The influence of brood on the pollen consumption of worker bees (Apis mellifera L.)

https://doi.org/10.1016/S0022-1910(98)00022-5Get rights and content

Abstract

(1) In midgut dry weight (tissue plus contents) of worker bees we found a representative parameter for pollen consumption. Midguts of bees of successive ages were analyzed and correlated with various parameters. The relative proportions of sugar, protein and water were either constant or negatively correlated with midgut weight. Only the relative pollen weight (percent of midgut dry weight) increased. (2) To investigate the influence of different levels of brood on pollen consumption of individual bees, midgut dry weights from 2 normally breeding control colonies and 2 brood-reduced experimental colonies were analyzed. In bees from control colonies the pollen consumption increased up to the nursing age (3–10 d), remained on an elevated level in middle-aged-bees (10–18 d) and decreased relatively sharply towards the foraging ages (>21 d). When queens were caged in the experimental colonies, the following decline of brood cells affected the consumption of pollen differently. After 6 days of caging, with a reduction of open brood only, no effect was seen. After 15 days, and even more pronounced after 23 days when no brood was present, the pollen consumption in young and middle-aged (10, 14, 18 d) worker bees was significantly reduced, while it was clearly elevated in older bees. We discuss pollen consumption as an adaptation to reduced necessity to nurse brood in young and middle-aged bees, and to enhance life span in older animals.

Introduction

In honeybees, brood plays an important role in the regular development of the colony. For providing the growing larvae brood food (jelly), rich in protein, the nurse bees secrete from their hypopharyngeal and mandibular glands (Rösch, 1925, Haydak, 1970, Huang and Otis, 1989, Crailsheim, 1990). For these proteinaceous secretions much pollen is ingested and utilized. Pollen is not only essential for the production of brood food, but also for the worker bees to build up their body tissues within the first days after emergence (Maurizio, 1954, Haydak, 1970). When they are a few days old, worker bees in normally breeding colonies start to nurse brood, after their first task of "cell cleaning". Nursing brood is the major task for some days only, but on a diminishing level it continues up to ages when workers transform from hive to field bees (Lindauer, 1952, Smith, 1974). During the last days of their lives, worker bees perform foraging tasks (Rösch, 1925, Rösch, 1930, Lindauer, 1952, Seeley, 1982). This sequence of duties, the caste ontogeny, is not strictly determined by age; it is rather flexible with genetic and environmental factors playing regulative roles (Calderone and Page, 1988, Calderone and Page, 1996, Robinson, 1992). So, rather than being based on her chronological age, the probability of when a worker task is performed is determined by her physiological status, which is altered by the environment a bee lived in before.

For honeybees pollen is the most important protein source. The collection of pollen is finely regulated to meet the needs of the colony in very subtle ways (Free, 1967, Fewell and Winston, 1992), probably through trophallactic interactions between the workers (Camazine, 1993, Camazine et al., 1998). Proteinaceous jelly is not only produced to nourish the immature members of the colony, but also to be transferred towards other adult worker bees, drones and the queen (Crailsheim, 1991, Crailsheim, 1992). Therefore, pollen consumption is dependent on the age and the function of the worker bee. In normally developing colonies during summer, nurse-aged bees feed on large amounts of pollen, while other task groups seemingly do not (Crailsheim et al., 1992). Juvenile hormone titer, which mediates task performance, also influences pollen uptake. When Jaycox et al. (1974)injected newly emerged worker bees, which were caged in small groups, with a juvenile hormone mimic, pollen consumption was significantly lower compared to untreated bees. Treated bees began to forage at younger ages and under laboratory conditions their life expectancy was significantly lower. Maurizio, 1950, Maurizio, 1954showed in experiments with caged young bees that a sufficient supply of pollen is a very important life-span enhancing factor for worker bees.

From observation hive experiments it is known that pollen consumption and pollen collection of bees varies with different worker-larvae ratios (Al-Tikrity et al., 1972, Hellmich and Rothenbuhler, 1986). More pollen is consumed when more brood is present and when bees are young (Eischen et al., 1984, Crailsheim et al., 1992). Gaps in the amount of brood are not only caused by human manipulations, but occur also in the natural life cycle of honey bee colonies. During phases such as swarming or absconding the level of brood declines successively in the mother colony and brood is absent in the swarm colony after establishment. In addition, adverse weather conditions, food shortage periods, the season, predators or pathogens may reduce the amount of brood in a colony and should have an impact on pollen consumption.

Because pollen plays such an important role for the economy of the hive, pollen consumption was estimated either by determining the weight of the pollen, which was supplied in a dish at specific intervals (cage experiments) (Jaycox et al., 1974, Eischen et al., 1984, Schmidt et al., 1987), or by counting numbers of cells in a comb containing pollen before and after a specific period (Hellmich and Rothenbuhler, 1986). More detailed investigations focusing on the pollen consumption of bees of various ages were performed by counting the numbers of pollen grains in the gastrointestines and measuring their diameters (Crailsheim et al., 1992, Szolderits and Crailsheim, 1993).

While the normal course of pollen intake by differently aged worker bees in colonies with normal amounts of brood is well known (Crailsheim et al., 1992), we know only little about how young, middle-aged, and old workers respond in their consumption of pollen towards altered levels of brood. Since pollen is ingested by workers to meet the needs of the larvae, to build up their own body stores and to feed proteinaceous secretions to other adult bees, not only should the behavioural task group of nurses be affected by diminished brood levels, but other groups as well.

Because counting pollen is time consuming, the first step was to find a parameter which is easily applicable to estimate pollen uptake in bees of various ages. The second step was to use this parameter to investigate the impact of changed levels of brood on the pollen consumption of differently aged worker bees.

Section snippets

Materials and methods

Fresh and dry weight, plus water, sugar, protein and pollen content of midguts were measured. To obtain these data, bees were collected from normally breeding control colonies, and from experimental colonies without brood, on experimental day 23 (see below).

Results

In the experimental colonies the caging of the queens resulted in a successive reduction of open brood first, and sealed brood later on, while in the control colonies open and sealed brood were present at any given time during the experiments (see right graphs in Fig. 2). To test the assumption that midgut weight is a representative parameter for pollen consumption, bees from experimental and control colonies were collected on experimental day 23, and their midguts (tissue plus content) were

Discussion

To find a representative parameter for pollen consumption in worker bees we measured the midgut parameters of water, sugar, protein, pollen content, and fresh and dry weight. Additionally water content, fresh and dry weight of recta were determined.

Acknowledgements

We acknowledge the support of Mr. M. Marek and Mr. J. Ulz from the Styrian School for Beekeepers for providing the apiary for managing the colonies. For technical assistance we acknowledge Mr. D. Riederer and for the help with marking bees we thank Ms. Cäcilia Grabenhofer and Msc. Naiem El Saied. We also thank Mrs. E. Pessl-Rossi and Dr. Moira van Staaden for linguistic corrections of the manuscript.

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