Evaluation of the floral rewards of Aloe greatheadii var davyana (Asphodelaceae), the most important indigenous South African bee plant

Abstract

The most important indigenous bee plant in South Africa is the winter flowering Aloe greatheadii var davyana, with a widespread distribution across the summer rainfall region. Beekeepers commonly move their hives to the "aloe fields" during winter, using the strong pollen and nectar flow for colony growth, queen rearing and honey production. In spite of its importance for the bee industry, no complete pollen analysis is available and, except for the popular bee literature, little is known about nectar production or pollinators. The aim of the study was therefore to evaluate the floral rewards of this aloe and to investigate the importance of these resources for honeybees. We analysed fresh, bee-collected and stored aloe pollen for its nutritional content (not previously done for any plant species). Addition of nectar and glandular secretions leads to an increase in water and carbohydrate content and a decrease in protein and lipid content. All the essential amino acids, except tryptophan, met or exceeded the minimum levels for honeybee development. In worker bees in queenright colonies, ovarian development is greater on aloe than on sunflower pollen, which may be explained by the exceptionally high protein content and high extraction efficiency during digestion. In assessing the nectar resource, we investigated the nectary structure and nectar presentation of two species belonging to different sections of the genus Aloe, A. castanea and A. greatheadii var davyana, but anatomical differences were not related to the nectar production. We looked at variation in nectar volume and concentration of A. greatheadii var davyana on various levels, from within the flowers to across the summer rainfall area. Nectar was continuously available and, although dilute (mean concentration 18.6%), the nectar of A. greatheadii var davyana is more concentrated than that of other Aloe species, making it an ideal source of energy and water for honeybees. Utilisation of dilute nectar by bees requires elimination of much excess water. We sampled crop contents of nectar foragers to determine if changes in nectar concentration occurred after collection and before unloading in the hive. Contrary to the common assumption that nectar is either unchanged or slightly diluted during transport, we observed a dramatic increase in concentration and a decrease in volume between the flowers and the hive. Bees may be foraging primarily to get enough water for their physiological needs. Using miniaturised data loggers, we showed that bees are able to adjust nest humidity within sub-optimal limits, in addition to efficient regulation of hive temperature. Humidity levels are influenced by trade-offs with regulation of temperature and respiratory gas exchanges. Although the dilute nectar and pinkish red tubular flowers are characteristic of bird-pollination, exclusion experiments showed that bees are the primary pollinators of A. greatheadii var davyana. This contrasts with other Aloe species which are pollinated by sunbirds and other passerine birds, but highlights the two-way interaction between the bees and the aloes.