Abstract
Effects of fenvalerate on spontaneous firings of the exposed abdominal nerve cord of the fourth instar larvae of the diamondback moth, Plutella xylostella LINNE, were examined and compared between a susceptible strain and pyrethoroid-resistant populations. In the nerve cords of the susceptible strain, no change was induced in the nerve activity by the application of fenvalerate at 10-7M and 5×10-7M, but the firing frequency increased with a latent time of a few minutes at 10-6M and without a latent time at 10-5M. On the other hand, the preparations of resistant populations were less sensitive than the susceptible strain, i.e., 10-5M fenvalerate induced the increase in firing frequency without the latent time on 16.7% of the preparations, with the latent time on 75% of the preparations, and caused no increase on the remainder, though the response was concentration dependent as in the susceptible strain. Decreased sensitivity at the site of action in the nervous system was suggested to contribute to pyrethroid resistance in this insect.
