Pollination can happen by chance. But great pollination – and a successful crop – requires management. Poor pollination will reduce crop size, and apples that are not properly pollinated will be oddly sized and shaped. Apple growers can take steps to ensure pollination occurs, including managing trees, orchard environment and bees.
“Without it, we wouldn’t get that initial fruit set,” Julianna Wilson, Integrated Pest Management outreach specialist, Department of Entomology, Michigan State University, said of adequate pollination. “We need bees to move the pollen in order for there to be fruit set. When we talk about pollination, growers really are focused on pollination for a short period of time.”
But pollination is not a one-shot deal. Managing orchards for prime pollination requires diligence year-round. To assist farmers in achieving the best pollination possible, the U.S. Department of Agriculture-funded Integrated Crop Pollination Project (http://icpbees.org) has the goal of investigating pollination strategies for various specialty crops. Wilson is a member of the ICP team, specializing in pollination of apples and cherries in Michigan and the Northeast.
Orchard design and pollination
Site selection has much to do with pollination. If the orchard is not protected from frost or from cold temperatures, buds coming out of dormancy can be damaged, preventing proper pollination. Use of frost fans and other protection to create inversion layers and “minimize the potential settling of cold air on the trees,” Wilson said, is the second line of defense. Increasing the orchard temperature through use of irrigation to form a layer of ice on the orchard trees or the orchard floor is another method of protecting critical flower buds. As water changes form, heat is released, increasing the orchard temperatures by a few degrees.
Even if buds survive the frosts and cold weather, the trees face other challenges. The flowers, only viable for a short period of time, may not be able to properly grow their germ tubes if the weather during bloom is too cold. One method of delaying bloom is to use overhead irrigation, particularly in high-density orchards, allowing temperatures to warm up before bloom occurs.
Because apples need to be cross-pollinated, intercropping crab apples or compatible apple cultivars with overlapping bloom time increases the chances of pollination success. Alternating rows of compatible cultivars within a block can help get the best results. Pollination with incompatible pollen will result in deformed apples.
Because cold weather also impacts bees, bloom time temperatures below 55 to 60 degrees Fahrenheit will cause reduced pollination. Native wild bees can operate at the lower numbers in this range, whereas honeybees are not normally active until temperatures reach 60 degrees Fahrenheit.
Attracting managed bees and wild bees to the orchard can enhance pollination. Wild bees do not interfere negatively with honey bee pollination, Wilson said. Instead, native bees are thought to cause honeybees to alter their straight-line pollination pattern. This is beneficial in the orchard, as it causes honeybees to visit other tree rows, with other cultivars, increasing the cross-pollination that occurs.
Honey bees from managed hives do well in apple orchards. They can fly a good distance, so hives can be placed outside of the orchard block, keeping the colonies a safer distance from any chemical applications needed. The density of hives for optimal pollination varies, with recommendations ranging from 0.25 hives/acre to 5 hives/acre, depending on the cultivar needing pollination and the presence of wild pollinators.
Orchard mason bees and bumblebees are also sometimes managed to promote pollination. Bumblebee colonies are active for about six to eight weeks. Managed colonies are “out of sync” with any native colonies, with purchased colonies providing more early-season activity than naturally occurs.
Mason bees are solitary bees that are active for about four weeks. They nest in aggregates, or mud tubes with caps, but work independently from one another. These Osmia spp. of bees include some native to the eastern U.S., and are naturally active in the spring. These bees “take a little finesse” to manage, with cocoons needing to be properly stored, but some Michigan growers are using this approach to enhance pollination, Wilson said.
Wild bee populations need a long season of flowers, with bloom pre- and post-apple blooming, to survive. Woodlands adjacent to orchards and wildflower populations – either planted or natural – near the orchard edges will bring these bees into the orchard during bloom. Ground-dwelling bees like to nest in the weed-free strips under orchard trees.
Although apple flowers are not specialized to certain bees, “solitary bees are not collecting nectar, and so since they are more focused on pollen collection, they are thought to be more efficient” than honey bees, she said.
Syrphid flies are another orchard pollinator. Although they do not collect pollen as bees do, they transfer pollen from flower to flower on their bodies, promoting pollination.