March 24, 2017

The Importance of Pollinators

by Katie Lee
Department of Entomology, University of Minnesota

Many fruit crops require an insect pollinator to help insure pollination (i.e. apples, blueberries, blackberries, cherries, cranberries, pears, plums, raspberries, strawberries). Having enough pollinators during bloom is essential to produce a sustainable crop. Proper pollination increases fruit size at yield, hastens maturity, and produces a more symmetrical fruit shape.

Pollination is the transfer of pollen from the anther (male part) of a flower to the stigma (female part) of a flower. The transfer of compatible pollen results in fertilization and the formation of seeds. The seeds stimulate growth of the surrounding ovarian tissue or the flesh of a fruit. Flowers with higher levels of fertilization produce more seeds and thus bigger fruit. Improperly pollinated fruits will often be smaller and misshapen because the ovarian tissue is not stimulated to grow around the areas without seeds (Figure 1).

Because plants are stationary, they have evolved ways to transfer and receive pollen. In plants that are not self-pollenizing, pollination is facilitated by wind or animals. In agriculture, animal facilitated pollination is mainly done by bees. Honey bees are the most common pollinator in agriculture due to the relative ease in which humans can keep and move them around. It is estimated that honey bees are worth $14.6 billion to the agricultural industry (Morse and Calderone, 2000) and contribute to 1/3 of the food we eat (McGregor, 1976). Honey bees may be more effective pollinators due to their colony size (up to 60,000 honey bee workers in the summer), but native bees are also important pollinators and in some cases are more efficient than honey bees at the individual level. However, the quantity of native bees is declining across the US (Status of Pollinators in North America, 2007), increasing agriculture’s reliance on beekeeper managed honey bees.

By renting honey bee colonies, a grower can compensate for the decrease in wild bees and increase the likelihood of proper pollination.

Figure 1: Diagram of asymmetry that develops due to incomplete pollination in raspberries. The left and middle berries were self-pollinated and the berry on the right was insect pollinated. (Photo by Jim Cane).

Fruit Crops that Benefit from Pollination and Pollenizers

There are three types of plants:

Self-fertile: The pollen from the anthers of a self-fertile plant is able to fertilize the stigmas of flowers from the same plant or other plants of the same variety. Self-fertile plants may require a pollinator (wind or animal) to transfer pollen from the anthers to the stigmas.

Self-pollinating: Self-pollinating plants are self-fertile and able to transfer the pollen to the stigmas without the aid of pollinators.

Self-sterile: Self-sterile plants require pollen from a plant of a different, but compatible, cultivar (pollenizer). Pollination of self-sterile plants can be accomplished by the transfer of pollen from the pollenizer to the self-sterile plant with the aid of a pollinator.

Table 1 shows crops that benefit from honey bee pollination.

To insure cross-pollination for self-sterile plants, it is important to select a pollenizer with the following characteristics: an annual bloom; a long bloom that overlaps with the main cultivar’s bloom; production of large quantities of pollen; and, if possible, production of a profitable fruit. Sufficient numbers of the pollenizer should be planted to insure cross-pollination. There are many ways to interplant the pollenizer. Growers can plant two rows of the pollenizer between every four rows of the main cultivar or a pollenizer every third plant in every third row. Honey bees work in rows, so intermixing the main cultivar and pollenizer may increase the level of pollination.

Table 1: Fruit crops that benefit from pollination (Delaplane and Mayer, 2000).
Fruit When to place honey bee colonies in crop Number of colonies needed per acre* Placement of Colonies Attractiveness to honey bees Symptoms of low pollination
Apples After 5% bloom 1.5 Large orchards: groups of 8-16 at 200-300 yard intervals

Small orchards: groups of 4-6 at 150 yard intervals

Medium; honey bees occasionally get nectar without pollination Misshapen, small, early fruit drop, lower concentrations on calcium leading to a shorter storage life, lower yield
Blackberries About 5% bloom; majority of pollination occurs in first 2 days 2.7   Very attractive pollen and nectar Misshapen, lower yield
Blueberries About 5% bloom; majority of pollination occurs in first 3 days 3   Attractive pollen and nectar, but difficult for honey bees to pollinate due to flower shape Smaller berry size, slower ripening, lower yield
Sour Cherries One day after bloom; pollination is best immediately after the flower opens 1 Groups of 4-12 at ≤100 yards apart Nectar poor attractant; pollen high attractant Lower yield
Cranberries Colonies do best when given time to acclimate to the bog prior to bloom. 3   Both pollen and nectar poor attractants Low yield
Pear 0-10% bloom 1.5   Poor attractant Misshapen, lower yield
Plum   1.3   Attractive Misshapen, lower yield
Raspberries 10% bloom 0.8 Best if placed throughout the field Attractive Misshapen, lower fruit weight, lower yield
Strawberries 10% bloom 3.5   Medium Misshapen, lower fruit weight, lower yield
* These numbers were found by averaging the recommended colony densities found in the literature (Delaplane and Mayer, 2000). The table gives general guidelines; individual growers should make their own decisions based on the needs of his or her operation.

