Stingless Bees in Crop Pollination
Dr. Madhurima Vinod and Dr. Sunil Kumar K
Stingless bees are a group of small- to medium-sized bees, with vestigial stings, found in tropical and many subtropical parts of the world. They are the major visitors of many flowering plants in the tropics. They show a level of social organization comparable to that of honey bees. Colonies are perennial and usually consist of hundreds or thousands of workers (Wille, 1983). The estimated several hundred species of stingless bees are arranged into 21 genera (Michener, 1990). The rank of the group has varied but recently has been placed at tribe. The most important genera are Melipona and Trigona. Melipona consists of »50 species, is confined to the neotropics, has more complex communication systems (Nieh and Roubik, 1995), and is capable of buzz pollination (i.e. ejecting pollen grains by vibration of the pollen-bearing anthers of flowers that dehisce pollen through pores). Trigona is the largest and most widely distributed genus, with »130 species in »10 subgenera, including the neotropical Trigona sensustricto and most of the Asian Meliponini.
It is often stated that stingless bees are important pollinators of crops in tropical and subtropical parts of the world. Stingless bees make an important contribution to pollination in crops such as coconut, mango, coffee, avocado, strawberry, sweet pepper, tomato, cucumber etc (Slaa et al., 2006). The use and management of non-Apis bees and other insects for crop pollination is important because of the almost total reliance of world agriculture on honey bees. In many locations and for many crops, the ability of honey bees to pollinate is threatened or limited because of factors such as Africanization, diseases and parasites, low efficiency on some crop species, climatic limitations, and economic pressures.
Recently, numbers of both managed and wild bees are declining rapidly, causing global concern for pollination services. Threats include habitat destruction or alteration, overuse of pesticides, parasites and diseases, and the introduction of alien species (Kremen et al., 2002). Management of honeybee hives is handicapped worldwide by infectious diseases and parasites such as varroa mites (Varroa destructor), American Foul Brood, (Bacillus larvae), and Chalk brood (Ascosphaera apis). Diversification of crop pollinators would help to achieve pollination services when the commonly used pollinator (specifically honeybees for most crops nowadays) is not available in sufficient numbers. In addition, honeybees are not always the most efficient pollinators due to various factors, e.g. a miss-match in body size and flower size, low nectar production and specialized pollen release mechanisms in some plants, including those with poridical anthers. When honeybees do not efficiently pollinate a given crop, it is probably economically beneficial to search for a better pollinator-plant match. It has been estimated that in the US alone, the commercial value of non-honeybee pollinators to crop yields may be as high as $6.7 billion per year (Kearns et al., 1998).
A-five year survey was undertaken to explore the diversity and distribution of Stingless bees in India. The study revealed that the survey area had two genera and six different species of Indian stingless bees that had been explored and redescribed. The distribution pattern of stingless bees in India, four species of stingless bees were prevalent in South India except Tetragonula bengalensis and T. ruficornis. In case of North East India, five species i.e. T. bengalensis, T. iridipennis, T. ruficornis, T. laeviceps and Lepidotrigona arcifera were dominant where as T. iridipennis and T. laeviceps were present in the North West India. The study also revealed that T. iridipennis and T. laeviceps were most commonly available species in all selected zone of India.
Why stingless bees?
Stingless bees comprise a highly diverse and abundant group of eusocial bees that inhabit the tropical and subtropical parts of the world. Stingless bees form perennial colonies from which they forage year-round. Worldwide several hundred species exist, which differ significantly in colony size (from a few dozen to tens of thousands of individuals), body size (from 2 to 14 mm; compare to 12 mm for honeybees). Many characteristics of stingless bees resemble those of honey bees. Some of the characteristics that influence their ability as pollinators are:
• Polylecty and adaptability: This enables them to pollinate multiple plant species and adapt to new ones.
• Floral constancy: A worker on a trip usually visits only one plant species.
• Domestication: Colonies can be placed in hives, inspected, propagated, fed, requeened, controlled for enemies, transported, and otherwise managed.
• Perennial colonies: This allow workers to forage continuously within climatic constraints and obviate the need to develop colonies each year.
• Large food reserves are stored in nests:This has the obvious benefit of allowing colonies to survive long periods of low food availability. Additionally, it means that workers will collect floral resources beyond immediate needs, resulting in intensive visitation of preferred flowers.
• Possibility of in-hive pollen transfer: This decreases the need for bee movement between plants of self-incompatible species. It has been found for honey bees and is equally likely for stingless bees.
• Forager recruitment:Workers recruit nest mates to rewarding floral resources and provide information on the position of those floral resources, which allows the rapid deployment of large numbers of foragers relative to other bees and insects in which each individual has to find the resource (Heard, 1999).
Advantages and Disadvantages:
Unlike honey bees, stingless bees are generally less harmful to humans and domesticated animals and are able to forage effectively in glasshouses. Propagation of their colonies contributes to preservation of biodiversity by conserving populations of species that may otherwise decline owing to human disruption of ecosystems. Their colonies rarely abscond as the old queen is flightless and they are resistant to the diseases & parasites of honey bees. Thus a honey bee epizootic that disrupted pollination would not affect the stingless bees in that system. However, there is a poor level of domestication technology for most of the species. There is a lack of availability of large numbers of hives and colony growth rates are low compared to honey bees. Some species are unable to be domesticated because of specific nesting requirements, some species damage leaves in search of resin and some species are territorial and fight when placed in close proximity.
At present, numbers of both wild and managed bee colonies are declining rapidly, causing global concern for pollination services. Diversification of crop pollinators would help to achieve pollination services when the commonly used pollinator (specifically honeybees for most crops nowadays) is not available in sufficient numbers. Besides physical properties hindering efficient pollination, honeybees may have other drawbacks in pollination services. First, honeybee colonies have seasonal cycles with a long inactive period in temperate regions which make them less suitable for the pollination of off-season products. Second, honeybees have a functional sting, and although the tendency to sting is quite low in several breeds of European honeybees, it may still cause problems for crop workers who are allergic. In addition, all honeybees present nowadays in the Neotropics are Africanized which requires additional safety measures due to their aggressive nest defence. Stingless bees play an important ecological role as pollinators of many wild plant species and seem good candidates for future alternatives in commercial pollination. These bees are especially suitable to provide pollination services in greenhouses as they forage effectively under enclosed conditions and posses several biological characteristics favourable in applied pollination indicating they are strong candidates in the search for alternative pollinators for our crops.
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Dr. Madhurima Vinod* and Dr. Sunil Kumar K#
Research Associate (Agril. Entomology), Research Associate (Agronomy), ZBNF Project, ZAHRS Brahmavar, Udupi Karnataka