SOME EFFECTS OF ABAMECTIN ON NEURAL ACTIVITY IN LARVAL INSTARS OF SPODOPTERA-LITTORALIS BOISDUVAL AND HELICOVERPAARMIGERA HUBNER (LEPIDOPTERA, NOCTUIDAE)

The spontaneous action potential (AP) frequencies recorded from an in-vivo ventral nerve cord (VNC) preparation of abamectin-treated insects not showing signs of paralysis (asymptomatic) were significantly (P < 0.05) less than those from untreated insects for fifth- and sixth-instars of Helicoverpa armigera (24 h) and Spodoptera littoralis (24 and 48 h) after topical application of abamectin. There were also significantly (P < 0.05) less APs from abamectin-treated insects showing signs of paralysis than from asymptomatic abamectin-treated insects. Significantly (P < 0.05) more radioactivity (and abamectin) was recovered from VNCs of larvae showing signs of poisoning (paralysis) than from asymptomatic-treated insects. There were also significant (P < 0.001) correlations between the levels of radioactivity (as log dpm) recovered from VNCs and the AP frequency (Hz) after 24 h for both instars and species and after 48 h for S. littoralis. AP frequencies did not decline significantly (P > 0.05) in abamectin-treated sixth- and fifth-instar S. littoralis larvae until respectively 4 and 8 h after treatment. In conclusion, significant correlations were found in the present study between electrophysiological events in the VNC, visible symptoms of poisoning, and accumulation of abamectin within the VNC. An inverse relationship was found between the levels of abamectin in the VNC and the level of spontaneous nervous activity. These observations are compatible with the VNC being a target site for abamectin. However, the relative importance of the VNC versus peripheral nerves, neuromuscular junctions and muscles as target sites is not known and requires further clarification. No differences in the sensitivity of the VNC to abamectin were found between fifth- and sixth-instar S. littoralis larvae, though there is limited evidence for the H. armigera VNC being less sensitive than the S. littoralis VNC to abamectin.