Heliothis zea (Boddie) (Lepidoptera: Noctuidae) density, development, and relationships with host crops were studied in eastern Virginia. Six instars occur. The larval population in corn is the reservoir for the generation in soybeans. An estimator, based on the reservoir population, permitted calculation of the larval density in soybeans. An index, based on blacklight trap and emergence data, permitted precise timing ot peak moth flight to soybeans. Regression analyses of biotic and abiotic factors revealed that soybean plant morphometrics best explained corn earworm densities. Discriminant analyses on empiricala priori soybean field susceptibility categories (described by plant morphometrics) classified five levels of anticipated maximum larval densities 80 - 98 percent correctly. Development of highest larval densities was synchronized with overall plant growth and specific pod development: third and fourth instars were present during rapid pod elongation, while fifth and sixth were present during rapid pod volume increase. / Master of Science

The processes that determine the structure of plant communities are of considerable practical and theoretical interest. Natural enemies such as herbivores, seed predators and pathogens provide one potentially important influence on plant diversity. I investigated the effects of natural enemies on plant diversity in two contrasting, species-rich plant communities (tropical forests in Panama and temperate grasslands in the UK), focusing on pre-dispersal seed predation by insects, and the mortality of seeds and seedlings caused by soil fungi. In Panama I found that pre-dispersal insect seed predators generate significant levels of mortality in multiple tropical tree species, with high heterogeneity in predation rates among individuals and at different forest sites. Insect seed predators were highly host-specific, consistent with a role in enhancing plant diversity. At Upper Seeds, a calcareous grassland site in the UK, I used manipulative experiments to show that soil fungi increase the diversity of plants propagating from soil seed banks. A parallel experiment in Panama, mimicking germination under light gap conditions, revealed differential effects of fungi among sites, with fungicide treatment appearing to increase the diversity of propagated seedlings at some sites but reducing it at others. These results suggest that the influence of soil fungi on pre-emergence mortality can alter plant diversity, even when post-emergence mortality from fungal pathogens is limited. In Panama, I also tested whether enemy-mediated mortality increases with rainfall, potentially contributing to the positive regional correlations widely observed between precipitation and plant diversity. In contrast to predictions, neither pre-dispersal insect seed predation nor the influence of soil fungi on seedling recruitment were affected significantly by site humidity, or (for soil fungi) with experimentally manipulated soil moisture levels. Overall, my results provide evidence that pre-dispersal seed predators and soil fungi can affect plant recruitment and diversity at early life stages, with potential consequences for the community structure of adult plants.

The main goal of this study was to evaluate the effectiveness of two biological control agents in reducing population growth and spread of the invasive tree Melaleuca quinquenervia, a subtropical tree native to Australia, and invasive in Florida, Puerto Rico, and the Bahamas. While in Florida two insects Oxyops vitiosa (weevil), and Boreioglycaspis melaleucae (psyllid) have been established as biocontrol agents, in Puerto Rico only psyllids are present, and in the Bahamas no biocontrol agents are present. This study combined demographic data, experiments and mathematical models to investigate the influence of the biocontrol agents on M. quinquenervia's spatial population dynamics. In the field, permanent plots were established and demographic data was collected in populations in the native and exotic ranges. Australian populations are comprised mostly of tall adult trees, while in the exotic ranges populations are comprised mostly of short trees (<1.3m in height), and small adult trees. In a shade-house, I performed an experiment to investigate the effects of insect type and density on survival and growth of M. quinquenervia seedlings. I found that high density of insects, independently or in combination, reduce seedling performance, thus having the potential to alter the seedling-short plant transition of the M. quinquenervia life cycle. Based on the demographic data, I developed integral projection models (IPMs) to determine population growth rates in each region. Populations in Australia and the Bahamas are increasing, while populations in Florida and Puerto Rico are decreasing. Population growth is most sensitive to the seedling-short plant transition in all regions, except Florida, where it is most sensitive to survival of tall plants. Simulations combining the results of the IMPs and experiment indicated the biocontrol damage results in reductions in population growth rate in Puerto Rico and the Bahamas. Seed dispersal and demographic data was combined to develop an integrodifference structured model of population spread. Simulations indicated that by reducing seedling performance, insects have the potential to reduce the rate of population spread. Overall this study shows that individual-level effects of biocontrol agents have translated into reductions in population growth rate and rate of spread of M. quinquenervia.