Species and age specificity of malathion towards the locusts Schistocerca gregaria and Locusta migratoria

The toxicity of malathion (2.5 % solution in kerosene) was determined by topical application to the hindwing base of mature adults of Schistocerca gregaria and Locusta migratoria migratorioides and of immature adults of S. gregaria. Mature adults of L. m. migrator ioides were slightly more resistant to malathion than those of S. gregaria. Immature adults of S. gregaria were approximately as susceptible as mature ones (at LD₈₉ level) when the criterion for toxicity was taken as the ultimate death of the dosed insects, but the symptoms of poisoning developed earlier and more strongly in the immature insects. Penetration of malathion was least in the mature L. m. migratorioides and highest in the immature S. gregaria. The higher rate of activation of malathion in the immature S. gregaria was, however, counterbalanced by a higher rate of hydrolysis. The toxicity reflected the rate of accumulation of malathion and/or mala-oxon inside the insect body.