Effect of diazinon on survival and growth rate of Bufo melanostictus and polypadates crucigergWl stage larvae

Abstract

Amphibiati populations arc declining globally at an alaiming rate and a growing body of evidence suggests thai pesticides may be a major cause. The present study is aimed at providing empirical evidence for the potential impact of pesticides on amphibian populations. The effects of diazinon. a widely used organophosphate pesticide, on the survival and growth of larvae of two amphibians, the common toad Bujb inelanoslicius and the Sri Lankan endemic common hourglass frog Polypadates cruciger are investigated. Larvae were laboratory bred from six egg clutches, three from each species, collected from ponds and wells located in home gardens in Delgoda and Malabe in the two districts of Gampaha and Colombo. Two separate trials were conducted using gill stage hatchlings{Gosner stages 20-22) of each species. The larvae were held in glass tanks and repeatedly exposed to 4 ug/L, 400 ug/L and 10 mg/L o f diazinon for seven days. Larval mortality was recorded daily, whilst the body length and activity levels were measured at the end of the experiment. Results showed that exposure lo 10 mg/L of diazinon caused significant elevations (p<0.05) in lar\'al mortality in both B, melanostictus and P. cruciger as compared to the controls. On the other hand, no significant increases in mortality were noted at 4 and 400 ug/L. The trends in mortality were significant and positive for both species. Growth retardation was also noted at the highest dose of 10 mg/L, these larvae being significantly smaller than those in Ihc controls (p<0,05). Larval activity was also seriously impaired at the highest dose. This study is one of the initial studies that have provided empirical evidence of the harmful etTects of pesticides on larval mortality, groulh and activity of native amphibian species in Sn Lanka. In view of these findings, we suggest that pesticides may facilitate the decline of"many amphibian populations in Sri Lanka.