forister, an expert on plant-insect ecology at the University of Nevada who did not participate in the review. “Especially the timing of it.”

It seems that extreme heat in late summer or early fall may be particularly devastating, Forister said. “In our last big look at butterflies across the West, we found that warming temperatures in the fall in particular predicted the worst butterfly season the following year,” Forister said, referring to his Science paper published last year analyzing decades of monitoring data from scores of locations in the West. 

As for why fall heat seems to be driving the declines, he added, “We don’t really understand it.”

Excessive heat can also upset the synchronized pollination schedules critical to producing the almonds and citrus that earn California farmers billions every year. If bees and other pollinators reach peak abundance before or after crops flower, the altered timing will leave the insects without food and crops without pollinators.

This can be a particular risk for ground-nesting species like squash bees. Rising spring temperatures heat up the soil, telling bees it’s time to emerge from their nests. Warmer than normal temperatures may send bees out to forage before flowers emerge, Walters said. “And they’re going to starve for some amount of time.”

Bees that survive will be malnourished and produce fewer eggs. Those eggs are likely to result in a higher ratio of males to females, because males require less pollen than females to develop. But it’s possible that pollen quantity and quality cause the male bias, Walters said. It’s a question she’s working on now.

Looking for Answers

Walters is also trying to figure out how excessive heat caused the 2018 blueberry crop crash. She started by exposing blueberry pollen to different temperatures in the lab to see how heat affects fertilization. Successful pollination triggers the growth of a structure called a pollen tube, which carries plant sperm to fertilize ovaries at the base of a flower. 

The results, which Walters hopes will be published soon, show that exposing pollen to extreme heat for a mere four hours—replicating what happened in 2018—was enough to significantly and permanently stunt pollen tube growth.

Walters dug into the scientific literature to understand how heat could do that. She discovered that pollen grains need proteins, carbohydrates and amino acids. Suddenly, everything clicked. The primary and almost exclusive resource for bee nutrition is pollen, which contains proteins, carbohydrates, amino acids and lipids, Walters said.

“So the same things that are fueling fertilization, this pollen tube growth, are the same things that bees are visiting these flowers for nutritionally,” she said.

Walters started thinking through the implications. If heat is inhibiting this physiological response in the plants, there has to be some sort of response in the bees as well. But what is it?

Again, Walters dug into the literature. “What I found when I looked into that was literally nothing.”

The vast majority of research in Walter’s field focuses on heat’s direct impacts on either  plants or bees, she said. She and her colleagues were shocked to find so little research on how these direct effects ripple through the complex interactions between pollinators and their host plants that are fundamental to the success of so many food crops. 

It’s absolutely critical to ramp up investigations of the cascading effects of these stresses right now to identify ways to enhance agricultural resilience, Walters said.

That will require not only reducing the fossil fuel emissions that exacerbate climate change, she said, but also boosting floral resources and habitat for pollinators.

Growers should invest in “rescue resources” for bees, like wildflowers, clover and other native flowering plants, Walters said. That will help supplement their diet, minimize their nutritional stress and reduce the negative effects of climate change.

On the bright side, University of Nevada’s Forister said, there’s a lot of resilience built into natural ecosystems. “It’s surprising.”

He recalled working on restoration projects in California’s Central Valley and seeing nothing but monocultures of pesticide-treated tomatoes stretch toward the horizon.

“And if the farmers have allowed just one field edge to go back to some kind of feral vegetation, where there’s flowers, you look down, and there’s a bee,” Forister said. “They’re always waiting around the edges for us to give them a chance to come back. We just have to give them that chance.”