Effects of aerially applied fenthion on survival and reproduction of the panacea sand fiddler, Uca panacea, in laboratory habitats.

Abstract

Sand fiddler crabs, Uca panacea, were exposed in laboratory habitats to measured concentrations of ULV-grade fenthion via simulated aerial spray at 5% and 50% of field rate application of 6-12 mg fenthion/m(2) (0.05-0.10 lbs fenthion/acre). Two habitats served as controls, and two habitats served as replicates for each of the test exposures. The aerial application was repeated 12 times between July 7, 1997, and August 15, 1997, during the period of the most active larval production of the crabs. The resulting measured concentrations of fenthion at the surface and in the water were consistently lower than what the application rate would have suggested. Statistical analysis of total, daily larval counts showed a periodicity of approximately 14 days that did not appear to be affected by fenthion, at least at the 5% application rate, where a 15% reduction in larval production was noted by the end of the second hatching cycle and a 25% reduction by the end of the third. Larval production in the habitats exposed to the 50% application rate was reduced by 18% at the end of the first hatching cycle, 77% at the end of the second, and 100% at the end of the third hatching cycle. At the end of the third hatching cycle, adult crab mortality observed at the surface was 0%, 3%, and 20% for the control, 5%-, and 50%-exposure habitats, respectively. Three weeks after the final fenthion application, survival of adult crabs was 100% in the control habitats, 75% in the 5%-exposure habitats, and 3% in the 50%-exposure habitats. Survival of unfed first-stage zoeae released during the night following the sprays averaged 4.6 +/- 0.9, 3.7 +/- 0.9 and 1.7 +/- 0.6 days for control, 5%-, and 50%-exposure habitats, respectively. Forty-eight-hour LC-50s (nominal) for fenthion exposure of adult crabs submerged in water and for first-stage zoeae were 215 microg fenthion/L and 0.8 microg fenthion/L, respectively.