Date of Award
Doctor of Philosophy
John J. Obrycki
The overall objective of this dissertation was to evaluate, both directly and indirectly, the natural enemies of the European corn borer (ECB). The direct evaluation component consisted of two parts: (1) Assessment of the level of naturally occurring mortality of the ECB and evaluation of the impact of biotic and abiotic factors that cause this mortality, and (2) Quantitative evaluation of the impact of augmentative releases of Trichogramma brassicae, a parasitoid of the ECB eggs, on reducing larval densities and damage on corn plants. The indirect laboratory-based evaluation, addressed two factors that affect the efficacy of natural enemies: prey suitability and intraguild predation;In the first study, the influence of predators, parasitoids, and pathogens of the ECB on its seasonal abundance and outbreaks was quantified. I used five treatments with differential levels of exclusion of natural enemies to assess the impact of natural enemies on ECB densities. There were higher rates of ECB egg predation in uncaged (>50%) than in caged treatments. The highest larval mortality occurred among the first and second instars; this mortality was not influenced by treatments, indicating that abiotic factors were responsible;In the second study, I released different numbers of wasps in 0.25 hectare plots to examine the relationship between release rates and parasitism levels. The total number of wasps released in each plot was: 0 (control), 25,000, 50,000, 100,000, and 200,000 wasps. Levels of egg parasitism were significantly affected by the number of parasitoids released;In the third study, I found that C. maculata developmental time and survival rates were similar on ECB eggs and pea aphids, indicating that ECB eggs are suitable prey. In the fourth study, I found that Chrysoperla carnea eggs are suitable prey for three coccinellid species that co-occur with C. carnea in corn fields. I also found that there is a potential for C. maculata and C. carnea larvae to negatively affect each other and reduce their ability to suppress pest densities.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Mpho Wycliffe Phoofolo
Phoofolo, Mpho Wycliffe, "Evaluation of natural enemies of the European corn borer, Ostrinia nubilalis (Lepidoptera: Pyralidae) " (1997). Retrospective Theses and Dissertations. 12231.