| Abstract: | Topically applied dilute solutions of malathion in kerosene extract ‘bottoms’ are more toxic and rapid in action than concentrated ones. Dilute solutions spread farther, penetrate more rapidly and consequently their rate of activation into the more toxic malaoxon is higher. With a single dose of 2·5% malathion the rate of penetration and activation varies according to the loci of topical application, presumably reflecting differences in sclerotisation of the cuticle. There are also inherent differences in the susceptibility of various body regions associated perhaps with the differential vulnerability of some vital internal organ (e.g. the central nervous system). Very rapid hydrolytic degradation of malathion occurs through its conversion into malaoxon; locusts survive the lethal action of malathion by way of such degradation only where activation into malaoxon takes place in areas away from the sensitive head region, or where the rate of activation is slow, or where the circulatory system delays its arrival at such sensitive sites. The higher rate of activation observable in immature adults of Schistocerca gregaria compared with mature adults is, however, counterbalanced by a higher rate of hydrolysis in the former. |
