The potential of weeds in arable fields to support pollinator assemblages

Abstract To what extent are weeds on arable land useful to pollinators in an arable‐dominated landscape? We sampled the weed flora in fields under conventional or organic farming in SE Sweden. More specifically, we noted the frequency of flowering among weeds that scored high on a pollinator index, henceforth ‘pollinator‐friendly weeds’. Furthermore, we sampled pollinating insects within cereal crops using transect walks and colour pan traps. As expected, weeds were ubiquitous and occurred in most sampling plots (0.25 m 2 ). In the pan traps, more than 100 species of pollinators were caught, including 26 Syrphidae, 19 social bees, 37 solitary bees and 22 other Hymenoptera. In the transects walks, the probability of encountering bees increased with flowering of pollinator‐friendly weeds, and there was a similar but weaker pattern among hoverflies. Organically grown fields differed from conventional ones by having more pollinator‐friendly weeds, more flowering of such weeds, and more bees. There was also a tendency for hoverflies and other Hymenoptera to be more abundant in organic fields. Three conclusions emerged. First, pollinator‐friendly weeds made up one third to almost half of the weed occurrences recorded. Second, substantial numbers of pollinators searched for flowers within arable fields, and some increased with the abundance of flowering of pollinator‐friendly weeds. Third, flowering pollinator‐friendly weeds and some pollinators were more abundant in organic fields than in conventional ones. Overall, we showed that weeds on arable land are a potential and sought after resource among pollinators, and that even conventionally grown crops should be considered a potential habitat for bees.