Pollination is a crucial part of growing quality apples. Apples require cross-pollination -- bees moving pollen from a pollen-donating tree to the receiving tree. Pollen-donating trees must be a compatible cultivar that has been intercropped (e.g., planted in alternate rows) or crabapple trees that have been interspersed within the apple orchard for this purpose.
Typically, honey bees visit flowers in the morning. Orchard management practices such as pesticide applications or mowing that disrupt their morning activity may significantly impact the success of pollination. Apples that do not receive adequate pollination can become malformed as they develop, or will result in early fruit drop. Apple ovaries are typically divided into five chambers, each containing two ovules available for pollination. A fully pollinated apple will contain 10 seeds; however, a minimum of 6 to 7 seeds per apple will succeed in good fruit development.
The average blossoming period for apples when pollination can take place is about 9 days. Cool weather during bloom will extend this period, whereas warmer weather will shorten it.
Honey bees (Apis mellifera), although not native to North America, are the most important and most commonly used managed pollinator of apples (read more on honey bee biology). To find a commercial beekeeper to hire for pollination services, contact the District Representative of the Michigan Beekeeping Association nearest you.
Other less commonly used managed bees for apple pollination are bumble bees and mason bees. Colonies of bumble bees can be purchased from commercial rearing facilities and must be ordered months in advance of when they are needed (read more on bumble bee biology).
Mason bees are solitary bees that will nest in large aggregations in nesting materials built out of cardboard or paper straws, cut pieces of bamboo, or blocks of wood with pre-drilled holes of a particular diameter. There are typically limited supplies of commercially reared populations of both native and non-native mason bees used for orchard pollination. The most common native species managed for orchard pollination is called the Blue Orchard mason bee (Osmia lignaria), and the most common non-native species is called the Horn-faced bee (Osmia cornifrons).
Growers can gradually build up their own native populations of mason bees by putting out nesting materials each season and following standard recommendations for keeping the bees cool for overwintering and then bringing them out of cold storage in time to be active for apple bloom.
For more information about mason bees and how to manage them, download the fact sheet, “Tunnel Nests for Native Bees,” from the Xerces Society. Or, you can purchase their excellent guide, "Managing Alternative Pollinators: Handbook for Beekeepers, Growers, and Conservationists".
Aside from managed bees, wild bumble bees and a variety of solitary soil- or stem-nesting bees can be found visiting and pollinating apples blossoms. Many of these bees nest directly in orchards or in adjacent habitat and are usually limited by the amount of non-crop flowering habitat that is adjacent to the orchard and the pest management practices used in nearby orchards. To build up populations of wild bees, growers are encouraged to provide non-crop flowering plants in adjacent habitats to the orchard – preferably in areas that will not receive pesticide applications or major drift from pesticides used in the orchard.
The Natural Resource Conservation Service has several conservation reserve programs that can help offset the cost of planting pollinator habitat and offer guidance about what to plant. There are also two Michigan State University Extension bulletins free as a PDF download or for purchase through the bulletin office for a print version: "Attracting Beneficial Insects with Native Flowering Plants" (E2973) and "Conserving Native Bees on Farmland" (E2985).
Pollinators & pesticide use
Pesticides, and in particular insecticides, can be harmful to pollinators. Most pesticide labels advise against their use during crop bloom for this reason. If an insecticide must be used during bloom, be sure to follow label directions and apply the pesticide when bees are least active, and so that the pesticide will dry before bees come into contact with flowers that have been exposed to it. For example, an application made at dusk or during the night will do the least harm to pollinators who visit flowers during the day. After the crop has finished blooming, be aware that pesticide drift onto non-crop flowering plants in adjacent habitat can harm pollinators on those flowers.
Read the following Michigan State University Extension article on "Minimizing pesticide exposure to bees in fruit crops."