Toxicological studies on spiders of Kuttanad rice Agroecosystem, Kerala, India

Spiders are obligate carnivores and hold the unique position of being the only large class of arthropods which are entirely predatory in nature. To form a basis for research into the role of spiders to determine the economic importance of them in the rice agro ecosystem of Kuttanad region of Kerala, a toxicity study of 3 commonly used insecticides on dominant spiders were conducted in the laboratory. Topical application (spraying) and dipping method were used for study. Of the three insecticides tested, Methyl parathion recorded the lowest lethal concentration values indicating its comparatively high toxicity in both methods. This is followed by Quinalphos and Monocrotophos. The exposure to Methyl parathion resulted in 80% mortality of experimental spiders compared to 65% and 40% of mortality with Quinalphos and Monocrotophos respectively. This is suggestive of the usefulness of Monocrotophos as a component of integrated pest management strategy for sustainable paddy cultivation. Of the three dominant species tested, Pardosa pseudoannulata was the least susceptible to application of insecticides both by topical application method and the dipping method under laboratory conditions. Tetragnatha mandibulata was the most susceptible to the insecticides tested under laboratory conditions. Among the two methods used, dipping method was found to be more fatal compared to topical application. Spiders are very important biological control agents in agroecosystems and play a major role as potential defenders by suppressing the pest population to a safe level which emphasizes the concept of Integrated Pest Management (IPM) in modern agriculture. Faced with the need to reduce pesticide usage on crops and optimize natural biological control, full investigation on the means by which spiders influence pest abundance and effect of insecticides on spiders is long overdue.