Does carbaryl increase fluctuating asymmetry in damselflies under field conditions? A mesocosm experiment with Xanthocnemis zealandica (Odonata: Zygoptera).
Previous laboratory experiments have shown that carbaryl reduces emergence success and increases fluctuating asymmetry in cell patterns of damselfly wings. These effects were validated using mesocosms. 20 artificial ponds, each containing X. zealandica, were exposed to 3 replicated (n=5) concentrations of carbaryl contamination plus controls. Emergence success, level of fluctuating asymmetry in meristic and metric traits of the wings, and average size of the damselflies were measured. The degradation of carbaryl was relatively constant for the first 5 weeks but later increased considerably, probably because of enhanced biodegradation. Carbaryl at 100 ppb (nominal concentration) reduced emergence success 10 days after application, whereas carbaryl at 10 ppb and 1 ppb had no effect. To investigate how the level of fluctuating asymmetry and size were affected by carbaryl, damselflies from ponds with the highest concentration where emergence success was not affected (10 ppb) were analysed over 4 time periods. Fluctuating asymmetry of the wings increased during the season, but was not affected by carbaryl at 10 ppb. Size, measured as average length of the front wings, was affected by date of emergence, but not by exposure to carbaryl at 10 ppb. Three main reasons for the absence of increased levels of fluctuating asymmetry as a result of carbaryl exposure are suggested.