Emergent body size of mayfly survivors

The authors examined the impacts of imidacloprid exposure on nymph abundance, emergence patterns, and adult body size in Epeorus spp. and Baetis spp. mayflies. They evaluated both press (20d continuous; 0-1 µg/L) and pulse (12h; 0-10 µg/L) imidacloprid exposures in mesocosms. Imidacloprid was administered to the mesocosms from a solution of the formulated product Admire (imidacloprid). Epeorus spp. nymph density was reduced in the 0.8 µg/L press exposure, and in the 9.1 µg/L pulse treatment. Baetis spp. nymph density was reduced only in the highest press exposure, 0.8 µg/L. The press exposure reduced Baetis nymph density resulted from reduced survivorship, while the reductions in Epeorus nymphs under pulse exposures could reflect increased emergence because of stress. Epeorus adult males that emerged in the treatment groups had reduced thorax lengths, and in the 0.25 µg/L and 0.8 µg/L press and 9.1 µg/L pulse groups, no Epeorus males emerged at all. Female Epeorus emergence and body size were not affected by imidacloprid, so some cohorts were entirely female. Baetis adult males had smaller heads in all treatment groups, but both males and females emerged throughout the experiment. Head length in newly emerged male Baetis declined over time throughout the 20-day experiment. Concentrations as low as 0.1 µg/L in the 12h pulse reduced head length in Baetis males and thorax length in Epeorus males.