Host Plant Adaptation in the Two Strains of Fall Armyworm (Lepidoptera: Noctuidae).

Abstract

The fall armyworm (Spodoptera frugiperda J. E. Smith) Lepidoptera: Noctuidae) consists of two genetically differentiated strains. The corn strain feeds on corn and the rice strain feeds primarily on rice and forage grasses. My research focused on host plant adaptations within each strain as assessed by larval performance and enzymatic responses to feeding upon different hosts. Three sets of experiments were conducted. The first experiment examined consumption and utilization in both strains reared on corn. In the second experiment, mixed-function oxidase (MFO) and general esterase enzyme activities were measured from larvae of both strains that were reared on corn, bermudagrass, or artificial diet. In the third experiment, larvae of both strains were selected for increased larval weight, and associated changes in MFO activities were measured after subsequent feeding upon several hosts. In the first experiment I determined that, when reared on corn, the corn strain had higher pupal weights and accumulated more biomass during the last instar than the rice strain. This resulted primarily from higher consumption in the corn strain. In addition, the corn strain was more efficient in converting digested food into biomass than the rice strain. Thus, there are differences in both feeding behavior and nutritional physiology between the strains feeding on corn. The results from the second experiment indicated that the corn strain had a higher MFO activity than the rice strain when reared on corn or bermudagrass. This result was predicted since there are more reported allelochemicals in corn than in bermudagrass. Within the corn strain, MFO activity was higher when reared on corn than on bermudagrass. Thus, corn plants affected expression of MFO activity only in corn strain larvae. In the rice strain, MFO activity was similar on both hosts. Esterase activity did not differ between strains or host plants. In the third experiment, lines selected on corn expressed higher MFO activity than either a line selected on bermudagrass, or a control. Differences in larval performance were minor. These results suggest there is genetic variation in MFO activity that could play a role in host plant adaptation.