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United States Department of Agriculture

Agricultural Research Service

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Pollinators at PI
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The following species of insect pollinators are utilized at the NCRPIS for controlled pollinations of germplasm within field and greenhouse cages:

  Honey bees Apis millifera L.
  Osmia bees

Osmia lignaria (Blue orchard bee)
Osmia cornifrons (Hornfaced bee)

  Bumblebees Bombus impatiens
  Flies Musca domestica L. (Houseflies)
Calliphora sp. (Blue bottle flies)
  Alfalfa leafcutter bees Megachile rotundata

Plant accessions are periodically grown in the field and greenhouse in order to obtain seed to distribute to requestors. Pollination of these accessions must be controlled in order to preserve the genetic integrity of each individual line. Although some germplasm such as corn and cultivated sunflowers can be increased with hand pollinations, much of the germplasm held at NCRPIS benefits from insect pollination. A cage system has been developed to contain desired insect pollinators and to prevent cross-pollination with other accessions of the same plant species, as well as to exclude pest insects.

For more information on controlled pollination at the North Central Regional Plant Introduction Station or for free plans of the Bee Nucleus (illustration shown here) contact Steve Hanlin at:

Honey Bee Nucleus

For more information on pollination cages used at the North Central Regional Plant Introduction Station (two are illustrated below) contact Larry Lockhart at:

Small Cage Frame


Large Cage Frame


Honey Bees


(Apis mellifera L.)

Honey Bee on DaucusHoney Bee on Frame of CombHoney Bee on Pastinaca

Honey bees were introduced to the U.S. by European colonists. Traditionally honey bees have been used to pollinate a wide variety of crops; this holds true at NCRPIS as well. Honey bees have been used to pollinate many accessions of most curatorial holdings with the exception of cultivated type Helianthus and Zea mays. These bees are used in ca 700 cages annually.

Honey bees are social insects (many bees live and work together in a single colony with one queen whose primary function is to lay eggs). A honey bee colony contains non-reproductive females called “workers” and productive males called “drones” along with the single productive female (the queen). Honey bee colonies are potentially “perennial” as the queen may live for more than one year.

Honey bees forage best from 60 to 90 F (15 to 32 C). They are considered aggressive and may sting when they feel threatened. Rearing of honey bees is well established but costly due to the equipment and continuous care required.

At NCRPIS, honey bees are maintained for pollination in small queen-right colonies referred to as “nucleus hives” or “nucs” which are constructed of pine wood. The bottom board features a unique sliding entrance which allows the bees to fly either within the cage only or outside of the cage (Ellis et al, 1981). This slide also allows for the bees to be held within the nuc box for short periods of time (e.g. for transport of nuc or pesticide applications to caged plants). Because of limited food supplies for bees within the cages, a feeding hole in the nuc lid allows for supplementary feeding.

Each nuc contains six 15.9 cm frames. Two of the frames contain honey and pollen, three frames contain brood; the final frame contains empty drawn comb. In addition there are ca 2000 workers and a mated queen bee.

Nucs are obtained through one of two methods. The first is the division of an over-wintered two story nuc in the spring several weeks after the hives are moved outdoors. One of the stories maintains the queen from the previous year, while the second story will have a grafted queen introduced shortly after the stories are separated.

The second method of obtaining nucs is to use three story honey bee hives referred to as “parent colonies”; the majority of nucs produced annually are created with this method. From April to August, nucs are established from parent colonies as follows: remove three frames of brood and adhering bees, provide two stored frames of honey and pollen and one frame of empty comb, and insert one ripe queen cell produced via standard queen-rearing techniques (Spivak and Reuter, 1997). Before nucleus hives are inserted into the cages for pollination, it is ensured that they are queen-right (i.e. the queen bee is producing new brood or eggs and young bee larvae).

