C Kameswara Rao

Foundation for Biotechnology Awareness and Education, Bangalore, India


Bees are heavily implicated in pollination of plants, more by popular belief than by rational science.  Honeybees actually need nectar and pollen from flowers for survival. 



Nectar, the sugary fluid produced by special glands at the base of the petals and/or the ovaries, collects around the ovaries in the cup like structures formed by the petals. The quantity of nectar produced in each flower varies from species to species, about a teaspoon in tulip tree flowers (Liriodendron tulipifera) to very minute quantities as in the flowers of white clover (Trifolium repens). Nectar rich flowers are often called ‘honey flowers’.  Honey bees collect nectar, fill the cells of the comb with it and plug the cells with compacted pollen mass. Nectar is dehydrated and converted into honey by enzymes.  Honey is the major source of energy for both the brood and the adults. 



Bee colonies would survive on nectar/honey or even on sugar syrup, but cannot lay eggs without pollen in the diet.   On the surface of the pollen there is a sticky layer known as the pollen coat, tryphine or pollen kit, which is a rich source of proteins, lipids, vitamins, phenolic compounds and minerals for the bees.  The extent and chemistry of pollen kit varies from species to species and the bees prefer pollen rich in pollen kit.


Flowers of many plants produce copious amounts of pollen, but the quantities vary very widely, from species to species.  The whole pollen component of a flower may not be available to bees. During floral visits, worker bees pack pollen in concavities on the hind limbs, called the pollen baskets’.  Because of the pollen kit, pollen stick to each other and also the different body parts of bees. When the pollen kit is poor (as in the grasses), the pollen tend to be dry and powdery, and drop off the vector’s body.


Once at the hive, worker bees pack the pollen, along with resin from plants, into the honey comb.  Pollen germination, bacterial growth, anaerobic metabolism and fermentation of pollen in the comb are prevented by several chemical compounds including enzymes the bees add to preserve the stored pollen.  The processed pollen comb is called bee bread’, later consumed both by the larvae and adult bees.


Royal jelly is secreted by the hypopharyngeal glands in the heads of young worker bees and is not related to the flowers the bees visit.  Rich in nutrients, royal jelly is used along with other diet, in feeding all the larvae in the colony, including those destined to become workers, but not the adults.


Bee colonies are perennial, though the life span of individual bees is about two or three weeks.  Bee colony populations are sustained through overlapping batch hatchings.


Bee foraging areas and Distances

The primary bee foraging areas are the various wild or cultivated plants in the vicinity of the hives.  Crop plant flowering being seasonal, bees depend heavily upon wild species of plants. Honeybees also visit a number of species of wind-pollinated plants which do not contain nectar, such as the willow, oak, some grasses, but only to collect pollen.


Bee foraging ranges seem to have a relationship to the body size, particularly body the length of the bee species, the smaller bodied going farther.


Studies on tagged bees indicate that, in general, the foraging distances range from 50 meters to two km from the apiary, occasionally to seven or eight km, and exceptionally nine to 10 km.  Maximum foraging distances between nesting site and food mostly vary between 150 and 600 m, for 16 bee species studied. 


Food Preferences of Bees

In general, bees forage on flowers of any species in the vicinity of the hive, on the basis of abundance of nectar and/or pollen and aggregation of flowers meaning fewer visits. However, there seem to be a few other criteria of food preferences.


Genetic differences among the bee populations seem to influence species preference, as for example more to apple pollen than to other pollen.


Bees prefer pollen rich in pollen kit.  Bees seem to have preferred oil seed rape pollen containing greater proportion essential amino acids, than field bean pollen, suggesting that bee food preferences may depend upon the nutritional quality, probably operating thorough chemical signalling and experience.  Bees have also shown preference to certain food odors, also learnt by experience.


Bees and Palynology

In general, Palynology is the scientific study of pollen and spores of all plant species, but normally the term refers to the study of pollen.  Bees, nectar and honey are studied for pollen composition from three different perspectives: a) the external body parts of the bees to identify the species of plants the bees had visited, basing on the pollen in the pollen baskets and on different parts of the bee body, b) bees captured immediately after floral visits and dissected to analyze the pollen in the stomachs to identify the plant sources of nectar sucked in, and c) analyze honey for pollen for characterization of honey.


