Plant Defensive Traits: Translation of Their Effects on Herbivorous Insects Into Reduced Plant Damage (Evolution, Feeding Compensation, Predation)

143 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.Plant traits can have four basic negative effects on herbivores: reduced individual growth, reduced population growth, directly lethal effects, and reduced preference for the plant. The first part of this thesis provides a theoretical overview of the potential mechanisms through which these negative effects, on herbivorous insects, can be translated into reduced herbivore damage to the plant expressing the trait. Some of the more important constraints on the manifestation of those mechanisms are identified, and the probable relative importance of these constraints is discussed. It is argued that the reduction of herbivore growth and perhaps reduction of population growth are more important bases for the evolution of defensive traits than is generally realized. However, the translation of these effects into reduced damage will generally involve very complex ecological interactions.The second part of this thesis presents an investigation of the ability of traits that reduce growth of an herbivorous insect to also reduce plant damage, depending on the density of predators of that herbivore. A model agricultural system was used that consisted of the predaceous bug, Podisus maculiventris, the Mexican bean beetle, Epilachna varivestis, and two soybean lines that differed in their capacity to support growth of beetle larvae. It was found that while beetle survivorship was significantly less on the "low-growth" plant than that on the "high-growth" plant, larval per capita damage was actually greater. However, the ratio of beetles surviving on the "low-growth" to that on the "high-growth" plant tended to decrease with increasing predation, an effect that apparently reflects a complex combination of increased larval susceptibility to pathogens and greater predator efficacy on the "low-growth" plant. The decrease in the survivorship ratio indicates that such growth-reducing traits will be most likely to offset an increase in herbivore per capita consumption, and so, reduce overall damage at higher predator densities. The existence of complex interactions even in such simple systems supports the assertion made in the first part of the thesis, namely, that our understanding of the evolution of plant traits as defenses against herbivores is still very meager.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD