ANTIFEEDANT AND TOXIC EFFECTS OF SESQUITERPENES FROM SENECIO-PALMENSIS TO COLORADO POTATO BEETLE

A bioassay-guided fractionation of the aerial parts of Senecio palmens is resulted in the isolation of two sesquiterpenes, 2, 10-bisaboladien -1-one and 11 beta-acetoxy-5-angeloyloxy-silphinen-3-one. The bisabole ne and the silphinene represented 0.012% and 0.024% of the plant dry w eight, respectively. Both compounds showed antifeedant activity agains t Leptinotarsa decemlineata larvae and adults in short-term choice and no-choice bioassays. Both compounds were also tested against differen t species of phytopathogenic fungi. The beetles were more sensitive to these compounds in choice than in no-choice assays, with a gradient o f increasing sensitivity from second instars to adults. Bisabolene was 45 times less active as an antifeedant than juglone, which was tested as a positive control. The silphinen was more active than the bisabol ene, with a range of activity similar to juglone. Furthermore, exposur e of fourth instars to these compounds over a 24-hr period resulted in reduced feeding and growth rates. To distinguish between antifeedant and toxic effects, growth efficiencies were calculated as the slope of the regression of relative growth rate on relative consumption rate. The comparison of these results with those of antifeedant simulation a nd contact toxicity bioassays indicates that feeding inhibition is the primary mode of action of the bisabolene, while the silphinene shows both antifeedant and toxic effects.