Impacts of fallow cropland winter weeds on honey bee pre-swarm colony growth

Lack of adequate floral resources is a key driver of bee population declines and honey bee colony loss, particularly in agricultural landscapes. Most studies evaluating floral resource use and land management consequences in honey bees focus on the period after new colony founding from reproductive swarms (spring-fall). Few studies have investigated nutrition in the late-winter/early-spring (LWES) period, the critical time when colonies break their winter dormancy and increase their colony populations in preparation for swarming. Growth during this timeframe influences beekeepers' ability to produce new colonies and predicts colony productivity for the remainder of the active season. In agricultural areas growing summer annual crops, many fields are fallow during LWES and anecdotally grow large quantities of early blooming winter annual herbaceous weeds. The value of these resources to honey bee colony health is unknown. In this study, conducted in central Kentucky, we characterized blooming winter annual plant abundance across different land cover types. We placed colonies on sites with varying quantities of blooming winter annual plants and assessed colony growth and individual worker immune system function over LWES. We found that blooming winter annual plants are most abundant on croplands and limited on forested sites and sites dominated by grasses (including lawns and pastures). Colonies on sites with the lowest quantities of blooming winter annual plants showed reduced LWES growth and patterns of individual worker gene expression consistent with nutritional stress relative to colonies on sites with greater abundances of blooming winter annuals. We used the National Land Cover Database to show that proximity to crop fields at larger spatial scales also had positive impacts on colony growth during LWES. Taken together, these results suggest blooming winter annual plants, and, more generally, croplands, may provide critical floral resources for honey bees during the critical pre-swarm growth period following winter dormancy.

Automated summary

Lack of adequate floral resources in agricultural landscapes during the late-winter/early-spring period can lead to a decline in bee populations and honey bee colony loss. This study in central Kentucky found that blooming winter annual plants, especially on croplands, play a critical role in honey bee colony growth during this critical pre-swarm period. Colonies on sites with fewer winter annual plants showed reduced growth and signs of nutritional stress in individual worker bees. Proximity to crop fields also had a positive impact on colony growth.