Effect of chlorpyrifos and monocrotophos on locomotor behaviour and acetylcholinesterase activity of subterranean termites, Odontotermes obesus

The acute toxicity of chlorpyrifos and monocrotophos to subterranean termites, Odontotermes obesus (Rambur), has been studied by a paper contact method. The LC50 values for chlorpyrifos and monocrotophos were 0.046 and 0.148 microgram cm(-2), respectively. Chlorpyrifos was 3.22-fold more toxic than monocrotophos. The effect of the pesticides on locomotor behaviour (velocity) and head acetylcholinesterase (AChE: EC 3.1.1.7) activity was estimated in LC50-exposed termites at intervals of 4, 8, 12, 16, 20 and 24 h. Chlorpyrifos- and monocrotophos-treated termites showed, respectively, 97 and 88% reduction in locomotor behaviour (velocity) after 24 h. At all time intervals the chlorpyrifos-treated termites exhibited more AChE inhibition and showed greater distorted behaviour than those exposed to monocrotophos. In vitro studies indicated that the I50 value (50% inhibition) for chlorpyrifos against AChE was 8.75 times that of monocrotophos.