Managing Alternative Pollinators

Release Rates, Methods, and End-of-Season Nest Removal

Release rates for bees vary between 10,000 (1 gallon) per acre and 20,000 (2 gallons) per acre (~25,000 and 50,000 bees per hectare) for newly seeded alfalfa fields and up to 30,000 to 40,000 bees per acre (~74,000 to 100,000 per hectare) for established alfalfa fields. At higher rates, more effective pollination occurs, but bee population increases are lower. It is estimated that individual bees can pollinate enough alfalfa flowers to produce 1⁄4 pound of seed (~113 grams). Also remember that under normal production practices, for every female bee there will be 1.5 to 2 male bees. Thus 20,000 bees may contain only 7,000 pollinating females.

There are currently few established release rates for other crops. However, canola growers have been stocking leafcutters at rates of around 20,000 bees per acre (~50,000 bees per hectare), as have blueberry growers in Maine and the Maritime Provinces. This release rate is probably also sufficient for other dense blooms like cranberries, oil and forage crops like clover, vetch, and sunflower, as well as herbs like mint. Less dense blooms would probably achieve adequate pollination at lower stocking rates.

Release methods for bees depend on the nest system used, the crop to be pollinated, and whether or not the bees were incubated. In standard loose-cell management systems for alfalfa pollination, the incubation trays are typically brought to the field on the 22nd day of incubation (or when 30 to 50 percent of female bees have emerged). The trays are placed inside the field shelters, often on shelves suspended from the ceiling inside the shelter.

Elaborate shelters may feature roofs that are painted black to absorb heat, which is radiated below, thus incubating the remaining loose cells. However, care should be taken to prevent excess heat, which can be lethal to developing bees. During transport to the field, trays should be loosely covered with a tarp to calm the bees and prevent wing damage caused by the bees flying against the screened lids.

In cooler climates, incubation trays are sometimes removed from the field shelters at night and returned to a room incubator. The trays are then brought back to the field shelters each morning for the duration of the emergence period. While this practice results in more even emergence, it may interfere with mating, as male bees disperse from the field shelter prior to female emergence.

Bees released as loose cells are likely to have significantly higher pre-nesting dispersal—especially under windy conditions. Therefore the trays should ideally be transferred to the field on days with fair weather in the morning or early evening when flight activity is reduced. Emerging bees are then more likely to orient to the shelter, rather than dispersing. It is also crucial that the incubation trays are protected from rain so that cells do not become flooded with water.

After emergence is complete, any remaining cocoons and debris in the incubation trays should be burned or buried to reduce the spread of disease. Incubation trays should also be cleaned with a bleach and water (1:3 ratio by volume) prior to re-use.

For bees allowed to develop inside nests, either under incubation or ambient field temperatures, a phaseout system, such as that described earlier, must be employed. Then, after all bees have emerged, the old nest blocks can be removed for cleaning. In this way old nest materials are phased out on an annual basis.