Bombus transversalis

While the Bombus transversalis is unique in its distribution, nesting, and foraging, it is still a social bee. It has been shown to have similar behavior to that of the Bombus terrestris, a well-studied bumblebee.[5] It was thence shown that although the two bees are distantly related, the Bombus transversalis still shows behavior of alerting its colony about food sources leading to increased activity in and out of the nest.[5]

It has been observed that B. transversalis uses walking trails similar to those of ants.[3] These trails extend out from the nest in opposite directions and are patrolled by at least 20 workers who collect materials and forage along the trails.[3] Workers on the trail cut leaves and debris into tiny pieces and push them to the side as they create trails. It has been observed that workers do this process in pairs, as they push debris to their fellow worker behind them.[4] As such, lines of five or more bees have been observed walking along trails nearby their nest. One purpose of these trails is to forage. This is unique because most bees forage via flight.[3]

The structure of the Bombus transversalis nest is very complex. It is cone-like in shape with two separate combs.[7] Oftentimes it is built off of existing nests, so there are many unused cells. The brood comb is built at the top and is surrounded by large honey silos. Some are filled with honey, but most are empty, as there is an abundant amount of storage.[3] There is one single entrance to the colony.[7] It is open and uncamouflaged but is usually guarded by 2-5 worker bees.[7] Entirety of the colony is covered in a heap of twigs, leaves, and roots which serve as a canopy. This heap is very dense and waterproof. It also provides an air space which helps the bees manage the temperature and humidity of the nest.[4]

Another purpose of the trails created by Bombus transversalis is to build up the structure of the nest. Using the twigs, leaves, and litter on the forest floor, worker bees cut up the debris into fragments and then bring these pieces back to the nest to build onto the canopy. As long as the trails are continuously being used, workers will continue to clear the trails even if leaves fall from the trees above. As such, the trails which have been observed of the Bombus transversalis serve a dual purpose in both foraging and nest construction.[4]

A queen Bombus transversalis must choose its nesting site very carefully if she hopes to have a successful colony. First, she must choose an area which has ideal soil and will not flood when it rains a lot.[4] Secondly, she must find adequate structural support for her nest. Oftentimes, this will take form in the roots of a large tree or among saplings and shrubs. This will give the workers a base to build the canopy for the colony and provide adequate shade for the nest.[7]

Since Bombus transversalis builds its nest on rainforest floors, it is very susceptible to predators on the ground. One such predator is the army ant Eciton hamatum. In a 100m² area on the forest floor, there is a 50% chance of being raided by army ants over an eight-month period.[2] The main defensive mechanism of Bombus transversalis against these attacks is through its nest structure. The colony is covered with a dense canopy of leaves and roots. So, the only way to invade the colony is through the single nest entrance which remains guarded if it is not raining. While this does not prevent all attacks, it is an effective mechanism at times.[2]

Bombus transversalis is known to be parasitized by moths. The process of this is puzzling since the nest only has one entrance and is usually well-guarded. Somehow the moths are able to sneak through the entrance or are willingly allowed to pass through into the nest.[8] At that the point, the moth larvae will then feed on the host cells. Through observation of the Bombus transversalis and these moths, a new parasitoid wasp in the genus Apanteles was also discovered.[8]