Degree Type


Date of Award


Degree Name

Doctor of Philosophy




Studies of the ecology of green cloverworm (GCW), Plathypena scabra (F.) (Lepidoptera:Noctuidae), populations and associated natural mortality agents, especially Nomuraea rileyi (Farlow) Samson (Hyphomycetes:Moniliales), were divided into six parts;In Part I, significantly fewer GCW larvae occurred in till-plant and no-till soybean plots than in fall moldboard plow and fall chisel plow plots during the first year of the study. Substantially more larvae occurred in plots that were fall moldboard plowed than in reduced-tillage plots in the second year. The differences in tillage plot GCW numbers were not explained by analysis of moth activity nor by the occurrence of natural biotic mortality agents, including N. rileyi. In Part II, consumption patterns of soybean leaf tissue by N. rileyi-infected larvae, up to the times of their deaths, were very similar to those of healthy larvae. The annual occurrence of N. rileyi-infected larvae and data presented in Part III suggested that N. rileyi conidia and stroma survive Iowa winters to initiate mycoses in host larvae the next season;Part IV compared GCW larval populations and incidences of biotic mortality agents in alfalfa and soybean habitats. In central Iowa, alfalfa fields may act as "nurseries" for biotic mortality agents. Early-season N. rileyi infections of alfalfa GCW larvae may supply inocula for infections of soybean GCW larvae later in the season. In Part V, analyses of 10 years of GCW collections in soybean provided several quantitative expressions that help explain GCW population dynamics in Iowa. The dates of first N. rileyi occurrence may be predicted by regression equations that use the accumulated number of larva-days during the season and the accumulated precipitation from 1 June as independent variables. Also, the seasonal increase in rate of N. rileyi-caused mortality in outbreak GCW populations is described as a function of the number of days after the date of initial detection of infected larvae. Part VI traced N. rileyi mycoses in infected GCW larvae. After cuticular penetration and hemocoel colonization, the pathogen invaded all body tissues and caused larval death in ca. 6 to 7 days. Conidiogenesis produced a covering of green conidia on larval cadavers.



Digital Repository @ Iowa State University,

Copyright Owner

Harlan Gene Thorvilson



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186 pages

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Entomology Commons