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Two bees or not two bees? How wild bees feel the sting of honeybee competition

Researchers from the University of Florence and University of Pisa conducted an experiment demonstrating that managed honeybees (Apis mellifera) on Giannutri Island significantly deplete nectar and pollen availability, leading to around an 80% decline in wild bee populations. Results provide strong causal evidence that honeybee competition, rather than environmental change, is responsible for wild bee decline.

Both wild and managed bees are essential pollinators. High densities of managed honeybees have previously been linked to wild bee population declines, yet studies have struggled to separate correlation from causation in understanding the impact of honeybees. Most prior research has relied on observational data rather than direct experimental manipulation.

Giannutri Island, a 2.6 km² protected ecosystem within the Tuscan Archipelago National Park, has hosted managed honeybee colonies since 2018.

With 18 hives introduced annually from December to June, honeybee densities exceeded the European average, raising concerns about competition with native pollinators. Researchers sought to determine whether honeybee competition directly contributed to wild bee population declines.

In the study, "Island-wide removal of honeybees reveals exploitative trophic competition with strongly declining wild bee populations," in Current Biology, the team conducted a field experiment to assess the impact of managed honeybees on wild bee populations.

Experiments took place across Giannutri Island and focused on two wild bee species: Anthophora dispar, a solitary wild bee, and Bombus terrestris, a social bumblebee.

To measure competition, researchers performed temporary honeybee-exclusion experiments by closing hive entrances before sunrise for 11 hours per day during the peak wild bee foraging season.

Floral resource availability was monitored by measuring nectar and pollen levels in two key plant species, Teucrium fruticans and Salvia rosmarinus. Wild bee behavior was recorded, including flower visitation rates, foraging times, and movement patterns. Long-term population trends were tracked using standardized transect surveys from 2021 to 2024 to assess overall species decline.

Short-term honeybee exclusion increased floral resource availability. Nectar volume rose by 52.5% in T. fruticans and 71.1% in S. rosmarinus. Pollen availability in T. fruticans increased by 35.7% near the apiary, with a smaller increase observed farther away.

Wild bee foraging behavior changed significantly in response to honeybee presence. Without honeybees, wild bees increased nectar intake, reduced search times, and altered their foraging schedules, indicating prior resource limitations. Interaction patterns within the pollination network shifted, with wild bees assuming a more dominant role in flower visitation during honeybee absence.

Population monitoring from 2021 to 2024 revealed a severe decline in both wild bee species. A 77% reduction in A. dispar and an 87% reduction in B. terrestris were recorded.

Climatic data from the same period showed no significant changes in temperature or precipitation during wild bee activity months, indicating that resource competition rather than environmental shifts was the primary driver of decline.

Experimental results demonstrate that managed honeybees directly compete with wild bees for nectar and pollen, reducing resource availability and leading to long-term population declines.

Short-term exclusions confirmed that honeybees significantly alter wild bee behavior, leading to increased foraging costs and potentially lower reproductive success.

The authors recommend that beekeeping should not be allowed in protected areas unless there is clear evidence demonstrating no harm to wild bee populations.

Their findings suggest that high densities of managed honeybees can amplify environmental stressors, particularly in fragile ecosystems such as small islands.