Persistence on cotton foliage of insecticide residues toxic to Heliothis larvae

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Abstract
Control measures for Heliothis spp. on cotton are usually applied against eggs and newly hatched larvae that infest plant terminal growth. In order to investigate the rate of degradation of a range of insecticides, leaf disc samples were collected, at intervals after application, from both fully expanded cotton leaves and unfolding terminal leaves. The samples were subjected separately to bioassay with newly hatched larvae, and to chemical analysis of the surface deposits.On old leaves, larval mortality caused by two biological insecticides declined by half after field exposure for 36 h, but for the synthetic insecticides, the interval ranged from 4.5 days with endosulfan to 18 days with fenvalerate. On new leaves, the longest interval was 7 days with fenvalerate. Chemical half‐life on old leaves ranged from 1 day with endosulfan to 12 days with DDT, and was also shorter on young leaves than the equivalent biological half‐life. Bioassay appears to be the most reliable method of assessing insecticidal activity over time. Dosage/mortality data were integrated to give LC50 and LC90 values.Leaf expansion may have the greatest effect on insecticide persistence during the 6 week period of rapid growth commencing 2 months after sowing. Expansion was slower when plants were water stressed, and accelerated following irrigation. The implications of the above findings for cotton pest management are discussed.