Food demand for colony development, crop preference and food availability for Bombus terrestris (L.) (Hymenoptera: Apidae) : a thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology
Eight Bombus terrestris (L.) colonies were reared in laboratory observation hives in 1986 and foraged in cages. Pollen and sugar consumption were correlated and could be estimated from larval area and number of live workers. Pollen consumption peaked within two weeks of maximum larval area in 6-8 week old colonies and coincided with peak worker emergence. Sugar consumption peaked with the emergence of males and queens two weeks later. The optimum period for maximum pollination efficiency for a colony would be from one week prior to peak larval area to two weeks after. There was a positive correlation between the total productivity of a colony ('Productivity Index', a biomass estimate from the number and size of empty cocoons) and the maxima of live worker numbers, larval area and rate of food consumption, and the total food consumption and biomass (but not sex ratio) of reproductives produced. Colonies with higher consumption made greater investment in reproductives, but larger colonies did not invest proportionally more into reproduction than smaller colonies. Larger colonies grew faster with more workers emerging per unit time than smaller colonies. Food consumption and development of indoor colonies was compared with ten colonies maintained in the same observation hives but free foraging outdoors on flowering crops in 1987. Maximum weekly pollen consumption was 12.6 times less in free foraging colonies and sugar energy consumption was 43 times lower with no queens produced in colonies foraging outdoors. The pollen consumption/cm2 at maximum larval area was 14 times lower in free foraging than indoor colonies so consumption (rate and total) for free foraging colonies could not be predicted from maximum larval area using indoor consumption data. The order of B. terrestris nectar gatherers' and queens' crop preference over the whole season was: borage, Borago officinalis > fodder radish, Raphanus sativus > swede, Brassica napus > broccoli, Brassica oleracea. Flower preference was not correlated with flower density or production. B. terrestris males and honey bees preferred borage with broccoli as second choice. Honey bees were on average seven times more abundant on crops than B. terrestris workers and with a similar tongue length honey bees provided the greatest competition for food. On calm, warm days honey bee numbers on borage exceeded 2/m2, nectar and pollen became depleted and B. terrestris switched to nectar gathering on fodder radish. The long corolla tube of fodder radish excluded nectar collection by honey bees with a short tongue, whereas B.terrestris workers bit holes in the corolla base and 'robbed' nectar. Honey bees and B. terrestris males removed nectar from previously perforated fodder radish flowers. Borage secreted nectar throughout the season and had the most pollen and nectar per flower. Fodder radish had the highest flower density and pollen and nectar standing crop, producing nectar later in the season as weather improved. B. terrestris being less sensitive to poor weather, foraged for nectar and pollen each day before and after the peak in honey bee numbers. B. terrestris workers collected pollen on borage by vibrational pollen harvesting ('buzz' pollination) from poricidal-like anthers. On crucifers, incidental dusting of pollen while nectar collecting occurred. Early in the season borage pollen was collected throughout the day, but later in the season with increasing honey bees on borage B. terrestris pollen gatherers collected borage pollen early in the day and crucifer pollen during the rest of the day. Sugar concentration of nectar returning in foragers was highly correlated with sugar concentration of the most preferred crop at that time. With higher temperature and decreased rainfall, more pollen and nectar became available, more pollen and nectar gatherers and honey bees foraged on the crops and workers returned to colonies with more food. The high density of honey bees on borage did not reduce the food intake returning to B. terrestris colonies.