Toxic effects of current-use pesticides in Costa Rica on amphibians: Acute toxicity assays and meta-analysis identify gaps in tropical ecotoxicology

Background/Question/Methods

Amphibian populations are declining worldwide, and declines are particularly severe in the New World tropics where amphibian diversity is among the highest on the planet. Exposure to environmental pollutants such as agricultural pesticides has been identified as one of the ultimate causes of decline, yet ecotoxicological studies in tropical regions are extremely rare. This study aimed to assess toxic effects to amphibians of the ten most commonly used pesticides in Costa Rica using two approaches.  First, we conducted 8-day acute toxicity assays with commercial formulations of each pesticide using individually reared Agalychnis callidryas tadpoles at Gosner Stage 25.  We assessed daily survival, quantified growth and activity at day 8, estimated LC₅₀values for survival, and established LOEC values for growth and activity. Our second approach was to conduct a meta-analysis of available LC₅₀ data from the EPA ECOTOX database to allow comparison of those findings with our own.

Results/Conclusions

LC₅₀ values from our assays ranged from 59.36 µg/L for chlorothalonil to 536.2mg/L for 2,4-D. Eight of the ten pesticides showed significant sub-lethal effects on activity and growth, and in all cases these effects occurred at concentrations lower than or equal to the corresponding LC₅₀ values. The nematicides terbufos and ethoprophos and the fungicide chlorothalonil were very highly toxic, with evident effects below 100 µg/L. Our meta-analysis indicated a large number of toxicity values for herbicides that are commonly used in temperate regions, but data were very limited or unavailable for fungicides and nematicides that are heavily used in tropical agriculture.  Our LC₅₀ estimates for A. callidryas were consistent with existing data from well-studied herbicides, but our toxicity estimates for two of our three poorly studied fungicides differed by orders of magnitude from published LC₅₀ estimates, and no published data were available for comparison with either of our two nematicides.  Our findings emphasize the great need for research into the effects of many commonly-used yet widely unstudied pesticides in tropical countries.