Once nucs are ready for use and have been requested by curatorial staff, they are transported to field cages early in the morning. Nucs are placed in the northwest corner of small field cages. This encourages bees to traverse the entire cage as they are attracted to the southeast side, closest to the spring/summer sun. The cage screen is wrapped around the midpoint of the box leaving the feeding hole outside the cage. The screen is secured around the box with a polypropylene strap; soil is placed at both sides and on top of the nuc box to ensure bees are retained inside the cage.

Double story nucs are prepared for use in large field cages. In a double story nuc, the bottom box contains two frames of honey and four frames of brood; the top box contains four frames of honey and two frames of empty drawn comb. Double story nucs are placed entirely inside of a cage; the cage opening is normally positioned at the north end, so the nuc is placed in either the northwest or northeast corner of these cages. Beekeepers must enter the large cages to feed these hives.

The nucs are fed a 3:1 solution of high fructose corn syrup and water on a weekly basis through the field pollination season. Pollen patties (mix of bee pollen, soy flour, and undiluted corn syrup) are provided every other week.

At curatorial direction, nucs are removed from cages, and transported to bee yards where the bees are allowed to forage freely until needed for another cage or prepared for winter. When nucs are pulled from the cages, screening is secured with soil to prevent seed loss to rodents and birds.

At the end of the field pollination season, all colonies are prepared for over-wintering. Several feedings of diluted corn syrup are provided to assure adequate food stores within the hives. In December, hives are moved to an indoor over-wintering facility. This facility is maintained at ca 45 F (6 C), RH 60 %, 24 hour dark.

In both the spring and the fall as hives are prepared for the upcoming season, they are examined for diseases such American foul brood and mites. Affected hives are treated appropriately.


Ellis, M.D., G.S. Jackson, W.H. Skrdla, and H.C. Spencer. 1981. Use of honey bees for controlled interpollination of plant germplasm collections. HortSci. 16:488-491.

Spivak, M. and G.S. Reuter. 1997. Successful queen rearing. Short course by University of MN Dept. of Entomology and University of MN Extension Service.


Osmia Bees


Osmia lignaria (Blue Orchard Bee)
Osmia cornifrons (Hornface Bee)

Blue Orchard Bee on ErysimumSmall Osmia DomicilesOsmia in Cage

Osmia cornifrons was introduced to the U.S. from Japan in 1977; O. lignaria is native to the U.S. Osmia bees are excellent pollinators of early blooming plants such as Brassicaceae and fruit trees. They are used in up to 200 cages annually at NCRPIS.

They are a solitary bee species (individual female bees perform all the work required to maintain their own nesting cells) that work from 50 to 90 F (10 to 32 C) during the months of April through June. These bees are non-aggressive. Rearing is established for this bee; they are low cost and require little care.

In nature, Osmia bees rear their young in bamboo or reeds but this bee readily accepts artificial nests. At NCRPIS, Osmia are provided waxed paper straws (7.25 mm diameter and 152.4 mm long); the females use mud to create barriers between the up to eight cells formed in each straw. Straws are inserted into cardboard tubes to protect the developing larvae from parasitization. These straws are placed inside domiciles constructed from PVC pipe for protection from the elements.

Two different sizes of PVC domiciles are used at NCRPIS; small domiciles are used inside of field cages and large domiciles are used to obtain an annual increase of bees. Small domiciles are constructed from 5.1 cm diameter PVC pipe. The front of the domicile is cut at a 45 degree angle to protect the straws from rain; an end cap at the opposite end retains the straws within the pipe. Large domiciles are constructed from 7.6 cm diameter PVC pipe. The front of the large domicile is also angled; in addition a piece of mesh screen covers the opening to prevent injury to the straws by birds and other predators. Both sizes of domiciles are fitted with two eye-bolts on the top of the pipe; eye-bolts are used to secure the domiciles in place in cages or increase areas. The domicile is suspended from a piece of rebar by sliding the eye-bolts onto the short section of the rod. The rod is inserted in the southwest corner of the cage with the domicile entrance pointed to the south-southeast. Large domiciles are simply suspended from tree branches with twine run through the eye-bolts; domicile openings are directed toward the south-southeast.

