|B.lapidarius in my garden|
When people talk about the vital importance of bees for pollination and agriculture, the only species that is usually thought about is the honeybee. For many crops however, both those grown in field and those in glasshouses, bumblebees and other wild bees are vastly more important. While honey bees are pretty generalist feeders, visiting many types of flower, they are not good at pollinating many species and are incapable of pollinating some crops at all, of which the Solanaceae (tomatoes, chilli peppers, aubergines etc.) are the most obvious. These require “buzz pollination”, where the bees vibrate the anthers to release pollen. Other important crops which benefit from this type of pollinator rather than honey bees include blueberries, cranberries, and also kiwifruit. Even apples seem to be better pollinated by bumblebees that honeybees, as honey bees approach the flower in such a fashion that pollen is not efficiently moved from flower to flower. In addition, honeybees will not fly in cool or wet weather, which reflects their essentially tropical to subtropical origins. Bumblebees by contrast are adapted to cool climates, and will fly in cold weather or even in rain.
With many crops therefore it is extremely advisable to have an ample supply of appropriate pollinators available when the crops are in flower. Unfortunately while honey bees form large, long lasting colonies that can be transported by truck to where they are wanted, bumblebees form small colonies that only live a few months, and wild colonies cannot survive in sufficient numbers, if at all, in a modern agricultural landscape with large monoculture fields of a single species of crop which only flowers for a couple of weeks. In glasshouse grown crops, any bees outside have trouble getting in to pollinate them, and in the past this therefore required expensive hand pollination.
There are two solutions to this problem. Management of areas for bumblebees and other wild pollinators should benefit all insect pollinated crops, but for glasshouse crops it is now possible to buy in commercially produced bumblebee colonies. This last is now widely used, but this in turn has generated other problems. As there is little control over what species is used, there is a grave risk of introducing species outside their natural range, with severe risks to local wildlife. This is less to do with direct competition, and more a result of the commercial species bringing with them foreign diseases. In the UK alone for example, around 50,000 commercial colonies, mostly of B.terrestris subspecies, are imported each year, of which around 50% are of non-native subspecies. Globally around 1 million colonies are traded. There is strong evidence that a very high percentage of the traded colonies carry one or more bee diseases. As even glasshouse colonies inevitably leak workers to the outside, and mated queens from the glasshouse colonies also leave, these diseases can easily spread. Each bee will visit many flowers, and each flower will usually be visited by many bees, so parasites, viruses, and other diseases will spread to the native pollinator community, with sometimes catastrophic results. In North America, Franklins Bumblee, B.franklini, is only found in a fairly small area a few hundred miles across in Oregon. When surveyed in 1998 it was fairly widespread, but today it is certainly extremely rare and very probably extinct, almost certainly a result of introduced diseases. A similar fate appears underway in Argentina for the world’s largest bumblebee, B.dahlbomii, as a result of diseases transferred from the introduced B.terrestris.
The other threat to bumblebees is change in agricultural practises. Before the advent of modern nitrogenous fertilizers, the usual way to restore soil fertility was to grow a crop of clover. These fix nitrogen in their roots nodules, and also provide nectar and pollen for many bee species, especially the long-tongued species. Today artificial fertilizers remove the need for an interval of clover in crop rotation, and seed cleaning techniques and herbicides eliminate crop weeds that also provided food for wild pollinators. In the UK, many of the twenty four species are now very restricted in range, with only a few generalist species being seen over most of the country.
One species has now been reintroduced to the UK, the Short-haired bumblebee B.subterraneus at Dungeness. This species was last recorded in 1998, but in the last few years queens collected on emergence in Sweden have been released and are known to have successfully established colonies, as worker bees have been seen foraging in the restored habitat.
A second species has established itself naturally. The Tree Bumblebee, B.hypnorum, has a range from France across to Kamchatka in the east, and in 2001 was first detected in the UK. Since then it has spread widely and is often found associated with settlements, probably as a result of its tendency to use bird nest boxes. In 2008 it even reached Iceland, and is now regular around Reykjavik and other areas. It is a very distinctive species, with a ginger thorax and a black abdomen with awhite tip, so is easily recognised in your garden.
For more on bumblebees, and issues relating to them, these links may be interesting:
Short-Haired Bumblebee reintroduction: http://www.bumblebeereintroduction.org/
Bumblebees in the UK: http://bumblebeeconservation.org/
Next time, new arrivals
Images from wikipedia