Toxicity of insecticides to wolf spider (Pardosa pseudoannulata) and rice leaf folder (Cnaphalocrocis medinalis): assessing the risk of insecticides on spiders in the rice ecosystem

Abstract

Chemical insecticides complemented with the natural enemies result in successful pest management in crop fields. Hence, knowing the impact of insecticides on natural enemies present in the rice ecosystem is essential. The wolf spider (Pardosa pseudoannulata), a predator of plant hoppers and young larva of leaf folders and stem borers in rice ecosystem, plays an important role in pest suppression. The effect of certain efficacious insecticides, commonly used in rice on P. pseudoannulata through contact toxicity was evaluated and compared with its toxicity to rice leaf folder (Cnaphalocrosis medinalis). Calculations of median lethal doses (LD50) for adult spiders revealed deltamethrin as a highly toxic chemical with an LD50 of 0.055 × 10−3 µg spider−1 followed by clothianidin (0.001 µg spider−1) at 48 h after treatment. Chlorantraniliprole was found to be highly toxic to C. medinalis larvae (LD50 = 0.001 µg larvae−1) followed by deltamethrin (0.009 µg larvae−1). Based on the risk assessment estimator selectivity ratio, which compares the LD50 of pest and predator, acephate and chlorantraniliprole were found to be safe by causing low mortality. However, when the LD90 of the pest was subjected to probit substitution, acephate, and chlorpyrifos were relatively less toxic to the predator. The hazard ratio, which takes into account the realistic field dose, revealed that chlorantraniliprole and acephate were safe for P. pseudoannulata. Most of the insecticides commonly used in agriculture negatively affect spiders, with few exceptions like chlorantraniliprole and acephate with minimum contact toxicity.