Nerve insensitivity resistance to cypermethrin in larvae of the tobacco budworm <i>Heliothis virescens</i> from USA cotton field populations

Abstract

Abstract 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 the preparations were exposed to increasing concentrations of cis ‐cypermethrin in a cumulative dose‐response assay. A positive response was defined as an increase of at least five‐fold in the rate of neuronal activity over that seen during the control period. Up to 35 individuals of each strain were assayed and the responses used to construct a phenotypic profile categorising the individuals from nerve‐susceptible to highly nerve‐insensitive. An EC 50 for the action of cis ‐cypermethrin was also obtained. There was a positive, significant correlation between non‐synergisable resistance to cypermethrin and nerve insensitivity as defined in the neurophysiological assay. It was shown that nerve insensitivity to cypermethrin increased throughout the cotton‐growing season.