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United States Department of Agriculture

Agricultural Research Service

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Research Project: Pathogen Load in Bumble Bee Communities Across an Elevational Gradient

Location: Pollinating Insect-biology, Management, Systematics Research

2013 Annual Report

1a. Objectives (from AD-416):
The objective of this project is to quantify the prevalence of parasite infections in wild populations of bumble bees, and to determine if elevation has any affect on the incidence of disease in these bees. Bumble bees are important pollinators of wild plants and agricultural crops. The populations of some species of wild bumblebee are in apparent decline in Europe and North America, but the extent of the declines cannot be precisely ascertained in the United States because of a lack of baseline population data. However, at least three (3) species of North American bumblebee are now endangered or extinct. Suspected causes for the decline include parasites and pathogens, habitat fragmentation, and pesticides. In this project, we will address four questions: (1) What is the prevalence and intensity of Crithidia and Nosema infections among bumblebee species living at different elevations in Northern Arizona; (2) Are there differences in patterns of infection among these parasites and between elevation zones; (3) What is the phenology of Crithidia and Nosema at different elevations in Northern Arizona bumble bees; and (4) Does the phenology of Crithidia and Nosema differ at different elevations.

1b. Approach (from AD-416):
The unique geography of Northern Arizona offers an opportunity to study the affect of elevation on the prevalence, phenology, and infection intensity of Bombus parasites because it is home to a variety of bumble bee species living along an elevation gradient. The Colorado Plateau rises abruptly from surrounding terrain in a formation of escarpments known as the Mogollon Rim. Not far from the Mogollon Rim, the San Francisco Peaks, which are the highest mountains in Arizona, attain an elevation of nearly 13,000 feet. Bumble bees will be sampled at bi-weekly intervals at three different elevational zones. Bumblebees of all available species at these sites will be collected, but the elevation zones will be selected such that least three target species can be collected at multiple zones so that comparisons can be made within the same host. To quantify infections by two parasites, Nosema bombi and Crithidia bombi, the digestive tract of these bees will be removed and examined microscopically for the presence of Crithidia bombi and Nosema bombi. Pathogens from positive samples will be quantified microscopically to determine the intensity of infection. Bees will be collected until workers are no longer present in the field during the fall. The following spring, queens will be collected, killed, and examined for the Crithidia and Nosema infections. The prevalence and distribution of both the bees and their parasites will then be quantified over both time and elevation to reveal what patterns are occurring in natural settings.

3. Progress Report:
About 20 percent of bumble bee (Bombus) species are in decline in North America. The microsporidian pathogen Nosema bombi causes a disease in bees called nosema, and may be a factor in these declines. A comprehensive survey of bumble bee communities was conducted in northern Arizona, throughout the flight season and along an elevation gradient, to determine the extent and timing of nosema. Focusing on the two most common bumble bee species, Bombus huntii and Bombus centralis, the bees were evaluated for nosema using both microscopy and DNA detection methods. The microscopy method only detects the pathogen when it is in the sporulating stage, but has previously been the most common method used for bumble bees. High levels of non-sporulating infections were found in B. huntii (31-63 percent) and B. centralis (57-67 percent), while the prevalence of sporulating infections was low (3-12 and 0-13 percent, respectively). Conversely, a high prevalence of sporulating infections was found in a co-occurring but much rarer bumble bee, B. nevadensis (29 percent). The survey revealed that different bumble bee species within the same environment exhibited varying levels of susceptibility to this pathogen, and that the pathogen may be inhibited from sporulating in some species. Also, the DNA method was a much more sensitive measure of infection and revealed that wild bumble bees have a much higher prevalence of the disease than has previously been recognized.

4. Accomplishments

Last Modified: 06/23/2017
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