Why Do Honeybees Exploit so Few Plant Species as Propolis Sources?

Abstract

Introduction Propolis is a resinous material produced by honeybees (Apis mellifera) from beeswax and resins collected from plants. Propolis has been used medicinally since antiquity and is presently regarded a complementary food. The chemical composition of propolis is complex. The majority of the constituents isolated from its resin are phenolic substances, although terpenoids also have been frequently isolated. 1 A wide diversity of biological activities of propolis and its constituents are known, e. g. antioxidant, antimicrobial, anticancer, immune-stimulant, antidiabetes, antiulcer and wound-healing. 2 Two types of propolis have deserved the greatest attention: temperate propolis and Brazilian green propolis. 2 The former is produced by bees from exudates of apical buds of poplar trees (Populus spp.; Salicaceae), 3 while the latter derives from young leaves (leaf primordia) and vegetative buds of Baccharis dracunculifolia (Asteraceae), a species from Central and southeast Brazil. 4 The chemical composition is dependent on the locality of the hives and the local flora. Many other types of propolis, each with a characteristic composition, have been reported. In addition to the mentioned green propolis, other types of Brazilian propolis are known: i. red propolis from the northeast coast, derived from exudates of Dalbergia ecastophyllum (Leguminosae); 5 ii. black propolis from Amazonia, derived from exudates of flowers of Clusia (Clusiaceae); 6 iii. Yellow and brown propolis, produced in several regions of the country, mostly with unknown botanical origin. 7 Considerable variation on the composition of propolis may occur in the same geographic area. 8 Irrespective of locality and type, in general propolis has high contents of phenolic substances and high antioxidant and antibacterial activity. Some of these characteristics have been recommended for standardization 3 and evaluation of propolis quality. 9 It has been claimed that many plant species have been reported as sources of resin for propolis production. Indeed, if we take into account plant sources of all types of propolis from Europe, Asia, Oceania, Africa and Americas we come up with a considerable number of species. However, if we center our focus on Brazil, one of the top countries regarding propolis production and a country whose propolis is among the most studied in the world, it is surprising the low number of species which have been reported as propolis sources. In addition to the plants listed above (Baccharis, Clusia and Dalbergia), recently it has been shown that Mimosia tenuifolia (Leguminosae) from northeast Brazil is source of resin for a new type of green propolis. 10 Few other plant species have been suggested as sources of Brazilian propolis, among them Araucaria heterophylla 11 and Hyptis divaricata. 12 Such paucity of plant species providing resin for propolis production is amazing, especially taking into account that Brazil houses over 56,000 native plant species, amounting to about 19% of all world plant diversity. 13 Of course, the strong asymmetry between the wide available plant diversity and the narrow spectrum of plant sources of propolis holds not only for Brazil, but for all other parts of the world. A quite different picture characterizes the diversity of plants exploited as sources of nectar and pollen. Many species, distributed in a large number of plant families are bee pollinated. Requisites of flowers to be honey bee hosts are essentially an open corolla, olfactory and visual cues, such as blue or yellow pigments and/or ultraviolet absorbing phenolic substances. 14 Pollen and nectar, in combination, provide moisture, sugars, lipids, proteins, amino acids and other nutrients. Functional substances may also be provided by pollen, such as carotenoids, flavonoids and other phenolic substances. 15 Thousands of extant plant species have co-evolved with insects bearing lapping and chewing mouthparts, such as honeybees. Instead, the resin collected by honey bees from certain plants (generally different from those providing nectar and pollen) has a distinct biologic meaning for the hive. In addition to physical purposes of sealing holes and crevices on the nest, propolis is important as a means to provide resistance against health problems caused by microorganisms. It has been argued that propolis enhances the social immunity of the hive, by acting as an important antimicrobial layer. 16 After parasite infection of the hive, it was observed an increase of the number of individuals foraging for resin, a behavior that was interpreted as self-medication at the colony level. 17 This hypothesis is consistent with the multiple reports of antibacterial, antifungal and antivirus activities by propolis and its isolated constituents. 2 Among secondary plant metabolites, phenolic compounds are widely known to have antimicrobial activity. 18 Another class of secondary Abstract Propolis is a honeybee product containing beeswax and resin stemming from plants. It provides protection to the hive against microbial infection. Potential plant sources of resin with antimicrobial secondary metabolites are plentiful, but the mouthparts of honeybees enable them to exploit only a narrow diversity of species.