**3. Characteristics and composition of food sources**

### **3.1 Nectar**

Nectar is the dominant source of carbohydrates and is therefore an energy source immediately available to fill colony needs. However, while the hive workers frequently visit honey cells, the food gathering foragers do not as they are fed by nurse honeybees through trophallaxis (Crailsheim et al., 1996). Thus nectar reserves, progressively transformed into honey by the workers, fuel the global activities of the colony, including food collection itself. Honey reserves also enable over-wintering survival of adults, as no extra food is gathered at this period.

Nectar is generally produced in the floral nectary, but some plants also have extra-floral nectaries (Elias, 1983). Studies of more than 900 plant species revealed that both floral and extra-floral nectaries contain 3 main sugars: glucose, fructose and saccharose, in proportions characteristic of the plant species (Baker & Baker, 1982). In addition, small quantities of proteins, lipids and amino acids are also found in nectar (Baker & Baker, 1982), as well as other substances that may help its spotting by bees (Thorp et al., 1975).

While the sugar composition of nectar is relatively stable, the volume and concentration of the nectar are quite variable. In entomophilous plants (pollinated by insects) growing in temperate regions, the volumes are most frequently in the order of 0.5 to 7 µL/flower, with a modal value of 2 µL (Cruden et al., 1983), while sugar concentrations vary from 5% to 50% depending on the species (Baker & Baker, 1982; Cruden et al., 1983). Nectar secretion is subject to a specific rhythm, and, for each species, not only is there a large difference between secretion rate between day and night, but also nectar production can vary considerably during the course of a day (Maurizio, 1975). According to Maurizio (1975) and Corbet et al. (1979), optimal nectar secretion, in volume and sugar concentration, is produced during a period of 5 hours on average, either in the morning, or at the end of the afternoon in the majority of plants that are pollinated by hymenopteran pollinators. More rarely, certain plants show two secretion peaks : one in the morning and one in the evening. It is also noteworthy that even within a given plant species, large differences can exist in the secretion pattern between populations and varieties (e.g., oilseed rape *Brassica napus* L.; Pierre et al., 1999).

Apart from genetic factors, nectar production (volume and concentration) depends on the developmental stage of the plant, as well as the time of day and the age and position of the flower on the plant. Even more, it depends largely on environmental factors such as soil type and moisture level, cultivation practices, and weather conditions (wind, temperature, relative humidity).

From this brief review, it appears that honeybees are clearly in a situation where nectar supply can not only be very diverse in that it is usually provided by an array of several plant species, but also extremely variable in amount and composition. The colony can thus go from a situation when nectar supplies are totally absent, to a situation where there is an abundance of nectar over just a few days (Crane, 1975). Honeybees have therefore to adapt rapidly their foraging depending upon the nectar and pollen availability in their environment.

## **3.2 Pollen**

374 Ecosystems Biodiversity

readily over several kilometres around the hive (Decourtye et al., 2008; Steffan-Dewenter

Nectar is the dominant source of carbohydrates and is therefore an energy source immediately available to fill colony needs. However, while the hive workers frequently visit honey cells, the food gathering foragers do not as they are fed by nurse honeybees through trophallaxis (Crailsheim et al., 1996). Thus nectar reserves, progressively transformed into honey by the workers, fuel the global activities of the colony, including food collection itself. Honey reserves also enable over-wintering survival of adults, as no

Nectar is generally produced in the floral nectary, but some plants also have extra-floral nectaries (Elias, 1983). Studies of more than 900 plant species revealed that both floral and extra-floral nectaries contain 3 main sugars: glucose, fructose and saccharose, in proportions characteristic of the plant species (Baker & Baker, 1982). In addition, small quantities of proteins, lipids and amino acids are also found in nectar (Baker & Baker, 1982), as well as