One peculiarity of Osmia, is that once these bees have emerged from a domicile in a particular location, if the domicile opening is repositioned even a slight distance, the bees will abort use of the domicile and will die soon after. These bees cannot be confined to their domiciles without causing death either. Because of this habit of the Osmia, if movement of domiciles is required prior to the end of pollination (e.g. for pesticide application to caged plants), the old domicile must be retired from use and a new Osmia domicile or a different bee must be introduced to the treated cage.

The ca 7000 Osmia bees used at the NCRPIS annually are obtained through two methods. 1500 to 2500 bees are purchased from U. S. suppliers located in Utah. In addition, ca 6000 bees are obtained from the nest cells produced in the field cages as well as outlying increase sites.

Osmia domiciles are removed from the field in early July, once temperatures are too warm for bees to continue working. Domiciles must be handled gently to prevent dislodging developing larvae from pollen balls within the nest cells. The domiciles are placed in a rearing room maintained at ca 80 F (26.5 C) and 50 % RH until November. In mid-November the straws are removed from the domiciles and examined for completed cells. Groups of straws containing 100 bee pupae are wrapped in 2-ply tissues and then placed into large garbage bags with water-dampened sponges (two sponges per bag). The garbage bags of sorted straws are stored at 40 F (4 C) for the winter. In March, the straws are removed from winter storage and placed in PVC domiciles in preparation for the coming field season. Small domiciles receive 3 filled straws (ca 24 bee pupae) in a bundle of 16 total cardboard tubes. Large domiciles receive 5 filled straws (ca 40 bee pupae) in a bundle of 23 cardboard tubes. Domiciles are maintained at 40 F (4 C) until use. Domiciles are moved from cold storage to cages immediately as the first bees will emerge from domiciles within 24 hours after they are brought to warm temperatures.


Bumble Bees


Bombus impatiens

Bombus on MintBombus near SunflowerBombus Cardboard Domicile Box

There are a number of bumble bee species native to the U.S.; we use primarily Bombus impatiens at NCRPIS. Bumble bees are known to pollinate many plants, however they excel at pollination of plants with trumpet-shaped flowers due to their long tongues. At NCRPIS, bumble bees are used primarily for pollination of Baptisia sp. and Calendula sp. in approximately 12 cages annually.

Bumble bees are social insects whose colonies only survive one year. New queens mate late in the summer and hibernate individually over the winter; the following spring these queens each establish a colony. A bumble bee colony consists of a queen, numerous workers (female), and drones (male). Bumble bee colonies pollinate throughout the summer and die off in the fall. They forage at 55 to 90 F (13 to 33 C) in all types of weather. Unlike other bees, bumble bees will continue to work in cool rainy conditions. These bees are mildly aggressive when they feel threatened. Rearing is possible, but difficult. Colonies are available through several commercial sources.

NCRPIS purchases “research colonies” consisting of a queen and 30 to 40 workers (queen-right colony). The furnished domicile consists of a cardboard box containing a plastic chamber which rests on top of a plastic feeding bag. The chamber houses the nesting area; bees are provided upholster’s cotton within which they build the brood nest. A cotton wick bridges the area between the nest and the feeding bag below. The feeding bag contains high fructose corn syrup. Initially bees can only exit the box once a sliding tab has been opened and lightweight mesh fabric is removed. Once a hive has been opened, bees can be allowed to enter/exit the box as appropriate by setting the slides as recommended by the supplier.

If the queen-right colony is found to be too aggressive in working tender flowers, the use of a “drone-only colony” is recommended. Drones (male bees) tend to have a more “mellow” disposition; they cannot sting.

Bumble bee hives are placed entirely within the cage regardless of cage size. They are placed within a protective frame to reduce weather damage to the cardboard box. When bumble bees are no longer needed in a cage, the slide is set to “entrance only” and the hive is collected between the hours of 9 PM to 7 AM.

Bumble bees are allowed to forage freely when not in cages. They are not stored in any way over the winter; individual colonies are allowed to die off naturally.


