Study reveals flies more vulnerable than bees to climate change

Despite their reputation as buzzing nuisances, flies serve a critical role as some of the Earth’s most prolific pollinators—and new research led by Penn State scientists suggests they are increasingly at risk due to rising global temperatures.

In a study recently published in the Journal of Melittology, an international team of researchers looked at the heat tolerance for a variety of species of bees and flies in tropical and subtropical regions of the Americas. Their findings suggest that rising temperatures pose a greater threat to flies than bees, as bees can tolerate much higher temperatures than flies and have a wider habitat range.

“Bees and flies are essential for pollinating plants, both in the wild and in agriculture,” said Margarita López-Uribe, the Lorenzo Langstroth Early Career Associate Professor of Entomology at Penn State, extension specialist of pollinator health and lead author on the study. “However, these vital insects are declining due to habitat loss, pesticides, disease and the growing threat of climate change.”

Flies play a crucial role as pollinators, second only to bees in terms of the volume of crops and habitat they pollinate, López-Uribe explained. Flies are especially important for overall health and diversity of wild ecosystems, as they facilitate reproduction for countless plant species, which in turn provide food and habitat for other organisms. Flies are also increasingly contributing to agriculture. For example, flies are the primary pollinator for cocoa trees that produce the fruits used to make chocolate.

A 2020 analysis of global crops found that the 105 most widely planted crops that benefit from pollinators have greater than $800 billion gross economic value and include many of the most popular and nutritious fruit, vegetable and nut commodities consumed worldwide. The study also found that flies, specifically hoverflies and blowflies, consistently came right behind bees as a top pollinator.

“It’s time we gave flies some more recognition their role as pollinators,” López-Uribe said. “Flies have a significant role, but they don’t get as much attention—and they are vulnerable in all the same ways that bees are.”

Insects are particularly susceptible to rising temperatures, as they have limited ability to regulate their own body temperatures, López-Uribe explained. To understand how different pollinator species might cope with rising global temperatures, the researchers studied the bees’ and flies’ “critical thermal maximum,” or CTMax—the maximum temperature they can withstand before losing the ability to move.

The team found that bees can tolerate much higher temperatures than flies. On average, the CTMax for bees was 2.3 degrees Celsius higher than for flies. The researchers also found that time of day affected the heat tolerance of bees. Bees foraging in the cooler morning hours had a higher CTMax than those active in the warmer afternoons. The study also revealed that geography plays a role in heat tolerance.

The team collected data throughout lockdowns during the COVID-19 pandemic, meaning international students on the project, from Penn State and other universities, conducted research in their home countries. López-Uribe explained that the challenge wound up being an asset, because students were able to collect data on bee and fly species throughout the Americas.

“We sent out all of the equipment to do the study to students throughout the U.S. and South America,” López-Uribe said. “These students were collecting the data in their houses, using their kitchens to understand the thermal ecology these insects could withstand. We effectively were able to provide an international research experience without being able to travel internationally.”

The research team found that flies and bees from high-elevation tropical areas like Cajicá, Colombia, had lower CTMax values than their counterparts in subtropical regions like California and Texas. This suggests that insects in cooler, high-altitude environments may be more vulnerable to even small temperature increases.

“In alpine and subarctic environments, flies are the primary pollinator,” López-Uribe said. “This study shows us that we have entire regions that could lose their primary pollinator as the climate warms, which could be catastrophic for those ecosystems.”