Flies are more vulnerable to climate change than bees

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

An international team of researchers examined the heat tolerance of various bee and fly species in tropical and subtropical regions of the Americas. 

The results indicate that flies are more vulnerable to warming climates than bees, since bees can handle significantly higher temperatures and occupy a broader range of habitats.

“Bees and flies are essential for pollinating plants, both in the wild and in agriculture,” said Margarita López-Uribe, an expert in pollinator health at Penn State 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 are underappreciated yet vital pollinators

Flies, second only to bees in terms of the volume of crops and habitats they pollinate, play a crucial role in sustaining wild ecosystems. 

By facilitating the reproduction of numerous plant species, flies help maintain biodiversity and provide resources for countless other organisms. They also increasingly contribute to agriculture – for example, flies serve as the primary pollinators of cocoa trees, the source of chocolate.

A 2020 analysis of global crops found that the 105 most widely planted crops benefiting from pollinators represent more than $800 billion in gross economic value. Among these pollinators, flies, specifically hoverflies and blowflies, consistently ranked just behind bees.

“It’s time we gave flies some more recognition of 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.”

Flies, bees, and climate change

Because insects cannot easily control their body temperatures, rising global temperatures present a serious challenge.

To assess how different pollinators might cope, the research team measured the “critical thermal maximum” (CTMax) – the highest temperature at which insects can still move – of both bees and flies.

They found that bees can tolerate substantially warmer conditions than flies. On average, bees had a CTMax 2.3 degrees Celsius higher than that of flies. 

Additionally, bees foraging in cooler morning hours proved more heat-tolerant than those active in warmer afternoons. 

Geographic factors also mattered – pollinators living at high-elevation tropical sites showed lower CTMax values than their counterparts in subtropical areas.

Data collection across the Americas

The study’s data collection occurred during COVID-19 lockdowns, so international students from Penn State and other universities conducted research in their home countries. 

Instead of traveling, students worked with equipment sent to them, gathering measurements in their own kitchens and backyards.

“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.”

Flies battling climate change

The findings suggest that insects from cooler, high-altitude environments, such as Cajicá in Colombia, are particularly susceptible to even slight temperature increases. 

Since flies are often the primary pollinators in alpine and subarctic regions, their reduced ability to withstand warming could jeopardize entire ecosystems.

“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.”

As climate change continues to reshape global temperatures, understanding the vulnerabilities of flies – as well as bees – will be key in guiding conservation efforts. 

Without recognizing and protecting the essential pollination services flies provide, communities may face declining biodiversity, reduced crop yields, and unstable ecological networks in the years to come.

The study is published in the Journal of Melittology.

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