Mason bees have a limited effective foraging range from the nest—probably not much more than 100 yards (~90 meters). At higher distances, bees tend to prefer nest sites closer to the forage source. Depending on the area to be pollinated, multiple shelters scattered throughout the orchard or farm may be needed.
Current USDA Bee Lab recommendations call for stocking rates of 250 female BOBs per acre (~620 per hectare) for apple pollination, and 300 per acre (~740 per hectare) for almonds. Hornfaced bees are likely to be equally effective at these rates. Rates for other Osmia species and other crops are not commonly established.
Release of bees maintained in cold storage should be timed to coincide with the beginning of the bloom period. In orchard settings, females that emerge significantly before the first blooming flowers are likely to disperse in search of adequate food sources elsewhere. Early flowering, alternative forage sources are extremely valuable to prevent this. Plants like willow, dandelion, and Siberian squill provide excellent food sources in orchards for BOBs and hornfaced bees before fruit tree bloom.
Similarly, bees released too late in the season may emerge as the main crop bloom is in decline. Without alternative forage sources, many bees are likely to disperse in addition to missing the peak pollination period.
Flower bud development and weather forecasts should be monitored prior to placement of bees in the field. Assuming they have had an adequate cold storage period (roughly 100 to 200 days), emergence of male bees can be expected within one day to a week when outdoor temperatures reach the 70s (°F) (~21 to 26°C). Female emergence for BOBs and hornfaced bees typically begins after about half of the male bees have emerged—usually within three to five days of steady warm temperatures. At cooler temperatures, in the 50s or 60s (°F) (~10 to 20°C), emergence will still proceed, but over a longer period of days.
One method of assuring that emergence corresponds to the main crop bloom is to stagger the release of bees. Nests containing dormant bees can be removed from cold storage over several days, placing only half or a third of the nests in the field at a time.
Bees overwintered in the field, or at ambient outdoor temperatures (such as in an unheated barn or garage), will have a variable rate of emergence lasting over several weeks. Bees maintained under such conditions should be winter hardy for that region and have a diversity of forage sources available throughout the nesting period.
Ideally, mason bees should be released from their original nests, employing a phase-out system. Bees forced to chew out of their natal nest have a lower dispersal rate. If loose cocoons are used, they should be released in an emergence box consisting of a dark container with a single exit hole (identical to the phase-out system described earlier). Loose cocoons within the emergence box are at great risk of parasitism by chalcid wasps, and should be covered with an inch of sawdust or vermiculite (not perlite). Emerging bees can readily dig their way out and exit the emergence box, but smaller-bodied parasites have greater difficulty digging down to the loose cocoons.
Active nesting for BOBs and hornfaced bees lasts one to two months depending on weather and forage sources. Near the end of the nesting period few female bees will be visible; those remaining are often slow and have tattered wings. Other cavity nesting bee and wasp species may appear, as well as parasites such as large ichneumonid or smaller chalcid wasps. These parasites signal that it is time to remove nests from the field. At this stage, bees are still in the larval stage, and rough handling of the nests can cause them to be dislodged from their pollen-nectar food provision. Unnecessary jostling should be avoided. Nests removed from the shelter should be stored with the entrances facing up so that larvae remain in contact with their food at all times.
Locally acclimatized populations may be left in field shelters to develop naturally and emerge the following year. However, such populations are at greater risk to predation, parasitism, and overheating.