Insect pollinators and sustainable agriculture Peter G. Kevan, E. Ann Clark, and Vernon G. Thomas Abstract. Underestimation of the pivotal role played by managed and native insect pollinators is a key constraint to the sustainability of contemporary agricultural practices. The economic value of such insects to pollination, seed set, and fruit formation greatly outweighs that suggested by more conventional indices, such as the value of honey and wax produced by honeybees. Although the European honeybee has been widely regarded as the single most important pollinating species, the increasing spread of trachael and Varroa mites and Africanized bees threatens the distribution and magnitude of tradi- tional honeybeekeeping enterprises in North America. A number of other bee and insect pollinators, such as orchard bees, bumblebees, and squash bees, which are not affected by either the mites or the Africanized bees, are considered as likely candidates for management and use in commercial agriculture. An additional role can be played by native or wild pollinators, provided that attention is given to curtailing of population losses caused by both inadvertent insecticide poisoning and habitat destruction. To ensure a reliable source of pollinators, both managed and native, a more comprehensive strategy for management of crop pollination is needed. Elements of this strategy include an increased understanding of the biology and ecology of pollinating insects, as well as providing appropriate nesting habitat, and ensuring the availability of alternative sources °f 'forage 99 to sustain populations when the target crops are not in bloom. Examples are discussed to illustrate how private initiatives and changes to public policy can enhance pollinator habitat, and ultimately, agricultural productivity. Key words: orchard bees, bumblebees, squash bees, ecology, habitat conservation, private initiatives, public policy, alternative pollinators Introduction Agriculture that is sustainable explic- itly recognizes its ecological foundations (Commoner, 1972; Altieri, 1987). Con- versely, agricultural practices which fail to acknowledge and enhance flows of energy and material through natural processes, or worse, which eclipse and disrupt such processes, become increas- ingly dependent on exogenous subsidies of energy and material to sustain pro- ductivity (Clark and Christie, 1988). At present, design and management of sus- tainable agricultural systems are at risk because of ignorance of the contribution Peter G. Kevan is Associate Professor of Environmen- tal Biology, E. Ann Clark is Assistant Professor of Crop Science, and Vernon C. Thomas is Professor of Zoology, University of Guelph, Guelph, Ontario NIG 2W1, Can- ada. of managed and native insect pollinator species to the function of ecosystems and, specifically, to managed agroeco- systems. Principles of pollination Pollination is the first step in the sex- ual reproduction of plants. It is the transfer of pollen from the male organs or anthers to the female receiving ele- ments, the stigmata. As such, pollination is an essential prerequisite to seed and fruit development on most temperate and tropical crops. Furthermore, irre- spective of whether or not the crop com- modity is cereal, oilseed, vegetable, or forage, seed production for replanting most agricultural and horticultural crops also relies on pollination. Thus, it may truly be said that pollination is a pivotal, keystone process in both natural and managed ecosystems (Kevan and Baker, 1983, 1984). Self-pollination and cross pollination (Figure 1) may be effected by gravity, wind, or animals (mostly insects). Some crops, such as varieties of soybean (Gly- cine max (Merrill) L.) and barley (Hor- deum vulgare L.) are self-pollinating before the flowers open (cleistogamy). Others which are self-pollinating and en- tirely self-compatible are pollinated after the flowers open (chasmogamy), a com- mon situation for crop plants and weeds. Nevertheless, even some self-pollinating crops produce better when outcrossing occurs, as has been found in canola (Brassica napus L.) (Kevan and Eisi- kowitch, 1990), and in self-compatible varieties of sunflower {Helianthus an- nuus L.) (Freund and Furgala, 1982) and soybean (Erickson et al., 1978). In self-incompatible plants, self-pollination will not effect fertilization, and outcross- ing is required, as for many stone and pome fruits. Floral morphology reflects the means of pollination. For example, wind is the primary agent of pollen transfer in for- age grasses, which are equipped with large, feathery stigmata, prolific pollen production, and inconspicuous flowers. But for many species with colorful, scented, and conspicuous flowers, pol- lination is the outcome of the lengthy coevolution of plants and their pollina- tors (Crepet, 1983). The most abundant and diverse pollinators are insects, which may transfer pollen within or among flowers on the same plant, as in self-pollinating species, or between flow- ers on different plants, as is required by obligately cross-pollinated species. Insect pollination in agriculture The importance of insect pollination has been documented in treatises by Free (1970), McGregor (1976), and Pesson and Louveaux (1984). The economic value of insect pollination to all agri- cultural production in Canada and the Volume 5, Number 1, 1990 13 http://dx.doi.org/10.1017/S0889189300003179 Downloaded from http:/www.cambridge.org/core. 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