Renting Honey Bee Colonies

Finding a Beekeeper

Growers should contact beekeepers around February to assure enough colonies will be available to rent for pollination. Beekeepers can be found though various beekeeping organizations including local, state, or nation associations (see additional references for websites and phone numbers).

Number of Colonies to Rent

The estimation of the number of colonies needed per acre of crop depends on several factors. One of these factors is the attractiveness of crop flowers. Cultivars with highly attractive flowers produce both quality nectar and pollen. Attractive cultivars will need relatively fewer colonies needed per acre compared to those with poorer quality flower resources. Another factor is the presence of blooming plants other than the cultivars. These plants may increase the number of colonies needed per acre to compensate for the bees foraging on non-cultivar plants. Smaller farms may need more colonies per acre to ensure that bees pollinate the cultivar. This is because bees are more likely to find attractive blooming plants in the vicinity of the farm other than the cultivars. The weather can also affect the bee’s foraging behavior. During cultivar bloom, if the weather is generally cloudy and below 55°F, then the grower may need more colonies per acre.

An estimation of the number of colonies needed per acre for each type of fruit can be found in Table 1. The estimation includes the attractiveness of the flowers, but the other factors are more dynamic. Growers may want to increase or decrease the number of colonies depending on the operation.

Cost of Renting Honey Bee Colonies

Since beekeepers are providing an essential service to fruit producers, it is important that they be compensated for their work. The beekeeper determines the cost of renting honey bee colonies by considering several factors. These factors include the cost of transporting the colonies to and from the crop, and the cost of a decreased honey crop. Honey bees in crops often make less honey because cultivars are generally not as nectar rich. The loss depends on the time spent in pollinating the crop, the type of crop, and the time of year the crop is in bloom. The rental price should be less if the bees can make a similar amount of honey from the crop as compared to the colonies placed for honey production. Payment is generally either a flat fee per colony or a fee based on the number of frames of bees.

Colony Strength

Effective honey bee pollination is best facilitated by strong, healthy colonies. Colonies are typically kept in one or two main boxes (Figure 2a). Each box has 8-10 frames containing comb (Figure 2b). The comb contains a combination of pollen, honey, and developing bees (eggs, larvae, and pupae). Honey supers (smaller boxes used for honey storage by the bees) may be placed on top of the main boxes.

Figure 2a: Pictures of a colon looking at the outside of a colony with two main boxes, and (Gary Reuter, U of MN). Figure 2b: Pictures of a colony looking inside one of the main boxes with one of the frames pulled out. (Gary Reuter, U of MN)

To have enough honey bees for proper pollination, the colonies should have a minimum of 8-10 frames covered with bees (Figure 3b) and 6-8 frames with brood of all ages (Figure 4). To insure the proper strength, a grower should have 10% of the colonies inspected. This can be done either by a broker or the beekeeper with the grower looking on. If the idea of being close to bees makes the grower nervous, he or she can watch from inside a vehicle.

Figure 3a: Weak colony with only 3-4 frames of bees. (Gary Reuter, U of MN). Figure 3b. Strong colony, good for pollination. Bottom box has 9 frames of bees and 7-8 in the top box, for a total of 16-17 frames of bees. (Gary Reuter, U of MN).
Figure 4: A frame containing all stages of brood. Underneath the cells with a tan colored wax capping are the bee pupae. The cells that contain larvae are white inside. (Gary Reuter, U of MN)

Pollination Contract

A pollination contract is a method of communicating the expectations and needs of both the grower and beekeeper. The contract should include the following:

  • A set price and date of payment (often 50% on delivery and 50% after removal).
  • The number of colonies the grower is renting.
  • When colonies are to be put in the crop and when they will be removed (generally with a ≥48 hour notice period).
  • Information on the restrictions/notifications of pesticide use. Whether the grower agrees to sustain from pesticide use and, if not, then include the number of days before the pesticide application the beekeeper will be notified to allow for enough time to move colonies out of the crop.
  • The grower assumes liability of the colonies while in the crop (stinging incidents, vandalism, and bee loss due to pesticides).
  • Minimum colony strength required by the grower.
  • Where the colonies will be placed (colonies must be placed in an area the beekeeper can get a truck in and out of to transport the bees).
  • Any other concerns the grower or beekeeper may have.

Sample Contract (Appendix II) (Original source in The Hive and the Honey Bee, 1992)

The grower and beekeeper can create a continuous contract that is good for several years. If this is done, the grower and beekeeper should insert a clause that requires the beekeeper to notify the grower of any price change around February, allowing the grower to opt out early enough to find a different beekeeper if he or she does not agree to the price change (Traynor, 1998).

