Mae Carson, of Otaki, asks :-

What is the life cycle of the bumble bee and does it pollinate plants?

David Pattemore, a plant pollination biologist with Plant and Food Research, responded. He has a special interest in developing ways to use bumble bees for crop pollination and you can read about the bumble bee research programme he leads in the June 2015 issue of New Zealand Geographic or google his name to find the TEDx talk he gave in July 2015 in Tauranga.

Bumble bees are colonial bees. A queen and her daughters live in a colony, gathering nectar and pollen from flowers and raising young bees in wax cells, similar to honey bees. Like all bees, bumble bees have evolved a special relationship with plants, where they take sugary nectar and nutritious pollen from flowers and so pollinate the flower so that the plant produces seeds.

But a bumble bee colony only stores a small amount of honey, not like the large honey stores in a honey bee hive. This important difference in honey stores plays a role both in the difference between honey bee and bumble bee life cycles and also in the difference between the bees in their role as pollinators.

A bumble bee colony, unlike honey bees, lives for less than a year. While honey bee colonies survive through winter with their stores of honey, a bumble bee colony dies off at the end of each summer. The only bumble bees that survive are the newly hatched queen bumble bees who mate and then look for a hiding spot in the ground where they will hibernate over winter. All the rest of the bees in the colony die.

Each spring, the queens emerge from hibernation when the temperatures start to rise and the flowers start to bloom. Initially a queen will just forage for herself, building up her energy reserves with nectar. But soon they start looking for a new hole in the ground to start a nest. In spring, you often see these queens (which are much larger than worker bumble bees), zig-zagging low over the ground and even coming into buildings as they look for nest sites. Once they find a good site, they start building a colony from scratch: laying eggs, building wax cells and collecting nectar and pollen.

The eggs hatch into worker bees, who are all females; sisters. These workers start looking after the nest and forgaging for nectar and pollen to support the colony. As the colony grows, the queen stops doing much apart from laying eggs, and the workers take over all the pollen- and nectar-collecting work.

These colonies can grow until they have many hundreds of workers. While this is large, it is no way near as large as honey bee colonies, which have tens of thousands of workers.

But a strong bumble bee colony can make a big contribution to pollination, despite its smaller size. Part of the reason for this is the larger size of bumble bees, who need to visit more flowers in each trip. Visiting more flowers means that each bee pollinates more flowers than an individual honey bee.

Another important reason bumble bees make great pollinators is linked to the size of their honey stores. Honey bees store large amounts of honey, so that when the weather is cold, wet and miserable, they can stay warm and dry in the hive feeding off their honey. Bumble bees store much less honey, and only have a few days back-up supply. So in changeable spring weather, they still need to head outside to visit flowers even if the weather isn’t great. This extra pollination service they provide in poor weather (as well as mornings and evenings) can make a significant difference for many plants.

As the colony gets large in summer, changes start to occur. The queen gets older and weaker, and the workers start to get a little aggressive towards each other. At the same time, the colony starts producing males and the new generation of queens. These young queens and males leave the nest to mate with other bees from other colonies.

It is at this point that the colony starts to die. Eventually, by autumn, no more workers are produced, and the colony disintegrates. All that is left are the new queens who look for somewhere to hibernate and begin the cycle again.

Both honey bees and bumble bees were introduced by humans in New Zealand: they are not native. We do have native bee species, but none that form colonies like honey bees and bumble bees.

Four bumble bee species were introduced to New Zealand. The most common species, the buff-tailed bumble bee (Bombus terrestris), is found throughout the North and South Islands, and colonies of this species can be bought to place in your garden. The other three species are less common, with one restricted to just a few locations in the South Island.

As it is now spring, look out for the new season’s large queens as the fly around fueling up after their long winter and start searching for holes in the ground to set up their new colonies.

For more information on this topic you could see the June 2015 issue of New Zealand Geographic or google my name to find the TEDx talk I gave in July 2015 in Tauranga.

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