While the sugar composition of nectar is relatively stable, the volume and concentration of the nectar are quite variable. In entomophilous plants (pollinated by insects) growing in temperate regions, the volumes are most frequently in the order of 0.5 to 7 µL/flower, with a modal value of 2 µL (Cruden et al., 1983), while sugar concentrations vary from 5% to 50% depending on the species (Baker & Baker, 1982; Cruden et al., 1983). Nectar secretion is subject to a specific rhythm, and, for each species, not only is there a large difference between secretion rate between day and night, but also nectar production can vary considerably during the course of a day (Maurizio, 1975). According to Maurizio (1975) and Corbet et al. (1979), optimal nectar secretion, in volume and sugar concentration, is produced during a period of 5 hours on average, either in the morning, or at the end of the afternoon in the majority of plants that are pollinated by hymenopteran pollinators. More rarely, certain plants show two secretion peaks : one in the morning and one in the evening. It is also noteworthy that even within a given plant species, large differences can exist in the secretion pattern between populations and varieties (e.g., oilseed rape *Brassica napus* L.;

Apart from genetic factors, nectar production (volume and concentration) depends on the developmental stage of the plant, as well as the time of day and the age and position of the flower on the plant. Even more, it depends largely on environmental factors such as soil type and moisture level, cultivation practices, and weather conditions (wind, temperature,

From this brief review, it appears that honeybees are clearly in a situation where nectar supply can not only be very diverse in that it is usually provided by an array of several plant species, but also extremely variable in amount and composition. The colony can thus go from a situation when nectar supplies are totally absent, to a situation where there is an abundance of nectar over just a few days (Crane, 1975). Honeybees have therefore to adapt rapidly their foraging depending upon the nectar and pollen availability in their

**3. Characteristics and composition of food sources** 

other substances that may help its spotting by bees (Thorp et al., 1975).

extra food is gathered at this period.

& Kuhn, 2003).

**3.1 Nectar** 

Pierre et al., 1999).

relative humidity).

environment.

Pollen is produced and released by the anthers and it can be more or less accessible to the floral visitors depending on floral morphology. Honeybees rarely eat it in its natural state. Rather, it is first aggregated with nectar or diluted honey to form pellets (Vaissière & Vinson, 1994). Pellets are then placed in the cells, packed and covered with honey and it is transformed by lactic fermentation to make « bee bread ».

Fresh pollen contains proteins and amino acids, but also carbohydrates and lipids, including sterols. Each type of pollen can be characterised by its global caloric value, its protein content as % of dry matter, its nitrogen content, amino acid composition (classified as essential or non essential), and starch, sugar and lipid content as well as its vitamin and mineral elements. Specific techniques are needed for each of these measurements and results often differ between different authors. It is therefore difficult to obtain the complete biochemical profile for the pollen of a given plant species (Stanley & Linskens, 1974). For example, dandelion pollen (*Taraxacum campylodes* G.E. Haglund), has a protein content that ranges from 9.2 to 19.2% of its dry weight, depending on the authors, its main lipids are made up linoleic and palmitic acid, and its mineral element composition is known as well. We also know that it is deficient in arginine and is missing essential amino acids such as tryptophane and phenol-alanine (Loper & Cohen, 1987). A few other complete data exists on pollen of gymnosperms and corn *Zea mays* L., but generally such complete data sets are rare and does not allow a multi-criteria classification of pollen types.

#### **3.3 Nectar and pollen budget of the colony**

Large quantities of food are required by a honeybee colony. Food gathering at the height of the season must be enough to feed 50,000 workers and 9,000 larvae. Such colony may have an annual nectar budget of about 120 kg and a pollen budget of 20 kg (Seeley, 1995). It may stock from 60 to 80 kg of honey per year (Erber, 1992; Rosov, 1944; Seeley, 1995; Weipple, 1928. The larva of a worker honey bee consumes about 140 mg of honey during its development (Rosov, 1944; Seeley, 1985; Weipple, 1928; Winston, 1987).

An active foraging worker uses 0.5 mg of honey per km flown, and it can fly as much as 800 km during its lifetime (Gould & Gould, 1988). Others authors estimated that a forager consumed 11.5 mg of sugars per hour in flight, and only 0.7 mg per hour when inside the hive (Heinrich, 1979; Olaerts, 1956). During winter, bee activity is reduced, but the cluster must maintain a constant temperature in the centre of the nest of 34°C to 36°C, which requires a large energy expenditure. Thus, an average sized colony needs to stock ca. 25 kg of honey for winter consumption. Overall, the annual needs of a colony are estimated at about 80 kg of honey and 20 kg to 40 kg of pollen depending upon the authors (Crailsheim et al., 1992; Louveaux, 1954; Winston, 1987).

