Would you like Mustard on that? Brassica juncea as a late season cover crop for bumble bees

Large-scale conversion of natural habitats to monocultural croplands is a major driver of pollinator loss. In the corn-soybean region of the Midwestern U.S., fallow periods in crop rotations are used to break pest cycles and restore soil fertility. Planting a short-season, bee-friendly ground cover during these periods could enhance floral resources for pollinators and improve soil health. The mustard cover crop Brassica juncea offers potential benefits, as it boosts soil fertility, provides floral resources for pollinators, and acts as a biofumigant against fungal diseases in soil through its antifungal glucosinolates. These glucosinolates might also reduce fungal infections in bee guts, though this has not yet been investigated. We aim to evaluate the potential of Brassica juncea as a late season cover crop to benefit both farmers and pollinators. In this experiment we placed commercial Bombus impatiens colonies in agricultural field sites, either with 1 acre of Brassica juncea planted, or no Brassica juncea planted (control) for 3 weeks. All field sites contained at least 4 honey bee colonies. 3D printed pollen traps were used to collect pollen from foraging bumble bees, and pollen slides were created to investigate the proportion of pollen loads containing Brassica juncea. Late season bee pathogen acquisition and possible pathogen spillover were investigated by sampling honey bee and commercial bumble bee colonies before bumble bee colony deployment and after the 3 week foraging period using PCR. Pollinators from Brassica juncea fields were sampled once a week to investigate insect pollinators utilizing these late season floral resources.