Srinidi Mohan, sm285@msstate.edu1, Peter W. K. Ma, pma@entomology.msstate.edu2, Paul Williams, pwilliams@pss.msstate.edu3, and Dawn S. Luthe, dsluthe@ra.msstate.edu1. (1) Mississippi State University, Department of Entomology, Mississippi State, MS, (2) Mississippi State University, Department of Entomology and Plant Pathology, P.O. Box 9775, Mississippi State, MS, (3) Mississippi State University, Plant and Soil Sciences, 344 Dorman Hall, Mississippi State, MS
In past studies we have shown that novel 33-kD cysteine protease (Mir1-CP) accumulates in the whorls of maize inbred lines that are resistant to feeding by fall armyworm (Spodoptera frugiperda) and other Lepidoptera. We have shown that the peritrophic matrix (PM) of larvae that feed on plant material expressing Mir1-CP was damaged and treatment of isolated PMs with purified recombinant Mir1-CP increased PM permeability in a concentration dependent manner. This study was conducted to determine the effect of purified Mir1-CP on larval growth in bioassay and determine if there was a synergistic effect between Mir1-CP and Bacillus thuringensis (Bt-CryIIA) toxin. Dose- response bioassays were conducted with 2nd instar fall armyworm larvae. The relative growth rates (RGR) of fall armyworm decreased at concentrations greater than 600 ng/ml Mir1-CP or 0.75 µg/ml of Bt-CryIIA. When 600 ng/ml Mir1-CP was used in the bioassay the RGR was 0.362±0.037. When 0.4 mg/ml Bt-CryIIA was used the RGR was 0.574± 0.013. When these two concentrations of Bt-CryIIA and Mir1-CP were combined the RGR was 0.198 ± 0.019. The RGR for larvae reared on the control diet was 0.624± 0.054. These results suggest that there is a synergistic decrease in growth rate when larvae are reared on diet that contains sublethal concentrations of Bt-CryIIA and Mir1-CP.
Lepidoptera Noctuidae
Spodoptera frugiperda (fall armyworm)
Keywords: Dose response, Synergism
