Seasonal Variation in the Toxicity of Carbaryl to the Snail Lymnaea Acuminata: Implications for Dosing Regimes

The aim of the present study was to observe the effect of seasonal variations (abiotic factors) on the toxicity of carbaryl to L. acuminata. In each month during the year 2006-2007, the 24h LC50 values of the molluscicide carbaryl were determined in an out-door study with the snail Lymnaea acuminata, with concomitant estimation of levels of abiotic factors viz., temperature, pH, dissolved oxygen, carbon dioxide and electrical conductivity in dechlorinated tap and pond water. On the basis of 24-h toxicity assays, it was noted that carbaryl was most effective at killing the snails during the months of June, July and August, when the LC50 values were 9.9, 11.6 and 10.8 mg L respectively for animals in pond water. The carbaryl was least effective in the month of January when its 24-h LC50 values were 59.9 and 58.7 mg L for snails in pond water and dechlorinated tap water, respectively. In the replicate test of random sample, the concentration response lines are within the 95% confidence limits. The index of significance of potency indicates that the value of the mean is within the limits at all probability levels of 90, 95 and 99%. There was a significant positive correlation between the LC50 of carbaryl and the dissolved O 2 and pH of pond / dechlorinated tap water in corresponding months. In contrast, a negative correlation was observed between the LC50 and dissolved CO 2 / temperature in the same months of pond / dechlorinated tap water. In order to ascertain that this relationship between toxicity and abiotic factors was not coincidental, the nervous tissue of the snail was assayed for the activity of acetylcholinesterase (AChE) and acid (ACP) and alkaline (ALP) phosphatases, to sublethal concentrations (40% and 80% of the 24h LC50) during each of the twelve months of the same year. A significant positive rank correlation between AChE, ACP and ALP activity did exist following exposure to the corresponding sublethal concentrations. There was a maximum inhibition of 68.36% and 78.60% in ALP and ACP, respectively, in snails exposed to 80% of the 24-h LC50 in the month of June. A similar treatment caused a maximum inhibition of 55.26% of AChE activity in the month of August.