Seasonal variation in abiotic factors and toxicity of thymol against the snail Lymnaea acuminata

Toxicity of thymol against Lymnaea acuminata was conducted in each month of the year 2010-2011. The 24, 48, 72 and 96 h LC50 values of a molluscicide thymol were determined, with the concomitant estimation of levels of temperature, pH, dissolved oxygen, carbon dioxide and electrical conductivity, both in control and test water. On the basis of a 24h toxicity assay, it was observed that 24h LC50 value of 6.41 mg/l in month of May, was most effective in killing the snails, while the thymol was least effective in month of April, when its 24h LC50 was 15.25 mg/l. There was a significant positive correlation between LC50 of thymol and levels of carbon dioxide/ pH of water in corresponding months. On the contrary, a negative correlation was noted between LC50 of thymol and dissolved oxygen/ temperature of test water in the same months. In order to confirm that relationship between toxicity and abiotic factors is not coincidental, activity of acetylcholinesterase (AChE), acid phosphatase (ACP) and alkaline phosphatase (ALP) in the nervous tissue of control as well as sublethal thymol (60% of 24hLC50) treated snail, were assayed during each of the 12 months of the same year. A significant positive rank correlation was noted between AChE/ACP/ALP activity and corresponding sublethal treatment of thymol. Maximum inhibition of AChE, ACP and ALP activity was observed in the month of May. This study shows conclusively that the best time to control the L. acuminata population with thymol is during the month of May to July.