Species richness of Hymenopteran flower visitors in Brassica campestris var. toria in Assam, India: A comparison of five sampling methods

Abstract We studied the species richness of flower-visiting Hymenoptera assemblages in two varieties of Brassica campestris var. toria : in variety TS-36 during the normal season grown in the riparian ecosystem and in variety TS-67 during the late season grown in the rice-ecosystem of Assam, India using five sampling methods. We aimed to investigate to what extent (i) the Hymenoptera assemblages in the two ecosystems differed from each other, (ii) the population densities of three dominant Apis spp. differed from each other, and (iii) the effectiveness of the sampling methods used differed from each other. In total, we recorded 64 Hymenoptera species belonging to 42 genera of 9 families in the two toria varieties. We detected the presence of all species in TS-36 grown as normal season crop in the riparian ecosystem, and the presence of 48 species in TS-67 grown in the rice ecosystem as a late season crop. Vespidae had the highest species richness, followed by Apidae, Megachilidae, and Halictidae. These visitors may contribute to the ecological resilience of the toria ecosystem through their service as pollinators as well as because they are natural enemies of the crop-pests. Transect walk methods sampled a higher number of species than observation plot method and colour traps. Apis cerana was the most abundant species, with a relative abundance of 38.5% and 37.9% in normal and late season crops, respectively. The mean abundance of all three Apis species was 11.1% higher in the later variety.