The serotonergic system is involved in feeding inhibition by pymetrozine. Comparative studies on a locust (Locusta migratoria) and an aphid (Myzus persicae).

Pymetrozine inhibits feeding in aphids immediately after application without producing visible neurotoxic effects, as previously reported. In the present work, Locusta migratoria, though not a plant-sucking insect, was found to respond to pymetrozine by displaying unique symptoms, which were lifting and stretching of the hindlegs, in addition to inhibition of feeding. In locust, pymetrozine enhanced spontaneous spike discharge of the metathoracic and suboesophageal ganglia in situ at nanomolar concentrations. Similarly, pymetrozine increased the spontaneous rhythmic contractions of the isolated foregut with maximal effects also in the nanomolar range. The actions of pymetrozine were counteracted by biogenic amine receptor antagonists and mimicked by serotonin, not by dopamine and octopamine. Moreover, pymetrozine and serotonin strongly potentiated the effects of each other. Pymetrozine was inactive at all neurotransmitter receptors present on isolated locust neuronal somata, and at all other examined neuronal sites. In Myzus persicae, electrical penetration graph experiments revealed that serotonin, like pymetrozine, inhibited stylet penetration, and strongly enhanced the action of pymetrozine, comparable to the locust. Amine receptor antagonists were not specifically active in the aphid. We conclude from the present results that pymetrozine acts via a novel mechanism that is linked to the signalling pathway of serotonin.