Spinosad has a mode of action different from the older compounds presently used for the treatment of head lice.
Salgado et al ( Appendix 4 Part A Part B ) has reported that Spinosyn A directly excites the nervous system,
initially causing involuntary muscle contractions and tremors by acting on the neurons in the central nervous system of the insect.
Insects treated with Spinosyn A are eventually paralyzed, but only after prolonged periods of excitation,
suggesting that paralysis is not a primary effect, but instead is a secondary effect resulting from the prolonged over excitation of the nervous system.
Further, excitation of the insect nervous system by Spinosyn A appears related to alteration of the function of nicotinic and gamma amino butyric acid (GABA)–gated ion channels.
However, Spinosyn A does not interact within known binding sites for other nicotinic or GABA-gated insecticides. This appears to be the basis for the unique insecticidal mechanism and lack of cross-resistance with other insecticides.
In summary, as stated in the Spinosad Technical Bulletin, “Continuous activation of motor neurons causes insects to die of exhaustion.”