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United States Department of Agriculture

Agricultural Research Service

Title: Ecological Bottlenecks in Commercially Managed Populations of the Alfalfa Leafcutting Bee

Authors
item Kemp, William
item Bosch Gras, Jordi

Submitted to: National Association of Wheat Growers Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 17, 1998
Publication Date: N/A

Interpretive Summary: Like all biological organisms, alfalfa leaftcutting bees, require resources such as food and shelter to increase their numbers from generation to generation, or in the case of the alfalfa leafcutting bee, from one year to the next. In past years, and perhaps under certain existing conditions, alfalfa seed frowers have enjoyed, in addition to the pollination services of these busy bees, increases in overall bee populations that they manage. Increasing bee populations from year to year means not only reduced production costs, as fewer additional bees must be purchased to pollinate next year's crop, but possibly added income from the sale of surplus bees. In farming or ranching, we frequently refer to "bottlenecks" when we speak of factors that hinder particular aspects of farm or ranch operations, from field preparation, to planting, irrigating, harvesting, and ultimately, transport to market. Similarly, for commercially managed alfalfa leafcutting bees, which are of vital importance to setting a seed crop, there are production bottlenecks as well as ecological ones. Our research is directed towards the identification and management of ecological bottlenecks to improve alfalfa leafcutting bee population increases from year to year. During 1997, we studied nesting success and progeny survival of alfalfa leafcutting bees in commercially managed alfalfa seed fields in Idaho, Oregan, California, and Manitoba, Canada. Our results, to date, suggest that pollen ball mortality, of commercially managed populations of the alfalfa leafcutting bee, may be at least partially related to pollen and nectar availability and, thus, provision quality. The lack of pollen & nectar resources can be viewed as a bottleneck, and the percent of pollen balls would be an indication of the severity of that ecological bottleneck.

Technical Abstract: Our research is directed towards the identification and management of ecological bottlenecks to improve alfalfa leafcutting bee (Megachile rotundata) population increases from year to year. During 1997, we studied nesting success and progeny survival of alfalfa leafcutting bees in commercially managed alfalfa seed fields in Idaho, Oregon, California, and Manitoba, Canada. We found very high overall pollen ball losses in alfalfa leafcutting bee populations from Corcoran, CA (58%), Caldwell, ID (57%), & Nyssa, OR (64%), and the pollen-nectar provision in most of the affected cells appeared dry and cracked. Also, pollen balls appeared to increase as the season progressed, and presumably, as floral resources became more scarce. Parasitism in CA and OR populations averaged 5 and 1 percent, respectively, but was higher in ID populations at about 9 percent. Chalkbrood esimates ranged from about 2 percent in CA populations to 9 and 20 percent, respectively, in OR and ID populations (in the ID field, phase out boards were used). Lastly, for the three U.S. alfalfa leafcutting bee populations, the percent live larvae was uniformly low and ranged from 15 percent in ID, to 26 and 35 percent in OR and CA, respectively. Initial results from populations sampled in Fisher Branch, MB, near Lake Winnipeg in Canada, differ markedly from U.S. samples, however. Pollen ball losses were generally much reduced, averaging about 21 percent, paracitism and chalkbrood losses were estimated at 3 & 2 percent, and the percent live larvae was 74. Our results, to date, suggest that pollen ball mortality may be partially related to pollen and nectar availabitlity and, thus, provision quality. If true, the lack of pollen and nectar resources can be viewed as a bottleneck and percent of pollen balls would indicate severity.

Last Modified: 9/2/2014
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