Pollination Services, Colony Abundances and Population Genetics of Bombus Impatiens PDF Download

Author: Carlene McGrady
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Languages : en
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Although recent studies suggest that native bees are likely supplying sufficient pollination services in Cucurbita agroecosystems, commercial pumpkin growers in Pennsylvania are spending thousands of dollars renting managed honey bees to insure adequate pollination. Here, we evaluate the ability of native bee populations to respond to increasing floral resources in a mass-flowering crop such as pumpkins, and the effect of temporal and spatial variables on pollination services supplied by native bees. We catalogued a surprisingly large community comprised of 37 species of native bees foraging in commercial pumpkin fields. Honey bees (Apis mellifera), Squash bees (Peponapis pruinosa) and Bumble bees (Bombus spp., primarily B. impatiens) were the most active pollinator taxa, contributing over 95% of all pollination services. We then examine the effect of distance from field edge on flower sex preferences using chi-squared tests and visitation rates using regression for the most active taxa. While visitation rates were not significantly impacted by distance from field edge, A. mellifera and Bombus preference for female flowers decreased as distance from field edge increased. We also evaluate the effect of field size, day of year and floral density on visitation rates using regression. Bombus visitation rates decreased as field size increased. Both A. mellifera and Bombus spp. visitation rates exhibited a curvilinear response as the growing season progressed and both responded positively to increasing floral density. We synthesized existing literature to estimate minimum pollination thresholds per taxa and calculated that A. mellifera, Bombus and P. pruinosa were each providing 10x, 12.75x and 1.8x the necessary pollination services, respectively. The relationship between visitation rates and pumpkin yield metrics were examined with principal components and correlation analysis for each year separately. Bombus spp. visitation rates positively influenced seed set and pumpkin weight in some years. P. pruinosa visitation rates positively influenced fruit per square meter in some years. A. mellifera visitation rates were never positively associated with any yield metric. This study provides strong evidence that native pollinators are sufficient for pumpkin pollination services in most settings, but managed pollinators should be considered for larger fields (> 3 - 4 ha), depending on configuration. These results have important implications for pollination management decisions and further highlights the importance of monitoring and conserving native pollinator populations. To evaluate the reliability of pollination services provided by wild B. impatiens, we estimate the abundance of Bombus impatiens colonies providing foragers for pollination services using a genetic technique known as microsatellite analysis. Microsatellite analysis is an important genetic tool with previous studies publishing guidelines for optimizing multiplexes and checklists for monitoring potentially biased loci. In this study, we proposed a standardized workflow for evaluating microsatellite loci for 5 common issues and demonstrate using the workflow to trial 14 non-species-specific loci for use in Bombus impatiens, an ecologically and economically important pollinator. We examined the DNA of > 6000 B. impatiens individuals collected from 30 sites over 4 years. We evaluated each locus for evidence of allelic drift, monomorphism, frequency of null alleles, Hardy-Weinberg disequilibrium and linkage disequilibrium. During this process, we propose a new method to visualize and account for allelic drift, which enabled us to efficiently eliminate one locus from our multi-plex, BL15. We found that BT28 was an extensively monomorphic locus. Including the monomorphic locus predictably decreased overall genetic diversity, but it did not alter patterns of genetic diversity between sites. Furthermore, monomorphic loci did not substantially impact the ability to identify genetically related foragers. Five loci exhibited isolated instances of null alleles in less than 10% of sites. One loci, BTMS0081 exhibited universal deviation from Hardy-Weinberg, but it was driven by only 2 sites. Several loci pairs were universally linked, but each linkage was driven by only 1 or 2 sites and including linked pairs had little impact on subsequent results. Implementing this systematic workflow will promote standardized methods to evaluate the extent of potentially biased loci and report the severity of the impacts on subsequent analyses. Furthermore, we can now provide a rigorously tested and thoroughly optimized multiplex of 11 microsatellite loci for use in Bombus impatiens (and potentially other Bombus species), saving financial resources and research hours for future researchers. We analyzed the genotypes generated from this optimized multiplex to test hypotheses about the abundance of B. impatiens colonies providing foragers to pumpkin fields and population genetics. Current studies assume conserving and promoting wild bumble bee colony abundance will result in increased economic and ecological benefits in the form of stable or increased pollination services, but the relationship between colony abundance and pollination services is understudied, particularly in agroecosystems. Bombus impatiens, the common eastern bumble bee, is an important pollinator in the eastern United States with recent studies proposing that the agriculture industry integrate these native bees into their pollination management strategies. However, studies regarding B. impatiens population abundance and genetic status are limited.We used microsatellite analysis and statistical models to estimate the number of B. impatiens colonies providing foragers to 30 commercial pumpkin fields in Pennsylvania and found foragers from 543.7 21.7 SE (range of 291 to 891) colonies per field. Average colony abundance per field was not affected by year (n = 4), or geographic region (n=3), indicating a temporally and spatially stable population of native pollinators. We used our large sample size to evaluate the influence of low levels of polygamy on estimating colony abundance, and showed that monogamy is a reasonable and conservative assumption for estimating colony abundance of B. impatiens. We tested for evidence of genetic differentiation using G-statistics and analysis of molecular variance and evaluated genetic diversity using expected heterozygosity and allelic richness. We confirmed previous assumptions that B. impatiens is a single, panmictic population throughout our study region of 5,000 square km and is characterized by relatively high genetic diversity, indicating a genetically resilient population with the potential to respond to selective pressures in the future. We also measured Bombus visitation rates to pumpkin flowers in a subset of 24 fields and found 0.3 0.05SE bee visits per flower per 45secons. We examined the relationship between colony abundance on a per field and per hectare basis, against visitation rates as a metric of the ecosystem service of pollination. We found that colony abundance per hectare accounted for 23% of the variation in visitation rates, indicating that wild bumble bee colony abundance, mediated by field size, positively impacts pollination services in agroecosystems. We use these relationships to discuss the influence of a mass flowering crop on colony-level abundances of a wild, native, eusocial species.