The Importance of Crop Attraction to Bees

Ecologic theory supports the notion that bee visitation, pollination and seed yield. Likewise, low nectar quality and associated low levels of bee visitation are limiting factors in fruit-set in avocado. There is a clear and positive relationship among nectar sugar concentration, frequency of bee visitation, and resulting seed number in watermelon. Thus, theoretical work and supporting field studies strongly suggest that it is in the best interest of farmers to grow crop plants that are attractive to bees.

Nectar production is affected by ambient conditions and culturing practices. Nectar production is relatively high under conditions of low nitrogen supply, moderate growth, and high levels of sugar in tissues. It is lower under conditions of abundant nitrogen supply, high vegetative growth, and low sugar levels. Nectar production is generally higher in sunny weather because sugars accumulate in plant tissues during photosynthesis. These sugars may reach a surplus and be excreted as nectar if plants are not growing maximally. However, if nitrogen is available it encourages plant growth which diverts stored sugars into proteins and other products necessary for producing tissues. Thus, nectar production tends to be lower in plants that are growing rapidly. (In perennial plants that bloom before leaves unfold in spring, nectar production relies on sugars stored in tissues from the previous season.) The model of nectar production presented above is not universally applicable; for example, nectar production in the Mediterranean herbaceous perennial Ballota acetabulosa is relatively unresponsive to measured changes in solar irradiation.

There is evidence that nectar production also may be affected by plant genetics. There are measurable differences in nectar production within crop genera or species, as in pepper, cranberry, and watermelon, and sometimes these differences are apparent even when environmental or cultural effects are controlled. These studies suggest that nectar production is at least partly under genetic control and could be increased by selective plant breeding.

The time is right to give renewed attention to increasing nectar production in the world’s most important bee-pollinated crops. This is especially justified given the evidence for a generally decreasing pool of available bee pollinators. Nectar production has received comparatively little attention from crop breeders, agronomists, and horticulturists. Low nectar production may not be a problem in areas with abundant bee populations, but where bees are scarce a nectar-poor crop will have trouble competing with weeds for the
limited number of pollinators.

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