Bumblebees have declined at Wicken Fen. In the late 1920s Spooner found fourteen species of Bombus, the true bumblebees especially near the north-eastern margin of the Fen, but today there are only seven -, in addition to three species of' cuckoo bumblebees, Psithyrus. Between 1930 and 1978 seven species of true bumblebees disappeared from the Fen. A single specimen of Bombus distinguendus, a male. found on Buddleia outside the William Thorpe Building in 1980, was probably a casual vagrant.

The well-documented losses of bumblebees at Wicken Fen mirror those in much of east central England. Throughout the region, bumblebee species present before 196o have been lost, leaving only the seven common species that are widespread throughout Britain. These losses, and the parallel decline in Germany, Belgium and France, have been tentatively attributed to agricultural development. In Kent, where the species that have declined here still persist, they are associated particularly with open habitats undisturbed by human activity, such as salt-marsh. sand-dunes, shingle and old meadow. Evidently Wicken Fen, even if it is still suitable for these species, does not provide enough resource, to allow. them to persist locally and reinvasion is unlikely as there are now no nearby sources of colonists.

A population of bumblebees requires four major resources: flowers as sources of nectar and pollen; nest sites; hibernation sites; and habitats for the male ‘patrolling’ behaviour that precedes mating. Species differ from one another in their requirements: some prefer deep flowers while others visit open ones; some nest at the soil surface while others nest underground; the male patrol-routes of some species go from tree-base to tree-base while those of others follow hedges. However, all benefit from open sunny habitats where the absence of ploughing allows the establishment of nectar-rich perennial plants; from well-drained rough land for nest sites; from dry banks in which to hibernate, and from a diversity of sheltered habitats with microclimates suitable for patrolling males.

Generally, long-tubed flowers are used by the long-tongued species Bombus pascuorum and B. hortorum; the short-tongued species B. terrestris and B. lucorum, and the agile little B. pratorum, use shorter flowers unless they can reach the nectar by robbing, through a hole bitten in the corolla; and B. lapidarius favours massed flowers and is often abundant in the Breed Fen hay meadow on common knapweed (Centaurea nigra) until mowing removes its forage. Wicken Fen is well provided with nectar-rich perennial flowers with a range of corolla depths, suiting a range of bumblebee species. In the surrounding farmland, forage of comparable value is probably largely confined to the banks of lodes and ditches.

Common comfrey (Symphytum officinale), a major forage source for bumblebees throughout Britain, is particularly important at Wicken Fen because of its abundance and its long flowering season. It is a rewarding flower on which to watch bumblebees, because of the diverse patterns of flower manipulation shown by different bee species. Nectar is secreted at the base of the tubular corolla. Bombus pascuorum and B. hortorum can reach the nectar with their long tongues. B. terrestris and B. lucorum have such short tongues that they must bite a hole in the base of the corolla tube. These holes are sometimes re-used by the little B. pratorum, which must otherwise thrust its narrow head into the corolla to reach the nectar. The pollen of comfrey is fine and powdery when dry, and bumblebees shake it out of the anthers by hanging below the flower and vibrating their closed wings with an audible high-pitched buzz.

A typical bumblebee nest is made in a dry soft mass of grass or moss underground or at the surface. Most bumblebees cannot collect nesting material for themselves, and they often take over the disused nests of small mammals or even birds. Because of the high water-table, bumblebees are unlikely to find sufficiently dry nest-sites on the peat fen itself, and most nests found in recent years have been on well-drained banks off the north-eastern margin of the fen; one on the fen itself was in a bird's nest in a bush. Piles of litter or peat might offer dry nest-sites on the fen, but these habitats may be rarer now than thev were in the past. Today most of the bumblebees that forage on the Fen probably come from nests at the fen margin. Rough land in Breed Fen Drove and the Lode Lane area may be particularly important to them. Hibernation sites, like nest sites, need to be dry; a high water-table would largely confine hibernation to the fen margins.

The habitat favoured by male bumblebees for the patrolling flights associated with their courtship differs from species to species. Males of Bombus hortorum make regular visits to the base of the large willow tree in front of the William Thorpe Building; but, apart from this rather public display, rather little is known of the male patrolling sites used by bumblebees on the Fen.

The most conspicuous social wasp at Wicken Fen is Vespula rufa, which is distinguishable from commoner species of Vespidae by a rusty red area at the front of the abdomen. It is widespread in damp and neglected spots, but local in today's much-cultivated landscape. It nests on the ground within the Fen and gathers nectar from the flowers of alder buckthorn. which have a conveniently long season: the bush is still producing flowers as the first berries redden.

Solitary wasps, in particular the Eumeninae. near relatives of the social wasps, are well represented on the Fen. The commonest is Ancistrocerus trifasciatus, a species generally associated with swampy conditions. Many members of the large family Sphecidae are excluded from the Fen by the fact that they nest in dry, often sandy, soil. The others nest in rotten wood, beetle borings and hollow stems, and a long list of these has been assembled from the Wicken area. It is based mainly on records published between 1916 and 1938. A great many species recorded in this period have not been found here, since. There are two reasons for thinking that this may be due to lack of recording. One is that the dearth of repeat records applies to generally common as well as to more local species, and the other is that Jeremy Field’s recent work at Chippenham Fen shows that there is still a rich wasp fauna there. It is also possible that such efforts as there have been at collecting this group have not been directed to the best places. Nevinson described the Sedge Fen itself as ‘unproductive’ and, though it has plenty of dead wood in places, solitary wasps seem to be difficult to find there. Apart from old posts and sheds near the Lode in Nevinson’s time there were ‘many good species in Adventurers’ Fen’ but best of all were ‘portions near Burwell Fen’ on account of ‘the large amount of rotten wood, trees and posts’. Solitary wasps and bees, like many other animals, benefit if dead wood is allowed to remain on the Fen.

The modest lists of spider-hunting wasps (Pompilidae) and ruby-tail wasps (Chrysididae) probably reflect a similar history to those of the Eumeninae and Sphecidae. Among solitary bees, too, wood-nesting species are better represented than ground- nesting species, but there is apparently a contingent that corresponds with the bumble-bees, nesting outside the Fen but foraging within it. This is indicated by the number of Andrena (Apidae) species recorded, though one of these, A. labialis, was reported by Spooner to nest in clay soil in Breed Fen Drove. The other major groups of Hymenoptera, the parasitic wasps and the ants, also have very brief modern lists; the former due to lack of recording, the latter due to the absence of suitably dry ground for nesting.

Wicken Fen provides a habitat for a number of bees and wasps that have special requirements. Rhopalum gracile, a wasp species with a local distribution in south-east England, is found only in damp areas. Another sphecid wasp Passaloecus clypealis and the small black bee Hylaeus (Prosopis) pectoralis both nest in the galls formed by the fly Lipara lucens at the tip of reed stems. The reeds need to be left standing for two years for these ‘squatters’ to complete their life cycle. Macropis europaea, a bee rather smaller than a honeybee, can easily be spotted visiting the flowers of yellow loosestrife (Lysimachia vulgaris) in July and August. This flower is one of several unrelated plants scattered across the world that offer oil instead of nectar to the specialized bees that visit them. Macropis nests on the Fen in soil that is probably swamped in winter.