Wild bumble bee colony abundance, scaled by field size, predicts pollination services

Although bee visitation rate to flowers is often used to assess both pollination services and bee abundance, the abundance of social species needs to be assessed by quantifying the number colonies instead of the number of foraging individuals. Because accurately quantifying the number of wild bee colonies can be difficult, the relationship of visitation rates provided by foragers and the abundance of colonies contributing those foragers from the surrounding landscape is poorly documented for social species. Here, we use genetic methods and statistical inference to estimate the abundance of wild colonies of Bombus impatiens in the surrounding landscape providing foragers to 30 commercial pumpkin (Cucurbita pepo) fields over 4 yr across a 13,000-km(2) area in Pennsylvania, USA. We show that the abundance of wild colonies in the surrounding landscape providing foragers per field (colony abundance per field) ranges from 291 to 829 colonies per field. Furthermore, colony abundance per field has been stable across years, counties, and field size, resulting in a dilution of foragers from the available colonies across larger fields. Wild colony abundance, when scaled by field size, by expressing colony abundance on a per hectare basis, is predictive of visitation rate. Thus, we document the relationship of wild colony abundance per hectare to visitation rate at a level that provides sufficient pollination to a highly pollinator-dependent crop. As expected, genetic differentiation among sampled populations is essentially non-existent across different fields or regions, suggesting a panmictic population. Although many Bombus species are in decline, we document abundant and genetically resilient wild populations of B. impatiens associated with a mass-flowering crop under current agricultural practices and provide baseline information needed to monitor these wild populations at a time when they face similar stressors implicated in the decline of congenerics.

Automated summary

Using genetic methods and statistical inference, the study estimated the abundance of wild Bombus impatiens colonies providing foragers to 30 commercial pumpkin fields in Pennsylvania, USA. The results showed that the wild colony abundance per field ranged from 291 to 829 colonies, with stability across years, counties, and field sizes. The study documented a predictive relationship between wild colony abundance per hectare and visitation rate, providing valuable information for monitoring wild populations of B. impatiens in the face of environmental stressors.