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How Extreme Heat Puts Pollinators—and Crops—at Risk

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 directly reduce both pollinators and plants’ ability to reproduce, develop and survive.

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.

Jenna Walters, a doctoral candidate at Michigan State University’s Pollination Ecology Lab led a recent review of heat’s effects on the pollinators and their host plants. (Photo credit: Jenna Walters)

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.

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