Effect of certain oils on cypermethrin toxicity against Helicoverpa armigera (Hubner) larvae

Our findings on the increased larval mortality due to cypermethrin in oil fed larvae may be due to the oils inhibit the enzymes like mixed function oxidases (mfo), which are responsible for detoxification of cypermethrin in resistant population. This would lead to culmination of resistant properties and consequently increased larval mortality. The higher larval kill might be owing to reduced larval vigour on account of their exposure to toxic oils in early stage of their development. Keeping this in view, the toxicity, induction and synergist effects of oils namely peppermint oil, clove oil, terpene oil, neem oil, karanj oil and eucalyptus oil have been tested in present experiment against larvae of Helicoverpa armigera. It is significant to note that clove and neem oils proved to be more toxic to the larvae in the culture and these were the oils, which recorded the highest susceptibility to cypermethrin after 7 days of development. This indicated that the toxic effect of the oils have been dominantly responsible for making the insect more vulnerable to cypermethrin. The effect of peppermint and terpene oils proved least toxic to the young larval stages during culture but they were intermediate in enhancing the toxicity due to cypermethrin subsequently. This synergistic action might be due to their effect very similar to mfo suppressors like piperonyl butoxide or the carboxylesterase inhibitors. As a consequence, the cypermethrin is not degraded quickly to non toxic compounds and proved to be highly toxic.