PRELIMINARY STUDIES ON THE BIOLOGICAL ACTIVITY OF EXTRACTS FROM RHODODENDRON MOLLE AGAINST INSECT PESTS AND THEIR MODE OF ACTION

This paper reports experiments with extracts from flowers of the yellow azelea (Rhododendron molle) on several agricultural insect pests. It was found that the crude extracts of R. molle possessed high antifeedant and stomach poison activities against several species of agricultural insects. Dichloromethane extracts (FCb) from the flowers of R. molle showed high toxicity against the third instar larvae of the diamond-back moth (Plutella xylostella). A strain of the moth highly resistant to fenvalerate showed no resistance to the fraction FCb. the LC50 of the resistant and sensitive diamond-back moth populations to FCb was 0.33% end 0.37% respectively. The insects showed symptoms of excitment, spasm and vomiting quickly after eating the extracts of R. molle, but they were killed slowly. Among the extractions of R. molle, the fraction FC-22 was found to be the most potent. The fifth instar larvae of the oriental army worm (Mythimna separata) showed more than 90% inhibition of feeding when treated with 150 ppm of FC-22. The respiration of the fifth instar larvae of M. separata was markedly depressed after eating a small amount of the extract FC-22. Increased multifunction oxida-se (MFO) activity was induced by the extract FC-22. The results of our experiments also showed that the extracts of R. molle had a strong oviposition deterrent effect on the females of pieris rapae under green-house conditions. Results of potted trials demonstrated that cabbages were protected from the damage of the larvae of P. rapae by spraying with 0.5% of extracts from flowers of R. molle. Since R. molle showed efficacy in the control of agriculral insects, it seems to be promising as a botanical insecticide for use in integrated pest management programs of vegetable insects in China.