A colony gathers from 15 to 55 kg of pollen per year (Eckert, 1942; Hirschfelder, 1951; Louveaux, 1958; Ribbands, 1953; Seeley, 1985). Nursing bees are the ones that consume the most pollen as they eat ca. 60 mg of pollen over 10 days (Pain & Maugenet, 1966), to develop their hypopharyngeal glands which produce the 42 mg of food consumed by larvae during the first 5 days of larval development (Haydak, 1968). This food given by nursing bees constitutes a major part of the protein supply consumed by larvae since pollen is processed into brood food and only 5% of the protein derived from pollen are directly fed to larvae (Babendreier et al., 2004).

Why Enhancement of Floral Resources in Agro-Ecosystems Benefit Honeybees and Beekeepers? 377

American foul-brood and European foul-brood, respectively (Manning, 2001). Likewise, a diet with reduced pollen quantity and diversity can induce not only a protein deficiency, but also a decreased synthesis of detoxification enzymes. As a result, bees fed with a mixed

Within complex landscapes, honeybee colonies normally collect and consume pollen from a large array of plant species (Dimou & Thrasyvoulou, 2009; Louveaux, 1959; Severson & Parry, 1981). This generalist behavior for pollen collection (polylecty), as opposed to oligolecty or monolecty in other bee species, is further supported by experimental evidence that honeybees feed preferentially on plurispecific pollen mixtures rather than pollen from a single species (Schmidt, 1984). These results support the postulate that honeybees select a mixed diet when given a choice. However, today intensive agricultural landscapes most often provide colonies with a low variety of plant species. Indeed, honeybees used for the pollination service of large areas may even be forced to feed mainly on a single flowering crop, like sunflower that has pollen with a poor nutritional value and sesame that has pollen which contains only low levels of phagostimulants (Schmidt et al., 1995). Such practices may be stressful for colony health. In those cases, the presence of additional floral or food sources

The protein content of pollen (2.5 to 61%), its amino acid composition, its lipid content (1 to 20%), and that of starch, sterols, vitamins and minerals vary tremendously among plant species (Roulston & Buchmann, 2000; Roulston & Cane, 2000; Stanley & Linskens, 1974). Therefore, regardless how much monospecific pollen is consumed, it might miss nutrients essential to the health of honeybee colonies. The deficiency of a given pollen in essential nutrients could be compensated for by a more diverse pollen diet. Indeed, a mixed pollen diet increases the lifespan of honeybees as compared to a monospecific pollen diet (Schmidt, 1984; Schmidt et al., 1987). In addition, a diversified pollen diet might help the bees to fight against pathogens. Alaux et al. (2010) found that a diverse pollen diet can actually enhance some immune functions in worker honeybees. For example, their production of glucose oxidase, an enzyme involved in the synthesis of the antiseptic hydrogenperoxide in honey and brood food, was 40% higher in honeybees fed with pollen from a variety of plants compared to those fed with pollen from a single plant species – even if this monofloral diet had a higher protein content. However, whether polyfloral diets might increase the actual resistance to diseases and pathogens remain to be tested. In bumblebees *Bombus terrestris*, larvae fed with a mixed pollen diet were heavier than larvae fed with monofloral pollen of equivalent or higher protein content (Taséi & Aupinel, 2008), and so we might also expect a

similar beneficial effect of a diverse pollen diet on the health of honeybee larvae.

