Orchard Pollination: Strategies for Maintaining Pollination Services in Tree Fruit

This means that the flowers of these tree fruits (Photo 1) need to receive pollen from a different variety in order to set fruit. (See: Orchard Pollination: Pollinizers, Pollinators and Weather)

Peaches, tart cherries, and apricots are self-fruitful, which still means pollination is necessary, but can be from the pollen of another tree of the same variety. Hence, they can be planted in solid blocks of the same variety, whereas pears and sweet cherry need to be interplanted with rows of other varieties. In the case of apple, trees are interplanted in the same rows with crabapple pollinizers (pollen sources) at a ratio of around 1:16. All of our tree fruits, however, are almost completely dependent on pollinators to move pollen between flowers with very little wind pollination.

Also critical to a fruit grower are the pollination requirements necessary to produce a commercial crop. In tart cherries, and to a somewhat lesser extent in sweet cherries, at least 80% of the flowers at bloom are required to set fruit from adequate levels of pollination to give commercial yields at a profitable scale of tons/acre.

With sweet cherries, fruit size can also be a factor at the market, and they may need to be hand-thinned early in the season to develop larger fruits. In apple, pear, peach, apricot, and plums, fruit quality and size are at least as important as yield to make a profitable crop. Therefore, only a small proportion of the flowers are necessary to produce a commercial crop of large, well-shaped fruit, and additional fruit will have to be chemical- or hand-thinned to eliminate excess fruit. If these extra flowers are not removed, the fruit will be smaller to fit the less valuable juice or processing markets rather than the profitable fresh market.

An additional consideration for fresh market apples and pears is fruit shape, as part of the quality standard required by the public. Both of these pome fruits potentially have up to 10 seeds in each fruit, affecting the fruit's size and shape at harvest. Fruits with fewer seeds are generally smaller, misshapen, and often abort from the tree during mid-summer due to drought, heat, or from having too heavy of a crop load (under-thinning). Seed development in apple and pear is an important consideration for growers because the number of seeds can only be maximized by adequate levels of pollination by bees.

Therefore, pollination is considered a critical aspect of tree fruit production. This is especially true for seasons when many orchards experience severe frost/cold injury to the blossoms. Maximizing the pollination of the few remaining flowers may mean the difference between having a commercial crop or not.

The main group of pollinators of tree fruit flowers are bees because they are great flyers that effectively transport pollen from flower to flower with their hairy bodies. Bees must be present and active in the orchard at the time of bloom because flowers are only open for 2 to 4 days, and the bloom period only lasts up to two weeks. Tree fruit flowers are visited by over 40 species of wild solitary bees (including mining, cellophane, sweat, and carpenter bees) that can effectively pollinate them (Photo 2; Table 1).

Pollination is often supplemented with managed honey bees (Apis mellifera) that are rented as insurance to guarantee optimal pollination in orchards. However, honey bees are less efficient than wild bees at transferring pollen from one flower to the other on a per-visit basis. For example, mining bees (genus Andrena) deposit, on average, 2.5 times more pollen than honey bees when they visit apple flowers. In addition to their lower pollination efficiency, problems with honey bee health over the past 15 years have increased the rental costs of hives, which translates into higher costs of pollination services for growers. Therefore, the most effective way to guarantee optimal pollination in orchards is to combine the pollination services of wild bees, honey bees, and alternative-managed pollinators. (See: Orchard Pollination: Wild Bees)

How to enhance wild bees?

Unmanaged wild bees have two basic needs: food and nesting habitat. In addition to the flowers provided by orchards, wild bees need complementary food resources that can be found in natural areas, floral strips, or weeds growing around the orchard. Wild bees need well-drained soil, hollow stems, rotting logs, or cavities in trees for nesting habitat. Creating more pollinator-friendly habitats by planting wildflower strips and supplementing nesting resources is an important first step to enhancing wild bee abundance and diversity. The Natural Resources Conservation Service (NRCS) has several pollinator conservation farm bill programs that pay farmers to maintain natural fence rows around orchards. Because both complementary floral and nesting resources are found in areas adjacent to orchards, wild bees are usually more abundant at the edges of the field. If the orchards have large acreage, trees near the edges tend to benefit the most from wild bee pollination, while trees at the center of the orchard may need to be supplemented with managed pollinators.

Are honey bees necessary for optimal pollination?

Managed honey bees can be essential to supplement the pollination services of wild bees, depending on the type of tree fruit that is targeted and the landscape context of the orchard. Apples only need about 2 to 5% of the flowers/tree pollinated, while cherries need 80% of the flowers pollinated. Given the large demand for pollination of cherry trees, managed honey bees are critical to maximizing yields in this crop.

