The Susceptibility of Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) to Spinosad

The susceptibility of Leptinotarsadecemlineata (Say) adults to spinosad was determined by leaf disk and glassjar techniques under laboratory conditions. The mortalities of the treatedadults were recorded 72 h post treatments. LC50s values in leaf diskand glass jar methods for adults were 98.50 and 108.34 mg a.i/L, respectively. A significant linear relationship between LC50values of individual population across test methods was detected (r = 0.93,P < 0.05). Both bioassaytechniques were suitable for monitoring resistance to spinosad in Coloradopotato beetle adult populations. Glass jar technique, however, exhibited lessvariability in LC50 estimates and showed a higher degree ofsensitivity than the leaf disk method. Filter paper and leaf disk techniquesfor larvae were two bioassay methods used to determine spinosad susceptibilityin L. decemlineata population. Both bioassay techniques exhibited a differentlevel of susceptibility of the larvae to spinosad; however, the confidenceinterval values from filter paper method were narrow than the leaf disk assaytechnique. A significant direct relationship between LC50 values ofindividual population across test methods was observed (r = 0.96, P < 0.05). The glass jar and filter paper testingmethods are simple and sensitive test techniques for measuring susceptibilityof Colorado potato beetle adults and larvae to spinosad, respectively. Theefficacy of spinosad can be conserved for a long period of time if it isjudiciously rotated with other suitable insecticides in a spray program and thenumber of application is restricted.