Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

David J. Boethel


Acetylene reduction and nitrogen difference assays were used to evaluate effects of defoliation by soybean looper, Pseudoplusia includens (Walker), during reproductive growth stages on symbiotic nitrogen fixation by soybean. Defoliation (50-55%) of group VI cultivar Lee from full bloom into pod development stages caused up to 85% reduction of nitrogenase activity, whereas lower defoliation levels (32%) during seed development did not affect nitrogenase activity. Defoliation (74-94%) of group IV cultivar Clark caused 80-100% reduction of nitrogenase activity, regardless of whether defoliation occurred during bloom and pod development or during seed development. The nitrogen difference assay identified a reduction in the amount of nitrogen acquired from symbiotically-fixed sources in response to defoliation, but this occurred in only one experiment (Lee, 55% defoliation, full bloom into early seed development). Yield of Lee was reduced by 1492 kg/ha (50% defoliation, full bloom through pod development) or by 971 kg/ha (32% defoliation, early seed development). Yield of Clark was reduced by 800-1359 kg/ha (74-94% defoliation, full bloom through pod development) or by 750 kg/ha (94% defoliation, early seed development). Chlorosis of non-nodulating isolines occurred during seed development and was associated with reduced feeding and poor establishment of soybean looper in field plots. As a result, anticipated reductions in leaf area of non-nodulating isolines were not achieved during seed development, and three feeding bioassays were initiated to examine how chlorotic plants influenced soybean looper development. These bioassays used foliage from nodulating and non-nodulating isolines collected from field plots beginning at bloom and early seed development, and also examined foliage from greenhouse-grown non-nodulating soybeans supplied with six rates of nitrogen fertilizer (0-168 kg N/ha as $\rm NH\sb4NO\sb3$). Responses by soybean looper to reduced nitrogen included development of greater number of stadia, lengthened duration of larval development, and reduced survival. Extended development resulted in greater foliage consumption and maximum larval weights of survivors that equalled those fed foliage with higher nitrogen concentrations. These responses to fluctuations in host nutrition may be mechanisms that allowed soybean looper to evolve in association with such a wide hosts range (73 species in 29 families).