HOST-PLANT ADAPTATION IN FALL ARMYWORM HOST STRAINS - COMPARISON OF FOOD-CONSUMPTION, UTILIZATION, AND DETOXICATION ENZYME-ACTIVITIES

The fall armyworm, Spodoptera frugiperda (J. E. Smith), consists of two genetically different strains. The corn strain feeds primarily on corn, and the rice strain feeds primarily on forage grasses and rice. Possible causes for differences in larval performance between the strains were examined. The objectives were to establish whether behavioral or physiological factors could explain differences in performance on corn. A comparison between strains in food consumption and utilization indicated that poorer performance on corn by the rice strain is, in part, caused by a lower consumption rate. In addition, the rice strain has a lower efficiency of converting digested food into biomass than the corn strain when reared on corn. When larvae were reared on either corn or bermudagrass, the rice strain had lower mixed-function oxidase (MFO) activities than the corn strain. In the corn strain, MFO activity was higher when larvae were reared on corn than on bermudagrass. Corn and bermudagrass-reared larvae from the rice strain expressed similar MFO activities. Esterase activity did not differ between strains or between corn and bermudagrass-reared larvae. Thus, the results indicate that both behavioral and physiological factors associated with host use differ between the strains. Moreover, MFOs may play an important role in host-plant adaptation in the fall armyworm.