Flies (House & Blue Bottle)


Musca domestica L. (Houseflies)
Calliphora sp. (Blue Bottle Flies)

Flies on DaucusPupae Holder & CartonFly Pupae

Two species of flies are used at NCRPIS to pollinate umbelliferous crops (e.g. Anethum, Angelica, Camelina, Crambe, Dalea, Daucus, Hyoscyamus, Petroselinum, Pimpinella, Spireae, Torilis) in field and greenhouse cages. Flies are used in approximately 40 greenhouse cages in the winter and about 20 greenhouse cages in the spring/summer; about 80 field cages are supplied with flies in the summer.

Flies are considered “incidental” pollinators. As they move around flowers in search of nectar, they move pollen on their body hairs from one flower to another. When flies are included with honey bees in cages containing the above mentioned plant species, pollination is more effective than when only honey bees are present (Wilson et al, 1991). An additional greenhouse study conducted in winter 2003 – 2004 at NCRPIS showed that Pimpinella seed viability from cages provided with either houseflies alone or both species of flies was comparable and greater than cages with blue bottle flies alone.

Flies will work plants at average temperatures of 70 to 90 F (21.5 to 32.5 C). Both species of flies may be less active in inclement weather; blue bottle flies may be less active at very warm temperatures. Flies are non-aggressive toward humans, but may be considered “irritating”. Rearing of flies is well established and pupae are low cost to purchase.

At NCRPIS, both species of flies are purchased as pupae from outside sources. Houseflies are purchased locally from an Iowa State University Entomology project; blue bottle flies are purchased from a commercial source in Idaho. House fly pupae can not be stored for an extended length of time, so a fresh supply is picked up weekly. Blue bottle fly pupae can be stored at 32 F (0o C) for three weeks before adult emergence is significantly reduced; regular shipments are scheduled at the beginning of each pollination season (winter greenhouse and summer field).

Based on past NCRPIS research, ca 200 fly pupae are placed in each cage weekly (Wilson et al, 1991). In general flies will live 2 to 3 weeks but weekly replenishment ensures an adequate population for pollination. Fly pupae are incubated for 2 to 3 days in 0.2 liter paper cartons with screened lids at 80 F (26.5o C) 30 % RH before placing in cages to ensure higher percent adult emergence. Fly pupae holders constructed of 0.9 liter plastic containers weighted with plaster are placed inside each cage to receive the pupae. Adult flies also may use these containers to rest in during the evening or inclement weather. No further care is provided to the flies once they are placed in the cages.


Wilson, R.L., M.P. Widrlechner, and K.R. Reitsma. 1991. Pollination methods for maintaining carrot germplasm collections. FAO/IBPGR Plant Genetic Resources Newsletter, 85:1-3.


Alfalfa Leaf Cutter


Megachile rotundata

Alfalfa Leaf Cutter on CucumisAlfalfa Leaf Cutter CellsAlfalfa Leaf Cutter Domicile

Alfalfa leafcutter bees (ALC) were introduced to the U.S. from Europe about 1930. The ALC is a solitary bee meaning that each female lays eggs and provisions her own nest cells. Even though they are solitary, the ALC is also a gregarious bee which means it prefers to live close to other bees of the same species.

Historically ALC has been used for pollination of forage legumes and more recently utilized for blueberries. At NCRPIS ALC use is still in the experimental stage; in general the ALC bees seem to be most effective pollinating small to medium flowers of a “flat” nature. Germplasm pollinated by ALC includes Angelica, Brassica, Cucumis, Daucus, wild-type Helianthus, Melilotus, Ocimum, and Potentilla. These bees have been used in both late winter/spring greenhouse cages and in summer field cages.

ALC will work at temperatures of 80 F (26 C) or above as they prefer dry sunny climates; these bees will not pollinate as well in cool cloudy or rainy weather. The ALC is non-aggressive, but will bite if accidentally squeezed; the bite produces a stinging sensation. Rearing of ALC bees is well established; the pupae are low cost to purchase.