While the pollination needs of apple, pear, apricot, and peach trees are lower, managed honey bees may be necessary at the center of orchards where wild bees may not be able to fly to (most wild bees have flight ranges of 1,500 ft or less). The number of necessary hives will depend on the size and spatial arrangement of the orchard, but the rule of thumb is to begin with one colony per acre and modify as needed. Strong, healthy colonies are key to maintaining the high pollination activity in tree fruit that is cross-pollination dependent, such as sweet cherry and apples. It is essential for farms that require honey bee pollination services to have a pollination contract with a beekeeper a few months before the trees are in bloom.

What alternative managed pollinators are best?

The Japanese Orchard Bee (Osmia cornifrons) is the alternative managed pollinator most commonly used for apple and pear fruit pollination. This bee was introduced to North America from Japan in 1977 to supplement the pollination services of honey bees in apple orchards. Because apples, pears, and O. cornifrons are all native to eastern Asia, these bees prefer to use apple and pear flowers as their main source of pollen and are highly efficient pollinators of both crops. Indeed, O. cornifrons can be up to 80 times more efficient than honey bees for apple pollination by efficiently pollinating and setting 2,500 apple flowers per day compared to setting only 50 flowers per day for a honey bee forager.

The Japanese Orchard Bee has a very efficient method of carrying dry pollen on its belly that is easily placed on the flower's stigma during each floral visit. On the contrary, honey bees wet the pollen and pack it onto their hind legs, making the transfer of pollen between flowers more difficult. The Japanese Orchard Bee and other wild solitary bees can be managed by providing artificial nests in the form of wood blocks, cardboard tubes, or bamboo (Photo 3-Left; also see Orchard Pollination: Solitary (Mason) Bees).

These nests should be positioned near the edge of the orchard and can be naturally colonized by O. cornifrons and other wild bees. If the abundance of these bees is low in your area, nest blocks can be seeded with cocoons from orchards that are already managing mason bees. Because mason bees are susceptible to diseases and parasites (Photo 3-Right), it is important to provide new nesting materials for these bees every year.

The phenology of both stone and pome fruits on diversified farms provides high-quality and continuous floral resources to managed and wild bees, starting with apricot in early April, then plum, peach, pear, and ending with apple in early May. Tree fruit orchards and surrounding woods and fencerows offer nectar and pollen to a large diversity of bee species, with over 260 species collected near Pennsylvania orchards to date. (See: Spring Bees: Who Are They and Where Do They Live?)

However, before, during, and after bloom, pome and stone fruit trees are susceptible to various pests and pathogens that require the application of pesticides and fungicides for effective control. (See: Orchard Pollination: Honey Bees) Insecticides should never be applied during bloom of any fruit crop, and care must be taken to limit petal fall applications to when almost all of the bloom is complete, as most orchard pollinators have only a single generation a year and can easily be wiped out from a late pink bloom/king bloom application in apple. In the past, when pollination relied on honey bees that could be moved out of the orchards after peak bloom, petal fall was defined as 80% of the flowers falling. However, when relying on alternative managed bees or wild pollinators, a single early petal fall spray on the 20% bloom can wipe out an entire generation of solitary bees.

As part of the new Integrated Pest and Pollinator Management (IPPM) paradigm, it is critical to minimize the number of pesticide applications, use the least toxic pesticides and formulations, and time the applications to minimize the risk of exposure to bees (see detailed recommendations in Tree Fruit Production Guide).

It is important to note that many wild bees fly earlier in the morning than the honey bee, and thus, spraying early in the morning is not as effective in reducing exposure.

Also note that when using plant systemic products like neonicotinoid insecticides and similar products (i.e., Assail, Actara, Sivanto, Closer, etc.), care should be taken to apply at the early ½ inch green timing rather than pink as it has been shown that applications close to bloom may result in the insecticide translocating to the nectar and pollen of the flower at relatively low levels that still may impact bees through chronic exposure over several days. Balancing the management of pollinators and pest control is critical to maximize tree fruit yields and sustaining healthy agricultural and natural landscapes.

Please watch an enjoyable 20-minute presentation summarizing this information:

Pollination in Pennsylvania Apple and Cherry Orchards.

Further Readings

Heller, S., Joshi, N. K., Leslie, T., Rajotte, E. G., & Biddinger, D. J. (2019). Diversified Floral Resource Plantings Support Bee Communities after Apple Bloom in Commercial Orchards. Scientific Reports, 9(1), 1-13.

Park, M., Danforth, B., Losey, J., Biddinger, D., Vaughan, M., Dollar, J., ... & Agnello, A. (2012). Wild Pollinators of Eastern Apple Orchards and How to Conserve Them. College of Agriculture and Life Sciences, Cornell University.

Westwood, M. N. 1993. Temperate-Zone Pomology, Physiology, and Culture. Timber Press, Inc., Portland. OR. 523 p.

USDA, N. (2014). Preventing or mitigating potential negative impacts of pesticides on pollinators using integrated pest management and other conservation practices. Agronomy Technical Note 9. Washington DC.