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United States Department of Agriculture

Agricultural Research Service

Research Project: BEE DIVERSITY AND THE DEVELOPMENT OF HEALTHY, SUSTAINABLE BEE POLLINATION SYSTEMS

Location: Pollinating Insects-- Biology, Management and Systematics Research

Title: Nest establishment, pollination efficiency, and reproductive success of Megachile rotundata (Hymenoptera: Megachilide) in relation to resource availability in field enclosures

Authors
item Pitts Singer, Theresa
item Bosch, Jordi - BELLATERRA, SPAIN

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 28, 2009
Publication Date: February 1, 2010
Citation: Pitts Singer, T., Bosch, J. 2010. Nest establishment, pollination efficiency, and reproductive success of Megachile rotundata (Hymenoptera: Megachilide) in relation to resource availability in field enclosures. Environmental Entomology. 39: 149-158.

Interpretive Summary: The alfalfa leafcutting bee is commonly used to pollinate alfalfa for seed production in the U.S. and Canada. It is difficult to maintain commercial populations of these bees in the U.S. due to problems with disease, parasites and predators, and unexplained immature morality. One possible explanation for early immature mortality is that too many bees are released in fields where they must then compete for food and places to build nests. For two nesting seasons (2003 & 2004), we performed an experiment in large field cages to determine if too the use of many bees causes them to produce fewer or unhealthy offspring, and if using many bees is effective for making sure the alfalfa flowers are pollinated for obtaining good seed yield. Our results showed that relatively high numbers of alfalfa leafcutting bees could not be kept alive in the field cages. Our significant results indicated that using too many bees made them less efficient at pollinating flowers, and caused them to produce fewer healthy offspring, compared to using more modest numbers of bees. This study shows that bee reproductive success depends on the number put in the field cages, but open field studies are needed to determine what commercial bee densities can result in sustainable bee reproduction levels while at the same time effectively pollinate alfalfa for profitable seed production.

Technical Abstract: The alfalfa leafcutting bee, Megachile rotundata (Fabricius), is commonly used to pollinate alfalfa, Medicago sativa L., for seed production in the U.S. and Canada. It is difficult to reliably sustain commercial populations of M. rotundata in the U.S. due to problems with disease, parasites and predators, and unexplained immature morality. One possible explanation for early immature mortality is that superfluous numbers of these bees are released in fields where resources become limited. For two nesting seasons (2003 & 2004), we performed an experiment in field enclosures to determine if pollination efficiency and reproductive success of M. rotundata is density-dependent. Our results showed that relatively high numbers of foraging M. rotundata could not be sustained and never reached maximum establishment potential in the field enclosures, and effects on seed yield could not be measured because of poor field conditions and thus seed production. However, we found significant reductions as bee density increased in the percent of pollinated flowers per female bee and the number of nests and cells produced per female. We also found an increase in the percent of early brood mortality in the form of pollen balls with decreasing bee density. The number of progeny that lived to adulthood, regardless of progeny sex, was negatively affected by relative bee density. This study shows that bee reproductive success is density-dependent, but open field studies are needed to determine what commercial bee densities can result in sustainable bee reproduction levels while at the same time effectively pollinate alfalfa for profitable seed production.

Last Modified: 4/19/2014
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