Bee pollination of crop plants under environmental conditions unique to enclosures

Many high-value cash crops that in former times were exclusively cultivated in open fields are now grown in greenhouses and nethouses. This shift has occurred mainly to facilitate plant protection, to enable out-of-season production, to prevent environmental hazards and to isolate plants for production of true seeds. A wide variety of fruit and seed crops that are grown in greenhouses and net-houses are pollinated by insects (Sommeijer and Ruijter, 2000), mainly honey bees and bumble bees (Free, 1993; Velthuis and van Doorn, 2006). The main greenhouse crop pollinated by bumble bees is tomato (Lycopersicon esculentum), comprising about 95% of all bumble bee sales worldwide and involving over 40,000 ha of greenhouse culture (Velthuis and van Doorn, 2006). Most enclosure grown crops other than tomato can be pollinated by either pollinator, generally with similar levels of effectiveness, as was found with sweet pepper (Capsicum annuum) in an artificially heated greenhouses (Dag and Kammer, 2001), with strawberry (Fragaria ananassa) in an unheated greenhouse (Dag et al., 1995) and with squash (Cucurbita pepo) in a net-house (Dag, 2006). At relatively high temperatures (32°C and above), honey bees forage freely in greenhouses (Spangler and Moffet, 1979) and have an advantage over bumble bees, as bumble bees cease foraging activities above that temperature (Kwon and Saeed, 2003). Conversely, bumble bees will fly at relatively low temperatures (9–10°C), while honey bees do not forage at temperatures from 16°C and below (Heinrich, 2004; Dag et al., 2006). The bumble bee species mainly used for greenhouse pollination in Eurasia is Bombus terrestris. In North America, similar considerations have led to choosing B. impatiens for areas east of the Rocky Mountains and B. occidentalis for areas west of the Rockies. In East Asia, B. ignitus has been selected, while in China, both B. ignitus and B. lucorum are preferred (Velthuis and van Doorn, 2006). One species of honey bee, Apis mellifera L. , is used worldwide, with variations in genetic strain based on the genetic lines available in a particular country. Attempts have been made to use other bee species for pollination in enclosures, but this is not an easy task. The bee species that visit and pollinate the flowers of the target crop must first be identified, then efficient ways to propagate promising species must be learned, and finally, effective ways to combine crop plants and pollinator bees within enclosures must be determined. In Australia, an intensive effort was made to find native bee species for pollination of greenhouse tomatoes as an alternative to introduced bumble bee species. One attempt (Hogendoorn et al., 2000) involved two species of carpenter bee, Xylocopa aeratus and X. bombylans. Two other attempts in Australia involved the blue-banded bees Amegilla (Zonamegilla) holmesi (Bell et al., 2006) and A. chlorocyanea (Hogendoorn et al., 2006). Elsewhere in the world, attempts have been made to use such stingless bees as Tetragonisca angustula and Nannotrigona testaceicornis for pollination in enclosures in Costa Rica (Sánchez et al., 2001) and N. perilampoides for greenhouse tomatoes in Mexico (Cauich et al., 2004). In the United States, Waller et al. (1985) experimented with X. varipuncta on cotton. However, at the present time, none of these bee species are used on a commercial scale. The current review presents the effect of the unique environmental conditions prevalent in greenhouses on plant (crop) and pollinator (bee) interrelations.