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. http://www.pollination.com/managed/albincubationcalendar.cfm
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.