Bumble bees (Bombus species) are a globally recognized group of fuzzy, hard-working insects that perform irreplaceable ecological functions. The question of whether bumble bees are invasive depends entirely on the specific species and how it arrived in a new location. While most native populations are suffering decline, certain commercial practices have led to the intentional introduction of non-native species. These introduced species can escape, establish self-sustaining wild populations, and behave invasively, posing a serious threat to local ecosystems.
What Makes a Pollinator Species Invasive
An invasive species is defined as a non-native organism whose introduction causes or is likely to cause economic or environmental harm. For a pollinator, this status is earned through two primary mechanisms of ecological disruption. The first is direct competition for finite floral resources, where the introduced bee is a more efficient forager that effectively starves out native species. Studies show significant overlap in the plants visited by introduced and native bees, reducing the food supply for local populations.
The second mechanism is the introduction and subsequent spillover of pathogens. Managed or commercial bees, often kept at high densities, can act as reservoirs for diseases and parasites for which native bees have no natural defense. When commercial bees escape and forage alongside wild bees, they can transfer these pathogens via shared flowers. This pathogen transfer, or spillover, is a major contributing factor to the decline of native bumble bee species.
Native Bumble Bees and Their Ecological Importance
The vast majority of bumble bee species found in any given region are native and are ecologically beneficial, having co-evolved with local flora. These native populations are necessary for the reproduction of certain plants, including many wildflowers and crops. Native bumble bees are particularly important for a specialized pollination technique known as “buzz pollination” or sonication. The bee grabs the flower and vibrates its wing muscles at a specific frequency to shake pollen loose from the anthers, a method required for many plants like tomatoes, peppers, and blueberries.
The ability of native bees to perform this specialized task makes them highly valuable to agriculture, with their pollination services estimated to be worth billions annually. Many of these native populations are currently facing severe declines due to factors like habitat loss, pesticide use, and climate change. For instance, a recent study found that the populations of 11 out of 21 native North American bumble bee species have declined by 50 percent or more. Endangered species, such as the Rusty-patched bumble bee, illustrate the fragility of these native populations.
Harm Caused by Non-Native Commercial Introductions
The primary ecological threat comes from the commercial practice of rearing and transporting non-native species for greenhouse pollination. The buff-tailed bumble bee (Bombus terrestris), native to Eurasia, is the most widely used species globally for this purpose. These managed colonies are used to pollinate crops like tomatoes within greenhouses, but workers and queens frequently escape and establish wild colonies.
The escaped B. terrestris has proven to be highly invasive, such as in South America after its introduction to Chile in 1998. This non-native bee possesses characteristics that enable it to outcompete local fauna, including a higher reproductive capacity and a generalist foraging strategy. The most significant harm, however, stems from the pathogens carried by commercial colonies, such as the intestinal protozoan Crithidia bombi.
Pathogen spillover occurs when these infected commercial bees interact with susceptible native species on shared flowers. Transmission from commercial hives can infect a significant percentage of wild bumble bees within a two-kilometer radius of a greenhouse. The resulting infections severely reduce the colony-founding success and survival rates of native queens and workers, contributing to the decline of native species near commercial operations.