ALC bees are purchased from commercial suppliers in Canada or the western U.S. Bees arrive as late instar larvae enclosed in leaf cells; cells are sold in gallon quantities with one gallon containing about 10,000 cells. High quality cells will result in about 80% bee emergence (Logan UT Bee Lab, 2004). The ALC cells are stored in screen trays or vented pint glass jars at 40 F (4 C). Cells should be kept in layers of 3.8 cm or less to prevent reduced bee emergence; greater cell depths allow overheating which kills larvae in the bottom layer of cells.

The bee cells require about 30 day warm treatment before all bees will emerge (International Pollination Systems, 2004). After removing from cold storage, cells are placed in screen-lidded pint glass jars for several days at cool room temperature (about 70 to 75 F or 21 to 24 C), prior to placement in an 86 F (30 C) dark incubator for two weeks. Next the cells are transferred to an 86 F (30 C) chamber which provides light for 5 hours per day. The cells from up to 15 pint jars are placed in a single wood emergence box which is attached to a plastic collection dish via 3 pieces of thick wall flexible plastic tubing connected to rigid plastic tubing in the box and dish lid. Within several days of cell placement in the emergence box, bees will begin exiting from their cells. Attracted by the light in the chamber, the bees move from the box through the flexible plastic tubing and into the collection dish where they are provided “Binderboards” (wood cell blocks purchased through Pollinator Paradise, Parma, ID) and several 2.5 cm long cotton wicks soaked in 5% sucrose solution to keep them calm until they are collected mid-afternoon daily. Bees are transferred from the collection dish to vented 120 ml plastic cups along with sucrose wicks while in a dark cool room; bees are held in vented cups at cool room temperature for up to 3 days before being transferred to cages. About 20 bees are placed in each cup and 1 to 2 cups of bees are released to each cage depending on the number of open flowers present. Because female ALC bees (brown eyes) are thought to do more pollination than male bees (green eyes), it is important to include both sexes in all cages.

ALC bees do not normally emerge from their cells before springtime in nature, so bees used in late winter greenhouse cages must be replenished weekly from January until late March to early April. After April bees are replaced about every other week depending on the bee activity/life span noted in each cage. It is unclear if these bees will survive to pollinate crops in the late fall.

Developing ALC bee cells are subject to parasitism by several species of small Hymenoptera including Pteromalus, Monodontomerus, Tetrastichus, and Melittobia (Peterson et al, 1992). In order to control these parasitoids, traps consisting of black lights shone over open dishes of soapy water are placed in all ALC rearing areas. Traps are cleaned weekly.

Domicile design and additional plant nesting sources are still being tested. Domiciles appear to be important in extending ALC bee life span and activity level; two designs are currently in use. Commercial Styrofoam nesting blocks with pre-drilled holes of 0.635 cm diameter and 6.75 cm depth (the size preferred by ALC bees) are cut into smaller blocks; the small Styrofoam blocks are placed in one of two types of wooden structures. The ALC domiciles are placed in the cages so the cell block faces to the south – south-east for rapid warm up from the morning sun.

Plants such as alfalfa, roses, and buckwheat are known to be favored by the ALC bee for cutting leaf disks to create their nest cells. It is not yet clear if providing some of these favored nesting plants in combination with the accessions in NCRPIS cages (many of which have leaf shapes, sizes, or other characteristics such as hairs, textures, or thicknesses which would make them undesirable for ALC bee use) increases ALC pollination activity.


Logan UT Bee Lab, USDA-ARS Bee Biology and Systematics Laboratory, Logan, UT. Personal communication with all lab personnel. April 1, 2004.

International Pollination Systems. 2004. A Calendar of Incubation for Alfalfa Leafcutting Bees.

Peterson, S.S., C.R. Baird, and R.M. Bitner. 1992. Current Status of the Alfalfa Leafcutting Bee, Megachile rotundata, as a Pollinator of Alfalfa Seed. Bee Science 2:135-142.


Last Modified: 9/28/2007