Bees do not visit every species in flowering in the vicinity of a hive.  Besides, the flowers of several species are not morphologically favorable for nectar or pollen collection by honeybees, particularly those with very small flowers or those with long narrow tubular corollas, which the bees cannot enter. 


Bees only pack pollen in pollen baskets and do not consume raw pollen per se, from flowers they visit.  Since the floral nectar sources are usually close to the dehiscing anthers, some pollen from the anthers and pollen of other flowers on the bee’s body, fall into the nectar gathered by the bees.  A small component of pollen, which does not originate from flowers the bees had visited, is also found in the bee hives and honey.  Such pollen come in through wind, rain or accidental fall out.  Hence, pollen analysis of bees and hives should be carefully executed taking several factors into consideration, in order to be reliable. 


The bee sucks nectar through a slender tube that enlarges into a thin-walled distensible sac called the ‘honey stomach’. Once in the honey stomach, the nectar flows over a regulatory apparatus (the proventriculus), that filters and controls the entry of food into the bee's stomach. The nectar in the honey stomach is drawn back and forth into the proventriculus to remove debris such as pollen grains, fungal spores and dust. About 90 per cent of pollen sucked into a bee's honey stomach along with nectar are filtered out within 10 to 15 minutes after the floral visit. 


The posterior end of the proventriculus extends into the mid gut (ventriculus), where food digestion and nutrient absorption take place.  A valve prevents the filtered nectar from entering the bee's digestive system. However, this same valve will later allow pollen and debris removed from the nectar to pass into the bee's intestines, and retained in the rectum until excreted. 


The yellow rain’ that often alarms people is the result of rapid defecation of massive quantities of pollen, by swarms of bees retuning to the hive, leaving thousands of tiny yellow spots all over the place. 


Bees are captured immediately after floral visits and dissected to analyze the pollen in the stomachs to identify the plant sources of nectar.  An average of 7,100 pollen grains per ml of fluid were found in the honey stomachs of 38 bees captured and dissected immediately after each had completed feeding on the nectar of rabbit brush (Chrysothamnus nauseosus).   On the other hand, 30 honeybees that fed on orange blossoms (Citrus sinensis) and 32 bees fed on cotton flowers (Gossypium hirsutum) did not contain one single pollen grain of the respective species. 


The analysis of honey for pollen (Melissopalynology or Mellittopalynology), facilitates construction of ‘pollen spectra’, the guides to qualitative and quantitative pollen composition of honeys, which are a potential basis to determine the quality and geographical origin of honey, the species of plants whose flowers the bees had visited and so the nectar sources.


Bees collect pollen and nectar from all sources in the environment and do not normally have any special preference to a particular species of plants.  Pollen of melon, cucumber, rapeseed, polygonum, alfalfa, clover, mint, thyme, sage, blue bells, thistle, white acacia, fireweed, eucalyptus, chestnut, basswood, orange blossom, buckwheat, and of several common weeds in the apiary environment were found in honey samples, in different parts of the world.


The quantity of pollen in a honey sample is not always directly proportional to quantity of nectar collected from a particular source.  In the case fireweed (Epilobium angustifolium), that contributed to 95 per cent of nectar, the pollen component in the nectar was only 6.3 per cent.  Rape seed (Brassica rapa) pollen were found to be abundant in a sample of honey but nectar contribution of this species was only 2 per cent.  With 28.3 per cent of white clover (Trifolium repens) pollen, nectar contribution of this species was less than two per cent.


The number of pollen grains in honey varies from 200 fireweed pollen grains per ml of honey to 41,000 white clover pollen per mlUnifloral honeys, such as melon, clover, cotton, canola, citrus or apple honey, are produced by hives located in the respective crop fields or orchards.  They contain more than 45 per cent of pollen from one predominant species, implying that most nectar was sourced by the bees from that particular species.  Special therapeutic and/or nutritional properties are attributed to unifloral honeys which command higher market prices.


February 22, 2008