Orange-belted Bumble Bee

Appearance

"B. ternarius" is a small, fairly slender bumblebee. The queen is 17–19 mm long and the breadth of the abdomen is 8.5–9 mm. The workers are 8–13 mm, and the drones are 9.5–13 mm in length. Both the worker and the drone have abdomens about 4.5–5.5 mm in breadth.

The queen and workers have black heads, with a few pale yellow hairs. The anterior and posterior thorax and the first and fourth abdominal segments are yellow, abdominal segments 2 to 3 are orange, and the terminal segments are black.

The queen and the workers are close in resemblance, and the most striking difference between them is in the size of their fat deposits. Workers have very little fat, particularly in their abdomen, leaving plenty of room for the honey stomach, an enlargement of the oesophagus in which nectar can be stored on foraging trips. In contrast, in young queens, the abdomen is largely full of fat. This leads to queens being heavier for their size than workers.

The drone has a yellow head with a few black hairs. The coloration of the thorax and abdomen is similar to that of the females, with the exception that the last abdominal segments are yellow on the sides. The fur of the drone is longer than that of the females.

"B. huntii", another species of bumblebee common throughout the western United States, is nearly identical in coloration to "B. ternarius", though it has primarily yellow facial hairs rather than black.

Naming

"Bombus ternarius" was first named by Thomas Say in 1837. "Bombus" is Latin for buzzing, and refers to the sound the insects make. The specific name "ternarius" refers to the number three, which refers to the bumblebees' three colors.

Behavior

Queen and worker bumblebees can sting. Unlike honey bee stingers, a bumblebee's stinger lacks harpoon-like barbs on the end of the stinger, so "B. ternarius" can sting repeatedly without risk of disemboweling itself and dying. "B. ternarius" is not normally aggressive, but will sting in defense of its nest or when threatened or provoked."B. ternarius", as well as other members of the "Bombus" genus, live in eusocial colonies in which the individuals in the group act as a single multiorganismal superorganism. Eusociality may have evolved in the bumblebee ancestor as a result of offspring remaining in the nest as adults to help rear their mother's young. The evolution of eusociality can be explained by Hamilton's inclusive fitness theory. The mostly sterile workers forage for food and take care of the colony's needs, while the queen is in charge of reproducing and creating new generations of workers. Toward the end of the colony lifecycle, workers jostle the queen, eat her eggs, and attempt to lay eggs of their own. The workers are not completely sterile, despite their inability to mate, since they have ovaries. Worker eggs always develop into males. The queen usually retaliates by acting aggressively toward the workers and trying to eat the workers’ eggs. However, the queen's retaliation proves insufficient in some cases and the aggressive reproductive bumblebee workers kill her.

Reproduction

Drones have one function in life: reproduction. They fly in a circuit and deposit a pheromone on prominent places such as tree trunks, rocks, posts, etc., to attract the newly hatched queens. A new queen follows the pheromone trail and mates with the male.

Mating among "B. ternarius" typically occurs on the ground or in vegetation. The male mounts the female by grabbing her thorax, the queen then extends her stinger and the male inserts his genital capsule. Mating time varies widely from about 10 to 80 minutes, with the sperm being transferred within the first two minutes of copulation. While mating, both the male and female are vulnerable to predators. After the transfer of sperm is complete, the male secretes a sticky substance that hardens into a plug that blocks new sperm for about three days. This prevents other males from impregnating the same queen and competing to fertilize eggs. The plug helps reduce competition and increases the first male's evolutionary success.

Individual colonies have slightly different reproductive strategies. About half of the queens adopt an early switching strategy that produces mostly male drones, and the other half adopt a late switching strategy, giving rise to mostly new queens. Because of this, bumblebee female to male sex ratio differs from the standard Hymenoptera three to one ratio as proposed by the haplodiploidy hypothesis, and sits closer to an even sex ratio. One hypothesis proposes that the reason why bumblebee's sex ratio differ from the characteristic hymenopteran ratio is because of the queen's decision-making. She can decide to adopt an early switching, male-producing strategy, or a late switching, queen-producing strategy. The worker bees have no choice but to go along with the queen's choice. Normally, the workers would work to shift the ratio towards female prevalence, but in the case of "B. ternarius", this would disfavor the workers' evolutionary success.

Defense

In late April, the queen comes out of hibernation from under a few inches of loose soil or leaf litter, and begins to search for a nesting site. "Bombus ternarius" prefer to nest underground in small and shallow cavities like rodent burrows or natural crevices. She flies low to the ground, stopping often to investigate holes in the earth, and once a satisfactory nest site is found, she forages for pollen and nectar to support her future offspring. Next, the queen secretes a protective waxy coating and builds a grove where she lays fertilized eggs destined to be the first of the new workers. The queen straddles the eggs, allowing for close contact between the ventral surface of her abdomen and thorax and the eggs. This close contact allows the queen to incubate her brood with the heat she generates by pulsing contractions in her abdomen.

These eggs progress through four lifecycle stages starting as an egg, then larva, pupa, and after about a month after laying the egg, the adult workers emerge. Considering that the entire lifecycle of a colony is only about a season long, incubation is necessary because it hastens the development of the first workers. However, the generation of such vast amounts of heat is incredibly costly for the queen. The queen uses an estimated 600 mg of sugar per day to incubate her brood. To obtain this amount of energy, she may need to visit as many as 6,000 flowers. Naturally, during her absence, the brood cools rapidly, so the availability of plentiful and rewarding flowers near the nest site is vital.

The newly hatched workers take over the duty of foraging and expanding the nest. The workers also assist in incubation of the eggs and larvae. "B. ternarius" nests rarely exceed more than 200 individuals.

In late summer, the queen switches to laying unfertilized eggs, which develop into male drones that are meant for reproduction. Towards the end of her life, the queen reverts to laying fertilized eggs. These eggs give rise to new female queens. The new adult queens forage for food. They use the nest for shelter, but the new queens do not contribute to the nests food reserves. During this time, the new queens mate with roaming male drones, build up reserves of body fat, and fill their nectar crop with honey to survive winter hibernation. The rest of the colony, including the old queen, dies in mid-autumn.