As stated previously, increasing the number of pollen species in the diet probably provides a buffer against deficiencies of specific nutrients (Schmidt, 1984) as well as the presence of toxic compounds in some pollen (Mesquita et al., 2010). Indeed, several studies have demonstrated the importance of some specific nutrients in bee health. For example, de Groot (1953) found that 10 essential amino acids in specific proportions are required for optimal honeybee health: arginine, histidine, lysine, tryptophan, phenylalanine, methionine, threonine, leucine, isoleucine, and valine. Those essential amino acids cannot be synthesized *de novo* by honeybees and therefore need to be supplied directly in the diet. Some fatty acids, found at different levels in pollens ( Manning, 2001; Singh et al., 1999), can also be beneficial

pollen diet can be less sensitive to some pesticides (Wahl & Ulm, 1983).

**5. What are the benefits of diet diversity?** 

could to reduce these potential problems.

Clearly, the survival and development of honeybee colonies are influenced by the regularity, quality and quantity of the nectar and pollen supply. Within intensively farmed agricultural landscapes, nectar- and pollen-producing crops may provide a narrow window with mass flowering followed by a dearth of pollen and nectar resources. A typical example occurs with oilseed crops such as rape and sunflower (*Helianthus annuus* L.) for which nectar and pollen resources are usually abundant during the blooming, but only for a short period. The subsequent temporal dearth of resources could be partially filled simply by the creation and protection of additional non-cropped areas such as field margins (strips bordering crop fields), hedgerows (linear scrub along field boundaries), woodlands, ponds, ditches and fallow farm fields (Decourtye et al., 2010).

But, for both pollen or nectar , it is long and difficult to assess precisely their production in quantity and quality. Indeed, one must not only take into account the effects from the environment but also the effects of the genetic make-up of the plants at the species and variety levels. This last point is important to objectively assess the best flowering plant for bees, though it is rarely known.

#### **4. Impact of pollen supply on the life of the colony**

The majority of the protein intake of larvae comes from the hypopharyngeal gland secretions of nurse bees, and the development of these glands depends on their pollen diet. The development of hypopharyngeal glands is not dependant upon the essential amino acids present in the pollen consumed, but on the total quantity of protein ingested (Pernal & Currie, 2000). Therefore, a situation of pollen deficiency may have detrimental effects on the nurse bees' tasks and brood care, resulting in undernourished larvae (Blaschon et al., 1999). When such deficiencies occur, the nurse bees may also reduce the number of larvae to feed and cannibilize the eggs (Schmickl & Crailsheim, 2001). The young larvae are eliminated for the benefit of the older larvae which need more pollen input. In the case of extended pollen deficiency, another strategy is to reduce overall larval care (Blaschon & Crailsheim, 2001). Finally, pollen feeding during the larval stage has an important impact on the traits of the adults as it may determine the size of the future adult (Roulson & Cane, 2000) and its life expectancy (Schmidt et al., 1987).

Ovary development is another major process influenced by pollen intake. Experiments have been carried out on worker bees kept in isolation without queen, in order to evaluate their capacity to develop ovaries, a physiological process highly dependent protein utilization and pollen quality. This method has shown that pollen from apple (*Malus domestica* Borkh.) or sweet clover (*Melilotus* spp.) is favourable to ovary development whereas pollen from pine (*Pinus* spp.) is not and that of sunflower is below average (Pernal & Currie, 2000). Also, a diet consisting of several pollens of different nutritive value is not equivalent to their average nutritive value ( Taséi & Aupinel, 2008; Alaux et al., 2011). We can also assume that a pollen of low nutritional value, like that of dandelion, can be compensated by mixing it with another pollen that has a higher nutritional value for reproductive needs (Genissel et al., 2002). This highlights the importance of avoiding putting bees in a situation where they do not have sufficient floral variety, as is often the case amidst large intensive agricultural landscapes.

Finally, it has recently been shown that a high concentration of pollen lipids (mainly linoleic, myristic and dodecanoic acids) protects the brood nest from certain bacteria such as *Paenibacillus larvae* and *Melissococcus pluton* which are the pathogenic agents responsible for American foul-brood and European foul-brood, respectively (Manning, 2001). Likewise, a diet with reduced pollen quantity and diversity can induce not only a protein deficiency, but also a decreased synthesis of detoxification enzymes. As a result, bees fed with a mixed pollen diet can be less sensitive to some pesticides (Wahl & Ulm, 1983).
