Decreased nerve sensitivity is a mechanism of resistance in a pyrethroid resistant strain of Heliothis armigera from Thailand

Neurobioassays using cis-cypermethrin were conducted on the third instar larvae of a susceptible laboratory strain and a resistant Thailand strain of Heliothis armigera (Hübner). They consisted of recording the spontaneous multiunit activity (MUA) from the exposed peripheral nerves after superfusion with a saline solution of 4 μ M cis-cypermethrin. After treatment, the MUA first showed a marked excitation and then blockage in both the strains. A consistent pattern of MUA was obtained in all the susceptible insects, whereas the response of the resistant insects was variable and clearly divided into three distinct categories. As in the susceptible insects, half of the resistant insects showed an increased MUA above the control level in the first minute and then it decreased until blockage within 5 to 6 min. Forty percent of the insects maintained their enhanced activity for 15 min above the control level after insecticidal exposure, the activity then decreased steadily but continued for over 36 min before eventual blockage. Ten percent of the resistant insects behaved intermediately. This implies that around 50% of the insects from the Thailand strain appear to be resistant due to the presence of a mechanism of decreased nerve sensitivity.