Relative Influence of Plant Quality and Natural Enemies on Population Dynamics of Bemisia tabaci and Lygus hesperus in Cotton

Abstract

The bottom-up effects of plant quality and the top-down effects of natural enemies are two of the most important factors governing the abundance, distribution and performance of insect herbivores. Plant quality effects are generally manifested through changes in nutrients, morphology, size or host availability. Natural enemy effects are mainly observed through the action of arthropod predators and parasitoids. The relative influence of these effects has been well studied in several natural and forest ecosystems but has rarely been evaluated in agro-ecosystems. In manipulative field experiments, I examined the relative influence of plant quality and natural enemies on the abundance and performance of Bemisia tabaci and Lygus hesperus, two important pests of cotton.This dissertation begins with an evaluation of the relative influences of plant quality (manipulated through differential irrigation) and natural enemies (manipulated by insecticide exclusion) on the seasonal dynamics of B. tabaci in cotton. I found higher densities of all B. tabaci stages when natural enemy densities were reduced, regardless of level of plant quality. Generally, densities of B. tabaci did not vary with plant quality. Similarly, natural enemy densities and predator:prey ratios were not affected by plant quality, indicating that natural enemies exert a greater influence on seasonal dynamics of B. tabaci than plant quality. I then evaluated the mechanism responsible for herbivore density patterns using cohort-based life table studies. I found lower rates of predation and parasitism when natural enemy densities were reduced across all levels of plant quality, with predation the primary key factor associated with changes in generational mortality. Levels of B. tabaci mortality did not vary with plant quality indicating that the impact of natural enemies was not mediated by the bottom-up effects of plant quality. Finally, I evaluated these effects on L. hesperus and found that plant quality significantly affected seasonal dynamics with the greatest and smallest densities on the highest and lowest quality plants, respectively. These effects did not translate to natural enemy effects, as their density and impact were not affected by plant quality. The implications of these findings for the management of these pests in cotton are discussed.