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.

Resistance to cis-cypermethrin was examined in a range of laboratory and field strains of Heliothis virescens (F.). The PEG87 laboratory strain and the DuPont field strain which was reared in the laboratory for a number of generations were both highly resistant to cypermethrin whilst the other field strains were generally less tolerant. In all but the PEG87 strain, larvae were more resistant in the first instar than in the third instar. Resistance in the field strains was especially weak in larger larvae. Use of the synergist piperonyl butoxide (PBO) indicated that the PEG87 laboratory strain and the DuPont strain possessed a resistance mechanism based on enhanced monooxygenase activity and that this was particularly strongly expressed in the PEG87 strain. The mechanism was absent from the Snook, Hearne and Itta field strains examined. A proportion of individuals from the DuPont, PEG87 and all the field strains displayed nerve insensitivity to pyrethroid action. The DuPont strain was comparatively homogeneous with respect to nerve insensitivity with a high proportion of the insects being resistant. It is concluded that nerve insensitivity is widely distributed in the field strains and confers considerable tolerance to first instar insects. The monooxygenase resistance likewise confers high tolerance to first instar insects. Larvae with a combination of both major mechanisms probably possess high tolerance to the pyrethroid throughout larval life.