Bumble Bees Part 2: Life History

Note: This program first aired on July 9, 2016. 

A worker bee, temporarily interrupted from foraging.

I’ve been spending a lot of time lately thinking about bumble bees, watching which flowering plants they visit and which they don’t, looking at them trying to discern one species from the next, and learning when they like to fly and when they don’t. I’ve volunteered to survey bumble bees for the Maine Bumble Bee Atlas project and the learning curve has been steep. The entomologists who direct the project are evidence of the fact that the rabbit hole you can go down when you open your eyes and start to see the insect world all around you has no bottom. Arthropods are the most successful phylum of animals on the planet, and there is no shortage of wonder to be pursued in their diversity. For me it seems bumble bees may just be the beginning.

One place to start is with their life history. I’ve kept honey bees on and off for a few years and thought that bumble bees worked the same way, albeit on a smaller scale. I was wrong. Bumble bees, like honey bees are true eusocial insects, meaning they live in colonies that have castes of workers with behavioral specialization, communal care of brood, overlapping generations, and reproduction limited to a few specific individuals (often a single queen and specially raised males). Bumble bees live this eusocial life in a way very different from the European honey bees many of us bee keepers are familiar with.

Bumble bee colonies over winter as queens, the large, mated reproductively active females that are raised and mated the fall before. When the weather turns cool these individuals search out an overwintering spot, in leaf litter at the edge of the forest. These are well fed bees, having been raised on the bountiful nectar and pollen from the late season golden rods and asters. These queens emerge in the spring and are the stock from which the new colonies form. The only bumble bees that survive the winter are the queens, colonies do not over winter, workers do not over winter. Only single bees do, prepared to start a brand new colony of their own the next year.

So those first really big bees you see flying in the spring are the over wintered queens. They emerge and look for a spot in which to nest, a spot that will house their  modest colonies for the summer. Old rodent burrows are especially good spots, as apparently are the seat cushions of the old abandoned cars in the woods you see frequently in Maine as early 20^th century farmsteads are reclaimed by forest. Once a queen finds a good nest cavity, she lays the first of her eggs (remember she mated the fall before, so she has all the sperm she needs to lay fertilized eggs), She incubates them herself by generating heat shivering, and feeds the larva nectar and pollen when they hatch. One bee foraging to feed several hungry babies though does not quite cut it, so the first round of bees that are produced by the queen are quite small. These are the first worker bees you will see in the spring, they look stunted, and quite literally they are—nutritionally they got enough to survive but not really thrive. Once there are more workers in the colony, the subsequent larvae get fed better, and the resulting bees are bigger. This pattern continues throughout the summer, the queen lays eggs, staying in the nest once there are enough worker bees to do the foraging, the workers out in the field collecting the nectar and pollen needed to sustain the hive. Workers live around 25 days, and a typical hive has between 50-100 bees when up and running during the summer.

At the end of the summer, two different kinds of bees get produced by the hive. The first are males—these come from unfertilized eggs. They have one purpose only, that next year’s queens can get fertilized before hibernating. The others are the new queens. The last batch of worker bee eggs that are laid become the queens for next year. Theoretically the hive is at its highest capacity at the end of the summer, the land is covered with golden rod and asters and thus there is plenty of forage, and many workers able to feed these up and coming queens. Once these very large nascent queens emerge, they mate with the males which also unsurprisingly emerge at the same time. As fall progresses on, the old queen, the founder of the colony, dies. Her daughters, the worker bees, all die. Her sons, the males, all die. The only bumble bees that don’t die are the new queens, well fed and stocked with sperm, ready to over winter in the leaf litter until the process starts over again in the spring.

It’s a pretty remarkable process, and something to consider when you clean up your yard in the fall. Do the bumble bees a favor, leave those leaves where they are until the spring, in doing so you create safe space for the potential bumble bees of the future.

References:

On eusocial insects:  https://www.amentsoc.org/insects/glossary/terms/eusocial 

The Maine Bumble Bee Atlas project: https://ac41277bd54735c2b85336aff6fdfbfd153fcee3.googledrive.com/host/0B985dSJVRA1maGtVekhDUkJjYWM/