Field experimental approach to detect urban impact on Erpetogomphus designatus Hagen in Selys larvae (Anisoptera: Gomphidae)

This study attempted to design and conduct an in situ field experiment using E. designatus larvae collected from the reference site and then exposed at 4 potentially impacted urban sites; all in the sub-watersheds of the city of Denton, Texas, USA. Before placing them in the urban site enclosures, head width, total width, wing pad length and wet weight were measured. The surviving larvae were retrieved after 6 weeks and all parameters were measured again in order to assess the difference between the reference and urban sites. No survival was observed at 2 urbanized sites in both spring and summer, and at another urbanized site in spring. The differences in survival of the larvae may be influenced by the differences in hydrology and water quality, especially during the summer experimental period. In the spring, a statistically significantly higher growth rate (p <0.05) occurred at one of the urban sites compared to the reference site. The difference in growth rate may have been influenced by less fluctuation and higher minimum water temperature at the urban site. Although the experiment was only partially successful, it did indicate that the local common odon. taxa found at the reference site could be used for field biomonitoring experiments to assess water quality of urban sites. If fully successful, this type of in situ field experiment may indicate actual impacts rather than attempting to apply conclusions based on either laboratory microcosm or mesocosm-based toxicity tests.