It feeds on several food plants, favoring sunflowers and clovers,[6] and functions as a pollinator.

Males have an outward penis valve head with a broad banana shape[6] and often have extensive yellow on the thoracic dorsum posteriorly and beyond abdominal tergite T3[3] Whilst bees normally forage close to their nests, it has been observed that individuals can return from up to 1.5 miles away, though there is likely individual variation in homing ability and the time it takes a bee to find its way back to the nest.

The method utilized by the bees is most likely trial and error, not a sixth sense or homing instinct, because the time taken to return to the nest varies.

The species utilizes bundles of hay or long grass to create sheltered nests above ground.

The fertilized queen stays in hibernation until spring of the next year, waiting for the optimal conditions to search for a nest.

In March, the queen bee gathers pollen and nectar, as a source of nutrition and to build a wax pot, and establishes her colony.

[14] Female workers develop through the pollen collected as it stimulates the ovaries to create eggs, which are fertilized from the males of year before.

[13] Initial workers forage and increase the colony size by bringing resources for growth.

As worker male bees grow larger in average wing length, they become the dominant caste as the number of queens decrease.

Until July, the workers are the foraging caste and an enormous increase in body size is observed.

The worker proportions decrease when reproductive males develop, representing a turning point in the colony, as male size increase until active bees develop to the size of a queen near November and December when the worker population dissipates.

Queens maintain the least amount of standard deviation for average wing length and thus are the most stable caste in the colony.

It is postulated that this is because queens are made in a short time span when colony resources have reached its threshold.

[14] Although Bombus pensylvanicus maintains aposematic coloration and defensive stinger, it faces many predators.

Predation is likely to be caused by attack to gain the resources of the hive, which contains carbohydrate and protein abundant nectar, larvae, and pollen.

Mallophora bomboides is a robber fly species that preys specifically on B. pensylvanicus and uses it as a model for Batesian mimicry.

The fly then pupates and spends the winter inside the bee, fully developed, before it emerges the following year.

Tracheal mites (Locustacarus buchneri) leads to reduced foraging efficiency by living in the bee’s alveoli.

Certain protozoans and fungi consume the host tissue or gut substances of the bumble bee’s digestive tract, decreasing foraging efficiency, life span, and thus the colony fitness.

Foragers with short corollas and shorter proboscises (tongue) were discovered in mixed species stands.

Ranges of Bombus pensylvanicus have specifically decreased in Illinois, coinciding with agricultural investment within the state.

These results were expected because of the recent decrease in population, which would cause declines in genetic diversity for severe bottleneck situations.