As global commerce grows, so too does the movement of species outside their historic ranges. Some invasive species dramatically reshape invaded habitats and restructure the interactions between native species. Anthidium manicatum, the European wool-carder bee, may be one such invader. It is the most widespread unmanaged bee in the world, first appearing in North America in the early 1960s. Concern has been raised about its rapid range expansion, and its aggressive behavior toward native species. A. manicatum males defend floral territories, using evolved abdominal spines to attack intruders. Bumble bees (Bombus, spp.) are the most commonly attacked native species. Over the course of two years, we asked two main questions:
Year 1: Are A. manicatum successful in excluding native bumble bees from resources? If so, do bumble bees avoid males and/or females?
Year 2: Will bumble bee response to A. manicatum change with continued exposure?
To answer these questions we used experimental enclosures containing floral resources. Bumble bee hives were installed with tubes allowing access inside the enclosure and to the surrounding environment. Over the course of two years, various treatments of A. manicatum within these enclosures were used to determine if their presence influenced bumble bee foraging behavior.
Results/Conclusions:
In year 1, there was a significant decrease in bumble bees in all enclosures with A. manicatum present across three treatments types: male only A. manicatum, female only A. manicatum, and male and female presence (GLMM; Male only: AIC=852.3, **p=0.0035, Female only: AIC=502.4, *p=0.0143, Male and female: AIC=900.7, ***p<0.001). While an effect was seen in all treatments, the strongest effect was seen when both male and female A. manicatum were present. In year 2, half of the enclosures remained devoid of A. manicatum and the other half contained both male and females. Bumble bees again showed avoidance behavior, and this effect did not diminish with continued exposure over six weeks (repeated measures ANOVA, p=0.0155).
Average lifespan of a B. impatiens colony is between 8-10 weeks, and both species (B. impatiens and A. manicatum) share peak activity months (June-Sept). The effects of A. manicatum on local bumble bee foraging patterns is therefore likely to persist for a large portion of the colony life cycle. These findings are concerning as bumble bees are important native pollinators that already face habitat destruction from many sources (urbanization, agriculture, etc.). Additional exclusion from resources driven by A. manicatum will likely hurt bumble bee conservation efforts.