Effects of Bacillus thuringiensis Cry1C toxin on metabolic rate of Cry1C resistant and susceptible Spodoptera exigua (Hubner) were investigated using closed-system respirometry. Mechanisms of resistance to Bt toxin may be associated with energetic costs reflected as increased metabolic rate. We tested this hypothesis using third and fifth instar larvae and 1 to 7 d old pupae. Metabolic rate was measured as amount of O2 consumed and CO2 produced. Mass corrected VO2 and VCO2(ml g-1h-1) of third instar Cry1C resistant larvae reared continuously on diet containing 320mg Cry1C toxin per g diet (CryonT) were significantly greater than third instar Cry1C resistant larvae reared on toxin for 5d and thereafter on untreated diet (Cry5dT), Cry1C resistant larvae reared on untreated diet (CryReg), and a susceptible strain (SeA). There were no differences in VO2 and VCO2 among treatments for fifth instar larvae. CryonT larvae and pupae weighed significantly less than larvae and pupae receiving other treatments. All one and 2 d-old pupae exhibited high VO2 (mean ~0.34) with CryonT having a significantly greater value than all other treatments. Metabolic rates of all treatment groups declined on days 3 and 4, but there were no differences among treatments. For the entire 7 d experiment, the lowest metabolic rate attained by all treatment groups was on days 3 and 4 and was followed by a linear increase until adult emergence. These results demonstrate the difference in metabolic rates between resistant and susceptible S. exigua as well as the effects which exposure time to Bt toxin has on metabolic rate.
Species 1: Lepidoptera Noctuidae Spodoptera exigua (Beet armyworm)
Keywords: Metabolic rate
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