Hesitation behaviour of hoverflies Sphaerophoria spp. to avoid ambush by crab spiders

Pollinators possess several antipredator adaptations that minimise predation risk during foraging. In addition to morphological adaptations, hoverflies might have behavioural antipredator adaptations. We conducted three field experiments to investigate whether the "hesitation behaviour" of hoverflies Sphaerophoria spp., moving backwards and forwards in front of a flower, is effective in avoiding ambush predators on flowers. First, we compared the behaviour of different flower visitors, including several bees and other hoverflies, with Sphaerophoria spp. behaviour. Only Sphaerophoria spp. exhibited the hesitation behaviour in front of flowers. The flight behaviour was observed more frequently before landing on flowers than on leaves. Second, we investigated rejection by Sphaerophoria spp. to artificially placed corpses of the crab spider Thomisus labefactus. The rejection rate of flowers with a crab spider placed on or under it was significantly higher than that of non-treated flowers. Moreover, the presence of a spider on the flower decreased the number of hesitation displays, compared with non-treated flowers. Finally, to determine whether hesitation behaviour could be a consequence of floral assessment, we investigated hoverfly rejection of previously foraged flowers. Sphaerophoria spp. did not reject flowers that had been visited by the same individual or conspecifics within 3 min. We suggest that hesitation behaviour may be adaptive, enabling assessment of predation risk and hence avoiding ambush predators on flowers.