Pollinating apples and cherries
East Lansing, Mich. - Without bees doing their business among the blossoms, cherry and apple trees would yield poorly and the apples would be deformed and smaller.
" Apples and cherries would not be possible without the contribution of bees. Adequate pollination is essential for good yields and marketable fruit," Jullianna WIlson, Michigan State University tree fruit IPM outreach specialist, said during a "Integrated Crop Pollination" webinar on pollinating the two crops.
Wilson collaborates with research and extension personnel to develop sustainable solutions for producing tree fruit. Her presentation was part of a series being sponsored by eXtension's Bee Health Project and funded by USDA-NIFA Specialty Crop Research Institute.
Apples and sweet cherries definitely need cross-pollination and so do sour cherries which, although they are considered self compatible, to benefit by the movement of pollen by bees in the wind.
The window of pollination is very short for both crops and particularly so for cherries, with flowers typically viable for less than a week.
Crops can fail for a variety of reasons, including pollen failure. To be fruitful, Wilson explained, each apple or sweet cherry blossom needs pollen from a compatible variety. She advised visiting the University of Missouri Extension website, which offers charts of compatible pollinators.
Another major component of pollination and crop failure is the weather. Set the stage for success by planting in a site that minimizes cold damage, such as a hilly area that allows cold air to drain away. Avoid hollows, where frost can gather, and near wood lots where cold air will settle on the fruit trees.
In Michigan, growers add mechanical strategies like pulling the warmer down with helicopters or using frost fans, an increasingly popular option for mixing air. In high density plantings and pear orchards, they might apply water to form a layer of ice covering the entire tree.
Other orchards apply water to form a layer of ice on orchard floor, which can increase the temperature in the orchard by a few critical degrees. This is usually done before bloom in an effort to protect the cold susceptible buds.
It can also be too wet or too cold during bloom for pollinators to fly while the flowers are viable. Although bumble bees, who can generate their own heat, and native bees will fly when the temperatures are below 55 degrees, Wilson noted, honeybees wait until it's around 60 degrees.
Crops also fail for insufficient numbers of pollinators, but this problem is one of the easiest to fix. The most common approach is adding more managed pollinators, usually by renting honey bees.
They are active, as long as the weather is favorable, can pollinate many crops and come in portable colonies that allow thousands to be put to work. "Honeybees actually like going into orchards," Wilson said. "The flowers and nectar are very available."
The recommended stocking density for apples averages around 1.5 hives per acre, but varies from 0.25 to 5 hives per acre. For cherries, the average is one hive per acre, but varies from 0.5 to 5 hives per acre. The wide variance in stocking rates reflects the abundance or lack of wild pollinators.
Bumble bees are also available in purchased colonies. They are out of sync with natural colonies, where only the overwintered queen will be active at blossom time; are pollinators of many crops and will be active for 6-8 weeks.
In Japan, orchard mason bees have long been used for pollination. The solitary bees will nest in bamboo or hollow straws in large aggregations and are particularly suited to orchard pollination, since they naturally occur in early spring and are active for about four weeks.
Orchard bees are portable after nesting is complete by removing the cocoons and storing them for the next season. Some Michigan growers are presently sheltering them in pole barns over the winter. "They're really an interesting bee," Wilson observed.
In Michigan, over 80 species of wild bees are active during the spring bloom. Most are solitary bees, often nesting in the soil of weed-free strips in orchards. Their numbers are limited by the abundance of flowers that bloom before and after the crop.
Research shows that wild bees do not compete with honeybees, Wilson said, but actually improve the way honeybees work by disturbing their methodical pollinating patterns. The result is a better spread of pollen amongst the blossoms.
The most important of the wild bees is the digger bee (Andrena spp.). They are active for 3 to 4 weeks and spend the rest of the season underground, anywhere from 6 inches to 3 feet down, where they are well protected.
The big question is how fruit growers can minimize harm to honey bees, wild bees and other pollinators without compromising pest and disease management, Wilson said.
Fungicides are critical during bloom to manage diseases like cherry leaf spot, which causes severe defoliation and tree decline, and the foliar and fruit damage caused by apple scab.
In the past fungicides were considered safe based on lab-tested toxicity levels, but new research has identified sub-lethal effects on colony health. "We know there is increased toxicity of pyrethroids when applied in combination, and increased toxicity of miticides, used in the beehive, when combined with fungicides," she said.
A synergistic effect has also been documented, as well as interference with detoxification, Nosema immunity, gut activity and the fermentation forces required for producing the high-quality bee bread fed to the young.
New EPA labels are being put on pesticide products with a bee icon to help signal the pesticides' potential hazard to the pollinators. "It is mainly to alert people to give them more to work with," Wilson said.
Recommended best management practices
During pre-bloom, orchard owners need to clarify expectations with their beekeeper and keep records, on both sides, setting the delivery date and approximate removal date, where the hives will be placed and expected hive strength.
Between 6 to 8 frames with 70-75 percent capped brood cells per frame is reasonable, Wilson noted.
Hives can be dispersed through the orchard, but honey bees are excellent fliers. "They will go where the food is, so you don't need to put them in harm's way. A better location is upwind of the orchard where they won't be sprayed or in the path of drift," she said.
During bloom, select the least toxic pesticide, whenever possible, and calibrate sprayers according to the target orchard to maximize coverage and minimize drift.
Spray when the bees are less active, which is when temperatures are lower than 55 degrees or after sunset, and turn off sprayers when near the hives.
Post bloom, communicate with the beekeeper as to spraying times and minimize flowers in the orchard floor with selective herbicides or by mowing before spraying. Removing the hives is also an option and a good idea.
Plant non-crop flowers outside of the orchard. Wilson advised planting natives following the NRCS technical guide #20. The summer blooms will draw the bees out of the orchard and out of harm's way, she explained.
Since bees with access to diverse pollen and nectar sources are known to be healthier and live longer, Wilson proposes that orchards with space, especially low-lying areas and up against wood lots, plant native flowering plants or simply let the land go native. "Just not mowing an area allows for lots of floral resources," she said.
The plantings or fallow land will also provide more nesting resources and attract soil-nesting native bees. "Planting non-crop flowering plants outside of the orchard will benefit all pollinators," she added.
For more information and resources, check out MSU Bulletin E3245 (free pdf) for toxicity levels of pesticides, MSU Bulletin E3282 for bees of the Great Lakes and wildflowers to support them, as well as the ICP website at www.icpbees.org.