BUMBLEBEE FORAGING PREFERENCES: DIFFERENCES BETWEEN SPECIES AND INDIVIDUALS
2 MATERIALS AND METHODS2.1 Site characteristics
2.2 Pilot study
2.3 Measuring environmental variables
2.4 Bee identification
2.5 The "bee walk"
2.6 Marking bees and measuring tongue length and head width and length
2.7 Preference and constancy of bees
2.8 Measurements and characteristics of flowers foraged by bees during bee walk
The bee walk side was free from the influence of cattle grazing, but parts of the other side of the lane were within reach of cattle which were put into Field B at the end of June. Field A was set-aside this year and grass last year. Field B was grass this year and set-aside last year. The most common flower in the set aside field was Capsella bursa-pastoris, there was also a large number of Lamium purpureum and various grasses.
2.2 PILOT STUDY
2.3 MEASURING ENVIRONMENTAL VARIABLES
2.4 BUMBLEBEE IDENTIFICATION
2.5 THE BEE WALK
2.6 MARKING BEES AND MEASURING TONGUE LENGTH, AND
HEAD LENGTH AND WIDTH
The conical shape of the restraining device made it possible to fit it over almost any size of glass jar. The bee was persuaded to fly up to the net by excluding light from the sides and base. Once the bee was on the net the jar was removed and the aerosol cap placed under the opening to prevent escape. It was possible to hold the aerosol cap in place with one hand, while offering the bee 30:70 honey and water in a plastic pipette with the other. Not all bees were willing to drink the nectar and, as time was limiting, if they ignored the pipette after it had been within reach for over 10 s it was removed and just their head measurements were taken and they were marked. There was no apparent difference between species in the proportion that drank from the pipette. One noticeable thing was that if their buzzing sound was higher pitched than normal they generally refused to drink and struggled more when being marked, this became known as the "angry buzz". It is not known what caused some bees to emit this higher pitched buzz, but there was certainly a link between it and how co-operatively the bee behaved.
The end of the plastic pipette was marked in 1 mm intervals with a Staedtler fine lumocolor permanent ink pen. With a little practice it was possible to move the meniscus of the nectar up and down the pipette so forcing the bee to extend its tongue to foraging length, which could be seen through the plastic and its length measured. This method is similar to that used by Brian (1957), except that she used capillary tubing, not a pipette. After the tongue length was measured the aerosol top was moved up the inside of the pot until it touched the back of the bee, when the bee turned to walk on the aerosol top the flower pot was moved down with just enough pressure to stop the bee moving its thorax. Its head length and width were now measured, and then the bee could be marked.
The time taken to mark bees varied greatly, the shortest time was about two minutes, but a few bees took as long as five minutes. Waiting for a bee to walk on the surface of the aerosol cap was the main reason for the variation in time taken. Some bees walked over every part of the mesh before they decided to walk on the aerosol cap.
Water based Tipp-Ex in four colours, white, blue, green and pink, was used to paint a disc on the bee, carefully avoiding the tegulae (wing bases). A small blob of Tipp-ex was placed on the thorax using the brush provided, the blob was then smoothed down with a blunted cocktail stick, this also flattened the hairs on the thorax. Since Bombus pascuorum has a very hairy thorax, the layer of Tipp-Ex was insufficient to flatten the hairs, so before marking these hairs were either trimmed using a very small pair of scissors, or scraped off carefully using a scalpel. In August the weather was much hotter and drier and the Tipp-Ex dried much more quickly, so a slightly thicker layer of Tipp-Ex was applied to B. pascuorum, this allowed the hairs to be flattened without having to trim or remove them. It was felt that this caused less disturbance to the bee, and made the marking process quicker. Once the Tipp-Ex had dried, a number from 1 to 100 was written on the Tipp-Ex with a Staedtler fine lumocolor permanent ink pen, and the bee was set free.
Marking and measuring were done indoors, the bees were set free out of a window which was about 10 m at right angles from the 100 m mark of the bee walk. Nearly all the bees flew away immediately, but a few sat on the windowsill or below the window. This area was checked periodically and any bees still there were brought indoors, fed and released when they decided to fly.
The number and Tipp-Ex colour of all marked bees seen on the bee walk were also recorded. Occasionally a marked bee was seen but the number could not be read; this was recorded as an unmarked bee.
2.7 PREFERENCE AND CONSTANCY
Flower preferences were calculated using the program PREFER in Krebs
(1989) which calculates Manly's alpha index of preference. The variables
required by the program are:
The number of flowers was calculated by observing how the bees used each flower species, so as to find what could be considered a "functional individual unit", i.e. an area where one bee could forage but not two.
The full list of flowers used by bees during the bee walk is shown in TABLE 2 in the results. During each five day bee walk period, flowers were counted on the morning of the third day. For flowers with inflorescences and umbels, heads were counted; spikes were counted in the Labiatae; Hypericum perforatum flower clusters were counted; racemes of Vicia spp. and Lathryus pratensis were counted; and for all others individual flowers were counted or estimated. For Stachys lanata, originally spikes, made up of several individual flowers, had been counted but, as the bee walk proceeded, it became obvious that it was common to find two, or even three bees on a spike, but it was also possible to find only one bee on a small spike. Consequently it was eventually decided that on average a spike comprised two functional units. The number of visits to each species of flower by bees was taken from the bee walk data, and the PREFER program was run for each species of bumblebee. It was decided to class the resources as renewable, since few flowers secrete nectar at just one time period and so do renew their nectar. Pollen is not a renewable resource, but no single bumblebee is likely to remove the entire quantity of pollen from a flower in just one visit.
Originally the program was run with each spike of Stachys lanata counted as one unit, the results appeared to greatly over estimate the preference for S. lanata and, it was felt, did not reflect what had been seen in the field. Consequently the program was re-run with each spike counted as two "flower units".
To measure flower constancy, bees were picked at random, by following the first bee sighted after stepping outside. The species, caste, number (if marked), and sequence of flowers visited was then recorded on tape. When one bee was lost the next bee sighted was followed as a new sample run. Most of the bee following was within the area of the bee walk, but was not restricted to this area; the bee was followed for as far and as long as was possible.
2.8 MEASUREMENTS AND CHARACTERISTICS OF
FLOWERS FORAGED BY BEES DURING THE BEE WALK
The height range of the flowers, the height of the vegetation surrounding the flowers, and the effects of wind on both the flowers and the surrounding vegetation were recorded. The surrounding vegetation was noted as the site is very windy and the movement of the vegetation in the wind might affect foraging preferences. Where most of the vegetation was lower and only the taller grasses were higher, than the flower species being recorded, the surrounding vegetation was classed as mixed, otherwise the surrounding vegetation was classed as higher or lower.
Where possible flowers and inflorescences for identification and examination were picked from various heights and in various places throughout the bee walk. Identification of flowers was made using Rose (1981) and Clapham et al. (1993).
(C) Copyright 1999 L. Smith