Abstract
Laboratory and field studies provide evidence that pesticides may play a role in population declines, range reductions and species extinctions of amphibians. The present study examined the acute and chronic toxicity of four commonly used agricultural pesticides, chlorpyrifos, dimethoate, glyphosate and propanil on the survival, growth and development of malformations in the Asian common toad, Bufo melanostictus , under laboratory conditions. The 48 hour LC50 values of the chemicals were within the Pesticide Area Network (PAN) specified limits, except for propanil, which was less than the PAN specified value. Acute exposure to high concentrations of propanil may have a high direct toxic effect on the Asian common toad. The survival of the tadpoles with chronic exposure to ecologically relevant doses of the four pesticides was significantly reduced (survival in chlorpyrifos 39%, dimethoate 41%, glyphosate 36% and propanil 40% in the highest concentration) than in the control group (93%). Exposed tadpoles took more time to metamorphose but were larger in size than the control tadpoles. They also developed malformations at high frequencies (chlorpyrifos 30%, dimethoate 25%, glyphosate 35% and propanil 15% in the highest concentration). Malformations were mainly axial, including kyphosis (hunched back) and scoliosis (curvature) while skin ulcers and oedemas were also observed. Severe limb malformations such as extra or missing limbs as reported for other species of amphibians exposed to pesticides were not observed in the Asian common toad. None of the tadpoles in the control group had any malformations. Glyphosate exposed metamorphs recorded the highest mortality and malformations at high concentrations (1.0 ppm). However, a profound toxic effect was observed in chlorpyrifos exposed group even at low concentrations (0.1 ppm). The study shows that exposure to commonly used agrochemicals poses serious risk to amphibians in Sri Lanka and highlights the importance of investigating the level of agricultural pesticides in freshwater ecosystems and their effect on non-target organisms. Keywords : Bufo melanostictus , chlorpyrifos, dimethoate, glyphosate, malformations, propanil. Doi : 10.4038/jnsfsr.v39i3.3631 J.Natn.Sci.Foundation Sri Lanka 2011 39 (3): 267-276
Highlights
Amphibians are the most threatened of all vertebrate taxa in the world with more than 32% of the species threatened and more than 43% experiencing some form of population decline (Stuart et al, 2004)
This study shows that the tested concentrations of commonly used pesticides such as chlopyrifos, dimethoate, glyphosate and propanil have a significant effect on the survival and growth of the common toad
Based on LC values assigned to most sensitive and least sensitive animal groups, chlopyrifos and dimethoate are classified as moderately toxic insecticides (0.10 - 10 ppm) while glyphosate and propanil are classified as slightly toxic herbicides (10 – 100 ppm; Kegley, 2007)
Summary
Amphibians are the most threatened of all vertebrate taxa in the world with more than 32% of the species threatened and more than 43% experiencing some form of population decline (Stuart et al, 2004). Pesticides can have many adverse effects on amphibians, which can be direct or indirect at lethal or sublethal level. The sublethal effects to pesticides on amphibians include abnormal growth, development and behaviour, which in turn may alter susceptibility to predation and competition, and decrease reproductive success (Bridges, 1999; Sparling, 2003; Boone & Semlitsch, 2002; Reylea, 2005). The effect of pesticides have been widely studied in ranids such as the northern leopard frog (Rana pipiens), the mink frog (R. septentironalis), the green frog (R. clamitans), the bullfrogs (R. cateasbeiana) and the foot hill yellow legged frog (R. boylii) (Berril et al, 1994; Bridges & Semlitsch, 2000; Christin et al, 2003; Hayes et al, 2003; Sparling & Fellers, 2007; 2009). Lawrence River valley of Quebec, Canada (Ouellet et al, 1997)
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