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Pollinators: - first steps for their conservation
Dr Sarah Gess
Department of Entomology, Albany Museum
In Southern Africa as a whole, little thought seems to have been given to employing practices which preserve or enhance pollinator populations of importance to agriculture and even less thought has been expended on the maintenance of pollinator populations of importance for maintaining indigenous plant diversity.
Pollinators are a recognized conservation priority. Much is being said to popularize the cause. It is being said that "we can thank a pollinator for one out of every three bites of food" and "that pollinators are vital in sustaining both natural diversity and variety in our diets".
Africa has joined the initiative. (see earlier
article)
What is the next step? With increasing numbers of people putting increasing pressure on the land, what can be done to check pollinator decline?
The key is to generate an understanding of pollinator requirements and to build these requirements into land management. Most plants requiring pollen vectors to achieve pollination, are serviced by insects. Insects from several orders are involved, most notably bees and wasps
(Hymenoptera), butterflies and moths (Lepidoptera), flies (Diptera), and beetles (Coleoptera). The requirements for their survival are diverse and to a large degree poorly understood.
Not all insects seen visiting flowers are pollinators. Not all insects seen collecting pollen are pollinators. For an insect to be a pollinator it must, when visiting a flower with ripe pollen, receive pollen in such a way that, when it visits another flower of the same species, which has a receptive stigma, the pollen is transferred from it to the stigma. This is usually achieved by insects visiting flowers to obtain nectar, the pollen being deposited on them due to their positioning within the flower. The pollen deliberately collected for provisioning by bees and pollen wasps, packed away, held by special hairs on legs or the underside of the abdomen or swallowed and carried in the crop, is not available for pollination. Almost all adult aculeate wasps and bees drink nectar for their own nourishment and are therefore flower visitors and worthy of consideration as potential pollinators. However, that bees and pollen wasps provision their nests with pollen and nectar enhances their potential because they make more numerous visits to flowers than do the majority of wasps which provision their nests with insects or spiders.
It must be emphasized that there are no truly generalist pollinators. No bees, not even honeybees, visit all flowers and not all bees seen visiting flowers are pollinating them. For example, although honeybees visit legumes with "pea flowers" as a general rule they are not their pollinators but are thieves, taking
nectar without servicing the flowers. Thus to maintain pollination services the requirements of all wasps and bees should be taken into account.
What are these requirements? The majority of these insects build nests and therefore suitable nest sites must be maintained. Nests, depending on the species of the nester, are sited in the ground, in or on vertical banks, in or on plants, or on stones.
Some ground nesters require friable soil and others hard clayey soil, in which to excavate their nests. In all cases ground which is not heavily trampled by stock nor disturbed by regular cultivation or any other destructive activity must be available. If the soil chosen is clayey, then, in addition, water is required. The water is imbibed by the nest builder and regurgitated at the nest site to be used to soften the ground so that a shaft and cells may be excavated. Such nests are usually surmounted by characteristic, delicate superstructures - nest entrance turrets - constructed from mud pellets extracted from the shaft. The water used must not be polluted and the water source should have a shallow shore at some point because many species require to stand at the waters edge whilst imbibing water. Others stand on the water surface and so this cannot be fast moving.
Vertical banks of soil or rock are home to a diverse array of nest excavators and users of pre-existing cavities. For this reason, cutting back of an established bank may be disastrous for a wide range of species.
Plant stems used by some species may be green but most species excavating nests or using pre-existing borings in plant stems choose dry stems, these according to species may be large branches of trees, twigs of various diameters, pithy stems such as aloe inflorescences or hollow stems such as those of reeds. Bush clearing, the removal of trees, shrubs and reeds associated with watercourses and heavy collection of firewood can have grave consequences.
Users of pre-existing cavities and constructors of aerial nests, that is nests on a support, be it a bank, rock, stone or plant, require building materials. These may be, for example, mud, resin, plant fibres, loose soil, small pebbles,
discs cut from leaves or petals according to species. Non-availability of necessary materials will preclude nesting success.
In addition to the availability of suitable nesting sites and nesting materials, suitable provision for the larvae must be available. In the case of bees and pollen wasps there is considerable diversity in the degree of specialization in choice of nectar and pollen, some species being very limited in their choice and others much less so. In the case of the majority of wasps, those which provision with insects or spiders, there is similarly a range from species specific to group specific prey choice. Clearly presence or absence of provision is dependant on the presence or absence of the
requirements of the prey organisms. Therefore what appears to be small changes in an ecological system may have a cascade effect.
That the nesting requirements of solitary bees (and by extension of solitary wasps) can be successfully taken into account in the management of farmed land is not a new concept, already having been abundantly demonstrated in the United States of America at least thirty years ago. Pollination of crops was enhanced by leaving fallow strips for ground nesters and by providing bee boards (blocks of wood with suitable borings in them for nesters in pre-existing cavities). Similarly, the value of using bee boards to enhance populations of megachilid bees for the pollination of Lucerne (alfalfa) has been demonstrated in the Little Karoo,
 South Africa. However, in southern Africa as a whole, little thought seems to have been given to employing practices which preserve or enhance pollinator populations of importance to agriculture and even less thought has been expended on the maintenance of pollinator populations of importance for maintaining indigenous plant diversity.
Providing for the needs of pollinators could readily become part of farming and conservation practice. The needs of bees and wasps can be catered for when designing irrigation systems or stock watering points. In the case of stock farming and game ranching care can be taken not to overstock and nesting areas of ground nesting bees and wasps can be protected from trampling.
When flower reserves or general nature reserves are created, it should be born in mind that most flowering plants, even those which can self-pollinate in the absence of pollinators, require cross pollination, if they are not to degenerate over time. Before delimiting the reserves preliminary studies of the area should be undertaken to establish what insects are associated with what plants and what are the requirements of those insects. It should be possible, at least in the case of bees and wasps, to ensure that their nesting sites are included in the reserve and such requirements as suitable water sources supplied.
Since 1972 Fred and Sarah Gess of the Entomology Department at the Albany Museum have been conducting an ongoing study of the role of solitary bees and aculeate wasps in semi-arid areas, in particular the Karoo Biome, southern Africa, which aims to obtain a knowledge and understanding of diversity, distribution, roles in the ecological system, prey and plant preferences, importance as predators and pollinators, and nests and nesting requirements. A full list of publications by themselves and their erstwhile colleague, Allan Weaving, on the subject can be found at:
http://www.ru.ac.za/affiliates/am/ento.html
More Information:
Dr Sarah Gess, Department of Entomology, Albany
Museum Grahamstown, 6139 South Africa. E-mail: S.Gess@ru.ac.za
Particularly relevant to the present topic are:
Gess, F. W. and S. K. Gess. 1993. Effects of increasing land utilization on species representation and diversity of aculeate wasps and bees in the semi-arid areas of southern Africa. In: LaSalle, J. and I. D. Gauld, eds, Hymenoptera and Biodiversity. Wallingford: CAB International, pp. 83-113.
Gess, S. K. 1996. The Pollen Wasps: ecology and natural history of the Masarinae. Cambridge, Mass.: Harvard University Press. 340 pp.
Gess, S.K. 2001. The Karoo, its insect pollinators and the perils
which they face. International Pollinator Initiative (IPI) Case Study.
www.ecoport.org/EP.exe$EAFull?ID=35
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