NERVE INSENSITIVITY RESISTANCE TO CYPERMETHRIN IN LARVAE OF THE TOBACCO BUDWORM HELIOTHIS-VIRESCENS FROM USA COTTON FIELD POPULATIONS

Spontaneous neuronal activity was recorded from the peripheral nerves of third-instar larvae of strains of Heliothis virescens (F.) obtained directly from cotton fields in the USA. Following a control period th e preparations were exposed to increasing concentrations of cis-cyperm ethrin in a cumulative dose-response assay. A positive response was de fined as an increase of at least five-fold in the rate of neuronal act ivity over that seen during the control period. Up to 35 individuals o f each strain were assayed and the responses used to construct a pheno typic profile categorising the individuals from nerve-susceptible to h ighly nerve-insensitive. An EC(50) for the action of cis-cypermethrin was also obtained. There was a positive, significant correlation betwe en non-synergisable resistance to cypermethrin and nerve insensitivity as defined in the neurophysiological assay. It was shown that nerve i nsensitivity to cypermethrin increased throughout the cotton-growing s eason.