Two-spotted bumble bee

Male B. bimaculatus can help care for larvae during the first days, or even weeks, of their life. Though female workers are mainly responsible for brood care, males cannot fly for the first 24 hours of their life so they cannot leave the nest. Incubating larvae is a potential opportunity for males to exercise their flight muscles. They assume the same position on the cocoon as females would and pump their abdomen to facilitate heat flow from their thorax to their abdomen to the brood. Male incubation may be more significant towards the end of the season when there are fewer workers to incubate larvae.[11]

B. bimaculatus mate outside the nest with males patrolling in circuits, searching for a queen to mate with.[7] Most queens only mate once; however, there are some queens who mate multiple times and have offspring of multiple paternity.[12]

B. bimaculatus queens forage on Aquilegia flowers. The queens hang upside down on stamens, clutch the filament with their forelegs, and scrape off pollen using their middle and hind legs. Workers enter the Aquilegia spurs by pushing their head and part of their thorax into the spur's mouth. They then extend the maxillae and tongue into the spur to drink nectar, repeating this process on multiple spurs of the same plant before visiting the next one.[13]

B. bimaculatus queens' probosces range from 10.53 to 12.19 mm in length, which has little overlap with other Bombus species. This may potentially be the reason why B. bimaculatus are such abundant pollinators.[13]

Research comparing B. bimaculatus to Xylocopa virginica, a carpenter bee, found that the former learned faster and had more flexible foraging patterns. It was hypothesized that B. bimaculatus being a social bee could have individuals specialize in either foraging for nectar or pollen, instead of having to worry about both in overall food collection as the nonsocial carpenter bees needed to do.[14]

Bumble bees eat nectar and pollen from plants. B. bimaculatus is known to pollinate a wide variety of plants, but they seem to have favorites. Queens can be found on willow and plum. Workers are found on red clover and mint. Males are found on mint and sweet clover.[6]

Zenobia pulverulenta, a plant that B. bimaculatus forages on

As a species they have been found foraging at the following plants:

• Zenobia pulverulenta[15]
• Gelsemium sempervirens[16]
• Lyonia lucida[17]
• Solanum dulcamara[18]
• Aquilegia species[18]
• Dicentra species[13]
• Mertensia species[13]
• Pedicularis species[13]
• Aesculus glabra[13]
• Camassia scilloides[13]
• Delphinium tricorne[13]
• Hydrophyllum appendiculatum[13]

A Florida black bear, which preys on B. bimaculatus

Predators of bumble bees include crab spiders, Florida black bears, ambush bugs, robber flies, dragonflies, assassin bugs, and some wasp species.[7] Crab spiders ambush B. bimaculatus at flowers, paralyze them, and then eat them.[7] Florida black bears eat B. bimaculatus most abundantly in the spring, and continue to eat them to a lesser extent in the summer.[19]

B. bimaculatus will defend its nests against intruders, such as Psithyrus variabilis, a cuckoo bumble bee. In an experiment, a female P. variabilis was placed in a B. bimaculatus nest. Workers quickly recognized her as an intruder, halted their work and attacked her when she entered the inner part of the nest.[20]

Oftentimes though, B. bimaculatus are content to ignore intruders, such as Psithyrus labrosius. Like P. variabilis, P. labrosius also is a cuckoo bumble bee. It will attack Bombus vagans, but not B. bimaculatus though they are in the same subgenus, Pyrobombus.[20][21]

B. bimaculatus queens can kill each other when dueling. Queens can also be hostile to unrelated workers of their own species by squirting feces in their faces.[10]

Brachycoma sarcophagina is an ectoparasitoid that will consume B. bimaculatus bees from the outside. Female B. sarcophagina deposit young larvae on B. bimaculatus larvae. B. sarcophagina larvae will not begin consuming their host until the host has begun spinning its cocoon.[22]

Tracheal mites, will parasitize multiple Bombus species, but strongly prefer B. bimaculatus. Mites were recovered from the autosomal air sacs of bumble bees. These mites can affect behavior and reduce longevity, which may cause further stress to colonies already facing difficulties.[23]

Conopid flies also parasitize B. bimaculatus. Male bees were less likely to be parasitized than workers, and larger bees were more likely to be parasitized than smaller bees.[24]

B. bimaculatus is also parasitized by a bumble bee of the subgenus Psithyrus, Bombus citrinus, a brood parasite.[7]

Kuzinia, Scutacarid, and Parasitid mites were found on B. bimaculatus bees. Scutacarid and Parasitid mites were found in the propodia, the first abdominal segment in bees.[23] These mites, being phoretic, likely just use the bee as a means of transport. It is unknown what effect, either detrimental or beneficial, these mites may have on the bee.

B. bimaculatus can be infected with Nosema bombi, a microsporidian. This bee can also be infected by other fungal species. It is unknown how detrimental such an infection is to the bee's health.[23] Compared to other bumble bees that are in decline, B. bimaculatus has a lighter infection, which may be the reason why it is experiencing stable growth compared to the decline in other Bombus species. However, the reason for the difference in infection is unknown.[25]

B. bimaculatus can also be infected by Crithidia bombi and Apicystis bombi. Both are protozoans, but C. bombi is known to hinder colony creation, decrease host lifespan and colony fitness, and have a negative effect on workers' behavior. This can cause undue stress on colonies, leading to a species' decline.[23]

The gut bacteria of B. bimaculatus was isolated and include Snodgrassella alvi and Gilliamella apicola strains.[26] B. bimaculatus had more gut bacteria from environmental sources, and less core bacteria compared to other bumble bee species. B. bimaculatus had more core bacteria when collected from farms as opposed to collection from semi-natural habitats.[27]

B. bimaculatus is an important pollinator in temperate forest regions as it is still abundant, unlike many other species of honey and bumble bees. They also pollinate a wide variety of plants. In addition, bumble bees can continue foraging even under sub-optimal conditions such as rain or clouds.[5] B. bimaculatus is capable of flying even at 7 °C.[28] This makes the continued growth and stability of B. bimaculatus particularly valuable.

Only female bees have a sting, males do not. Bumble bees typically only sting when defending their nest or when captured. Allergies to bumble bee stings are much less common than allergies to honey bee stings though the venom composition is similar. B. bimaculatus venom contains additional proteins, including acrosin and a tryptic amidase related to clotting enzymes.[29]