To enrich or not to enrich? Are there any benefits of using multiple colors of pan traps when sampling aculeate Hymenoptera?

Abstract

The decline of Aculeata has raised concerns regarding their conservation and the maintenance of their function as important pollinators. Pan trapping is among the most frequently used methods for monitoring these organisms. Numerous researchers have suggested using pan traps of multiple colors in addition to the yellow traps. Here we provide the first data for the color preferences of European Aculeata by analyzing 5,421 individuals from 356 species of bees and wasps obtained using white, yellow, turquoise and pink pan traps. The total capture rates and diversity were higher in the white and yellow traps over the turquoise and pink ones. The species-specific abundance did not follow changes in the total capture rates. Nevertheless, only seven (of the 96 most commonly captured species) were captured at low rates in the yellow traps, but were obtained at high rates in the white traps. Only two species (Apis mellifera and Priocnemis minuta) were captured preferentially to other than white or yellow traps. Importantly, caution is needed when assessing the sex ratios of Aculeata obtained by pan trapping because many Aculeata display sex-specific color preferences. For example, in Dasypoda hirtipes, we captured 14 of 16 males in the yellow traps, but obtained 34 of 38 females in the turquoise traps. We provide experimental evidence that the females of numerous oligolectic bees (collecting pollen for their brood on a few specific plant species) display more stringent trap color preferences than their males, which correlates with their observed flower color preferences.