Sustained warming alters Bombus impatiens foraging, colony performance, and pollination of zucchini (Cucurbita pepo)

Climate change is not just about rising global averages; it is about the increasing frequency and intensity of heat that living things must endure. For insects like bumble bees, which are vital for pollinating crops and wild plants, high temperatures present a direct physiological challenge. These bees generate their own body heat to fly and regulate their internal temperature, but when the air gets too hot, they struggle to cool down. This stress can limit their ability to fly, find food, and care for their young. At the same time, the plants they rely on are also changing how they produce nectar and flowers under sustained heat. Understanding how these two sides of the relationship—the bee and the flower—react to long-term warming is critical, especially for farmers who depend on bees to grow food. If the heat becomes too intense, the delicate balance between a bee's need to forage and a plant's ability to reproduce could break down, threatening food security.

In a controlled experiment designed to mimic these future conditions, researchers placed small groups of eastern bumble bees and zucchini plants into growth chambers to see how they would fare under different levels of sustained warmth. They set up three distinct environments: a control group kept at a baseline temperature of 29.5 degrees Celsius during the day, a group warmed by an additional 1.5 degrees, and a group warmed by an additional 3 degrees. These temperatures were maintained continuously for about ten weeks, covering the entire active lifespan of the bee colonies. Unlike previous studies that only looked at short, intense bursts of heat, this experiment tested what happens when the heat never lets up. The researchers monitored the bees' behavior, their physical health, how they organized themselves inside their hives, and how the zucchini plants responded in terms of nectar production and fruit yield.

The results revealed that the bees did not simply get worse as the temperature rose; instead, their reactions changed depending on how hot it got. At the moderate warming level of 1.5 degrees above the baseline, the bees showed a clear ability to adapt. They began fanning their wings much more frequently—about twenty times more often than in the cooler control group—to cool the hive. However, this effort came at a cost: the bees spent significantly less time caring for their young. They were actively compensating for the heat, but they were doing so by shifting their energy away from brood care. Surprisingly, the total number of adult bees in the colony did not change, and the size of the male bees remained the same, suggesting that the colony could maintain its population size even while its workers were under stress.

When the temperature rose to the extreme level of 3 degrees above the baseline, the bees' ability to compensate began to fail. While they continued to reduce their care for the young, the organized structure of the hive started to fall apart. In the cooler groups, bees naturally clustered together in specific areas, particularly near the brood and the hive entrance. In the hottest group, this spatial organization was disrupted. The bees scattered, spending less time in the critical zones near the entrance and the brood. Furthermore, the queen bee in these extreme conditions weighed significantly less than queens in the cooler groups, dropping from an average of about 0.63 grams to just 0.26 grams. This indicates that while the colony could survive the moderate heat, the extreme heat imposed a heavy physiological cost on the queen and broke down the social coordination that keeps the colony functioning.

The plants in the experiment told a different, yet equally complex story. The zucchini plants in the hottest chambers produced nearly double the amount of nectar compared to those in the control group. This surge in floral reward attracted the bees; visits to the flowers in the hottest group were about 70 percent higher than in the control group. One might expect that more nectar and more visits would lead to more fruit, but the outcome was not so simple. Despite the increased activity, the number of zucchinis produced per female flower was actually lowest in the moderately warmed group. In the extreme heat group, the yield did not recover to match the control group, even though the bees were visiting the flowers far more often. This suggests that the heat was damaging the plant's ability to set fruit directly, regardless of how many bees were visiting.

The study concludes that sustained warming creates a disconnect between pollinator activity and crop production. The bees were working harder and visiting more flowers because the plants were offering more nectar, but this increased activity did not translate into a proportional increase in food yield. The bees' ability to thermoregulate and maintain their colony structure has limits, and once those limits are exceeded, the system becomes inefficient. For farmers relying on these bees, this means that simply having more bees visiting the crops may not be enough to save yields if the heat is too intense. The research highlights that as the climate warms, the relationship between bees and the crops they pollinate will become more fragile, with the bees' physiological limits and the plants' reproductive needs potentially pulling in different directions.