Establishing a good relationship with a beekeeper can be important in insuring years of pollination.

Tips on Getting the Most out of Rented Honey Bees

  • Reduce weeds; bees can be attracted by the nectar and pollen of non-cultivar plants like dandelions and clover. Also, these plants can pick up residual pesticides that are harmful to the bees.
  • Place colonies in the crop at designated percent bloom (Table 1). If colonies are placed in the crop too soon, the bees will have time to find other plants to forage on and may choose those plants over the cultivar when the cultivar comes into bloom.
  • Have colonies placed in a location where they are exposed to the early morning sun to encourage bees to forage earlier in the day, increasing the total hours the bees forage.
  • Cultivars needing pollination need their pollenizers blooming simultaneously for pollination to occur. However, if the pollenizer is not in bloom at the proper time, then growers can place the pollenizer’s pollen (that has been previously collected or purchased) in the entrances of colonies. This pollen dusts the bees with pollen before they forage on the cultivar. Alternatively, growers can put branches of the pollenizer close to the entrances of the colonies.
  • Pheromone based honey bee attractants can be used to increase bee visitations to flowers that are relatively unattractive to bees or during poor weather conditions (below 55ºF or cloudy).

Pollinator Safe Pesticide Use

Pesticide treatments on crops are a big concern for beekeepers. Bees are vulnerable to many of the pesticides used to control insect pests, and consideration for bees is often not acknowledged when a pest is detected. Any un-notified pesticide application that affects bees while rented colonies are on the property will likely result in the immediate termination of the pollination contract, withdrawal of the colonies, and a very upset beekeeper. Misuse of pesticides is not only harmful to the relationship of the grower and beekeeper, but the application of a pesticide can be counterproductive to the grower if he or she relies on bee pollination for a sustainable crop.

Understanding what pesticides to spray and when to decrease the number of honey bee and native bee deaths is extremely important in assuring proper crop pollination, as well as maintaining good relationships with beekeepers.

Pesticides that are harmful to bees have a warning on the label.

General guidelines for protecting pollinators include:

  • Avoid using pesticides that come in the form of dusts, wetable powders or mirco-encapsulated pesticides. These are more likely to be harmful to bees (DNR, 2006).
  • Use pesticides with a low Extended Residual Toxicity (ERTs).
  • Avoid spraying when the crop is in bloom. If it is absolutely necessary, then use a pesticide with low toxicity to bees.
  • Spray pesticides in the evening when bees are not foraging on the bloom.
  • Spray at a low percentage bloom (10% instead of 100%).
  • Give the beekeeper at least a 48 hour notice prior to spraying.
  • Use Integrated Pest Management (IPM) to decrease the overall number of pesticide applications.
  • ALWAYS follow the label.

Poster on “Protecting Pollinators While Using Pesticides”

Native Bees

Native bees can be encouraged to help pollinate your crop in two ways: facilitating the growth of wild populations or by keeping native bees.

Native bees can be very efficient pollinators of fruit crops. Different species of bees have different foraging habits and different methods of pollination. Many native bees fly at lower temperatures and in cloudier weather than honey bees and some can be more efficient pollinators of some cultivars than honey bees. For example, bumblebees hold on to the flower and vibrate their muscles, dislodging pollen form the anthers that falls either on the bee on the plant’s stigma, called buzz pollination. This is especially effective in cranberries, blueberries and tomatoes. The blue orchard bee (Osmia lingnaria) has a high preference for fruit tree flowers, meaning these bees will likely focus on the fruit crop and not be distracted by other flowering plants (Bosch and Kemp, 2001). For example, 250 female blue orchard bees are needed per acre of apples compared to one or two honey bee hives with 30,000-60,000 worker bees needed per acre (Bosch and Kemp, 2001).

By including native bees in your pollination plan, you can: decrease the number of rented honey bee colonies; decrease your risk of improper pollination; and improve the general quality of the environment.

Figure 5: Images of native bees. Starting in upper left hand corner and moving clockwise: bumblebee (Bombus species) (Photo by Elaine Evans), mason bee (Osmia species) (Photo by Eric Mader), squash bee (Peponapis sp.) (Photo by Jim Cane), blue orchard bee (Osmia lignaria) (Photo by Pavel Klimov), alfalfa leafcutter bee (Megachile rotundata) (Photo by Eric Mader), and an alkali bee (Nomia melanderi) (Photo by Jim Cane).

