Extreme heat threatens solitary bees and global food security

The scorching heatwaves that have swept across the southwest United States and southern Europe in recent months are not just a concern for humans. These extreme climate events are disrupting the delicate balance between pollinators and their resident pathogens – an imbalance with potentially severe consequences for global economies and food chains.

In a recent study, an international team of researchers led by Penn State University delved deep into the relationships between heat waves, solitary bees, and the protozoan pathogen, Crithidia mellificae.

The results, published in the journal Frontiers in Ecology and Evolution, shine a light on an issue often overshadowed by the broader discourse on global warming.

Alarming statistics 

Alarmingly, solitary bees, which comprise over 90% of the approximately 4,000 bee species in North America, display a decreased inclination to forage following exposure to intense heat and prior infection. 

“We are now experiencing the highest temperatures in recorded history,” said study lead author Mitzy Porras. “These heat waves are lasting three, or even four days, which is a long period of heat tolerance for bees.”

“Then, when you combine that with prior infection from a pathogen, we’re looking at two factors that can severely negatively impact pollinator populations.”

Thermal boldness 

To delve deeper into these observations, the team created an innovative experiment to gauge what Porras terms “thermal boldness,” a measure of how much heat a bee can endure while seeking food. 

The results were startling: bees previously infected by the protozoan pathogen demonstrated a significantly lower threshold of heat tolerance, and were hesitant to brave higher temperatures even for vital nourishment.

While both the bee and its pathogen were adversely affected by the heat, the bee shouldered a disproportionate burden. The consequences of the heat were more pronounced on the bee’s thermal boldness and overall heat endurance, while the pathogen’s growth rate was only marginally impeded. 

Overlooked connections

“These asymmetrical relationships between organisms are often overlooked when studying climate impacts, but they are essential if we want to understand what is really going on,” said Porras. 

“When we looked at the host and pathogen in tandem, we found that infection greatly reduces heat tolerance in the host – a finding we wouldn’t have discovered if we had only been studying bees.”

This was a particularly concerning discovery – the tangible impact of infection on a bee’s heat endurance. A healthy bee could withstand temperatures of up to 109.4°F. However, this threshold plummeted to 98.6°F post-infection.

Study implications 

“Our results shed light on the implications of extreme heat waves on host–pathogen dynamics under a warmer world,” said co-author Professor Ed Rajotte. “We’re not going to see a simple, linear change as the climate warms.” 

“Every organism will respond differently and the relationships between organisms will be fundamentally altered. If we’re going to try to predict the impacts of climate change, relationships matter. There are real consequences to changes in our ecosystems and we must understand the subtleties if we are going to prepare ourselves for the reality of a changing climate.”

Solitary bees, often termed the “workhorses” of the pollination world, live for about a year. Yet, their active outdoor span lasts only two to four weeks, typically in early spring. This means that for these bees, a three or four-day heatwave can drastically curtail their pollinating activities, mating potential, and offspring production. Infection further exacerbates these challenges.

“We have to think about the big picture,” said Porras. “Climate change is not just impacting species; it is impacting the relationships between species and that could have huge implications for human health and the planet as a whole.”

More about solitary bees 

Solitary bees are a diverse group of bees that, unlike honeybees or bumblebees, do not live in colonies. 

Key points

Lifestyle 

As their name suggests, solitary bees live and work alone. Each female typically builds her own nest without the assistance of worker bees.

Nesting

They often nest in the ground, in hollow stems, or in holes in wood. Many gardeners provide “bee hotels” to support them.

Diversity

There are over 20,000 species of bees worldwide, and the majority are solitary. Some common types include mason bees, leafcutter bees, and sweat bees.

Pollination

Solitary bees are efficient pollinators, often more so than honeybees. They play a crucial role in pollinating flowers, fruits, and vegetables.

Stinging

While some solitary bees can sting, they are usually less aggressive than social bees because they don’t have a colony to defend. Many can’t sting at all, and those that can often have a sting that’s less painful than that of other bees.

Provisioning

Unlike honeybees that store honey, solitary bees provide their offspring with a mixture of pollen and nectar called “bee bread”.

Conservation

Solitary bees are under threat from habitat loss, pesticides, and climate change. Providing habitats, reducing pesticide use, and planting pollinator-friendly plants can help support their populations.

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