Pollination Recommendations And Practices:
The alfalfa flower must be tripped and cross-pollinated by insects for maximum production of high-quality seed. The majority of the western alfalfa seed producers now use either honey bees, leafcutter bees, alkali bees, or some combination of the three. Honey bees are usually rented from beekeepers. Leafcutter bees are usually purchased in the pupal stage, either in bulk (1 U.S. gallon contains about 10,000 pupae in cells) or with the cells intact in the prepared holes in boards. The grower usually prepares his own alkali bee bed and cares for it as a perennial holding.
Recommended rates for usage of honey bees vary from 1 to 10 colonies per acre. Jones 9 recommended two colonies per acre, plus one colony for each additional 100 pounds of seed expected in excess of 250 to 500 pounds. Later, Jones (1958) recommended a colony concentration that would provide two to seven nectar collectors per square yard. Todd and Crawford (1962) recommended that they be distributed about 0.1 mile apart in the field. Most growers use two to four colonies. From 2,000 to 3,000 leafcutter bee nests, or 10,000 individual leafcutter bees have been recommended, with a bee shelter and nests on each 4 acres. A well- populated alkali bee bed, 30 by 50 feet for each 40 acres of alfalfa, or 2,000 female alkali bee visitors per acre is recommended. The data supporting these recommendations are surprisingly meager.
Many factors influence the degree to which the grower follows these recommendations. Also, many variables influence the effectiveness of the pollinators in the field. As a result, one field may be adequately pollinated while another, in which the grower tried to follow the same recommended treatment, may suffer from lack of adequate pollinator activity. Such factors as competing plants, pesticides, adverse weather, bee diseases, strength of colony (of honey bees), and agronomic manipulations can alter effectiveness of the pollinators.
When the grower elects to use honey bees, each colony should have a minimum of 800 in2 of healthy brood in all stages and sufficient bees to blanket 15 to 20 combs (Todd and Reed 1970). There should be three to six honey bees per square yard of flowering alfalfa during the more active part of the day, to provide maximum pollination to every bloom. This may mean some colonies should be moved into the field at the beginning of flowering and augment their numbers as flowering progresses. Water for the bees should be within one-quarter mile of any colony, and shade should be provided in warmer areas.
When alkali bees are used, an equivalent of about 40 ft2 of a well- populated nesting site should be provided per acre of alfalfa. The nesting site should be protected from flooding, exposure to pesticides, trampling by livestock, or damage by predators and parasites. In the field, there should be about one bee for each square yard of blooming alfalfa.
When leafcutter bees are used, from one to five boards, bearing about 2,000 nest-filled holes, or 1 to 5 gallons of pupae should be placed for emergence, and nesting holes should be supplied on each 4 acres of alfalfa. The nesting areas should be protected from hot sun, rain or irrigation water, parasites, and predators. There should be one female leafcutter per 5 yd2 of alfalfa flowers (Bohart 1967). (1)