Increasing wild bee populations

There are 3500 species of native bees in North America. Native bees include leafcutter bees, sweat bees, andrenids, alkali bees, mason bees, bumblebees, carpenter bees, and squash bees (see Figure 5 for some examples). However, their populations are declining due in part to habitat loss and pesticide use (Status of Pollinators in North America, 2007). By providing a habitat for wild bees and using pesticides in a pollinator friendly way, you can encourage populations of wild pollinators. Here are some suggestions on what to provide to create a habitat for native bees (Conserving Native Bees on Farmland, 2007):

  • Nesting sites: woodland, overgrown ditches, underground nesting boxes, grasslands around crop. In general, this can be a plot of land that is left un-manipulated.
  • Flowers with quality nectar and pollen sources when the fruit crop is not in bloom that are not sprayed with pesticides (spring: willow, elderberry; fall: aster, goldenrod).
  • Access to clean water.
  • Nesting material: mud, waxy leaves.

Keeping Bumblebees and Blue Orchard Bees

There are a few ways to keep bumblebees or blue orchard mason bees. Both of these species can be purchased through distributors and bumblebee colonies can be rented like honey bee colonies. Both of the species can be reared as well.

Bumblebees are common, large, slow moving bees. They are excellent pollinators of raspberries, cranberries, blueberries, among other crops. They are social bees that live underground or in cavities. Each spring, mated queen bees emerge from the ground and start new nests. The queen makes wax pots in which she uses to lay eggs and raises her young, mostly sterile female worker bees. These bees go on to forage, maintain the colony, and feed the young as the queen shifts to just laying eggs. Bumblebee colonies reach 100-300 individuals in the summer. In the fall, the queen will lay eggs that develop into male bees and virgin queens. These queens will mate, lie dormant through the winter, and emerge in the spring to begin a nest. The rest of the colony, including the original queen, dies.

Unlike the social honey bees and bumblebees that live in colonies, blue orchard bees are gregarious solitary bees, meaning the bees nest alone but prefer to nest close to other bees. The females build nests in holes such as hollow reeds or holes that have been drilled into wood. A female uses the pollen she collects to make a pollen ball on which she lays an egg. She then seals off the egg with a wall of mud, creating a brood chamber. The bee will lay up to 35 eggs in this way, each with its own chamber. The eggs hatch into larvae that will eat the pollen ball, change into a pupa, and in the fall become a dormant adult that will emerge next spring.

To successfully rear either of these two bee species, consult one of the recent publications on the matter (see additional references for examples).


Bosch, J. and W. Kemp. 2001. How to manage the blue orchard bee as an orchard pollinator. Beltsville, MD: Sustainable Agriculture Network

Committee on the Status of Pollinators in North America. 2007. Status of Pollinators in North America. National Academies Press, Washington, D.C.

Delaplane, K.S. and D.F. Mayer. 2000. Crop Pollination by Bees. Wallingford, Oxon, UK; New York.

Graham, J.M. 1992. The Hive and the Honey Bee. Dadant & Sons, Inc. Hamilton, IL.

Isaacs, R. and J. Tuell. 2007. Conserving Native Bees on Farmland. Michigan State University Extension, Department of Entomology.

McGregor, S.E. 1976. Insect Pollinations of Cultivated Crop Plants. US Department of Agriculture, Agriculture Handbook 496.

Morse, R.A. and N.W. Calderone. 2000. The value of honey bees as pollinators of U.S. crops in 2000. Bee Culture 128: 1-15.

Spivak, M.S. and G. Reuter. 2007. Protecting Pollinators While Using Pesticides.  Accessed 2/11/2008.

Traynor, J. 1996. Pollination Guidelines: for Commercial Beekeepers.  Accessed 2/11/2008.

Trees, pollinators and responsible pesticide use for Minnesota’s woodlands. 2006. Department of Natural Resources.

Additional References

General Information on Crop Pollination by Pollinators

  • Free, J.B. 1992. Insect pollination of crops.  London: New York: Academic Press.

Pesticide Application

  • Johansen, C.A. and D.F. Mayer. 1990. Pollinator Protection: a bee & pesticide handbook. Cheshire, Conn., USA : Wicwas Press.

Keeping Bumblebees and Blue Orchard Bees

  • Evans, E., Burns, I., and M. Spivak. 2007. Befriending Bumblebees: A practical guide to raising local bumblebees. University of Minnesota Extension.
  • Bosch, J. and W. Kemp. 2001. How to manage the blue orchard bee as an orchard pollinator.  Beltsville, MD: Sustainable Agriculture Network.
  • Field Conservation Management of Native Leafcutting and Mason Osmia Bees
  • Fact Sheet No. 301  UMCE No. 2420. Accessed 2/11/2008.
  • Mader, E., Evans, E, and M. Spivak. In press. Managing Alternative Pollinators: A Handbook for Growers, Beekeepers and Conservationists. Natural Resource, Agriculture, and Engineering Service (NRAES) Ithaca, NY.

Finding Beekeepers

Regional Associations:

National Associations: