Assessment of mid-summer drainage regime on the potential for Sympetrum frequens conservation by using low threshold temperature and total effective temperature for development of larvae to 10sup(th) instar

The aim of the present study was to clarify the effect of a mid-summer paddy field drainage regime on the conservation of Sympetrum frequens. We investigated the theoretical lower threshold temperature and total effective temperature required for S. frequens egg and larval development under laboratory conditions. The lower theoretical threshold temperature and total effective temperature for egg hatching under early, typical and late inundation conditions were 5.7℃ and 55.2 degree-days, 5.6℃ and 36.8 degree-days, and 6.8℃ and 21.7 degree-days, respectively. The theoretical lower threshold temperature and total effective temperature for larval development to the 10sup(th)-instar stage were estimated at 5.6℃ and 909.1 degree-days. We then estimated the water temperature in paddy rice fields in Miyagi Prefecture based on meteorological data represented on a 1-km grid. Mean days to hatching and days required for larval development to the 10sup(th)-instar stage were estimated based on the paddy field water temperature, theoretical lower threshold temperature, and total effective temperature. These results were then represented in a GIS to show the mean time to hatching in days and the time required for development of 10sup(th)-instar larvae. Finally, we compared the estimated time required for the development of 10sup(th)-instar larvae and the timing of mid-summer paddy field drainage. We considered that the drainage of paddy fields in mid-summer would be conducted before the development of 10sup(th)-instar larvae. Based on these findings, we consider that the adoption of a mid-summer drainage regime by rice farmers after S. frequens larvae development to 10sup(th)-instar stage would promote the conservation of this species. The risks associated with mid-summer drainage are also discussed.