Investigating host use and migration by the corn earworm (Noctuidae: Helicoverpa zea) using stable isotope and morphometric techniques.

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Title: Investigating host use and migration by the corn earworm (Noctuidae: Helicoverpa zea) using stable isotope and morphometric techniques.
Author: Rennie, Tracy Lee
Advisors: Fred Gould, Committee Chair
Abstract: The commercialization of cotton and corn, genetically engineered to produce a caterpillar-specific insecticidal protein (Bt toxin) has focused renewed attention on the issue of southward migration of Helicoverpa zea because it could have a major effect on the rate at which this pest evolves resistance to the Bt toxin. Corn is a major host of H. zea and since there is presently limited use of transgenic Bt corn in the Northern corn-belt southern migration from this corn could result in a larger overall refuge for susceptible H. zea and that would decrease the rate of resistance evolution We applied a stable carbon isotope technique to test the hypothesis that H. zea migrates North in early summer, feeds on corn, then migrates back South to feed on cotton. We analyzed the relative abundance of the naturally occurring isotope, 13C, incorporated in wing cuticle of a moth as an indicator of the photosynthetic pathway (corn=C4, cotton=C3) of the plant that had been fed upon by it when it was a larva. Our carbon isotope analyses of H. zea, captured in pheromone traps, in the Brazos River Valley, Texas, demonstrated that at any given date from late summer to early fall in 2000, 40-100 percent of the moths developed on C4 host plants and were present as adults in a cotton growing area where there were no known C4 hosts suitable for development. Our investigation has also shown significant morphometric differences in moths from C3 and C4 hosts. Moths that fed upon a C4 host during the larval stage typically had larger wing length and were heavier than moths that fed upon a C3 host. Host plant quality during larval development may affect fluctuating asymmetry in adult characteristics, such as wing length, causing greater asymmetry in moths whose larvae fed on sub-optimal hosts. Apparently, there was no effect of host plant type on the degree of wing length asymmetry measured in the Texas moth samples, however, there was a significant effect of DATE, and an interaction effect of HOST and DATE. The moths from early in the season were more asymmetrical than moths later in the season. We have provided circumstantial evidence that suggests that H. zea moths are migrating into cotton growing regions of the Brazos River Valley from northern regions where corn or some other C4 host is suitable for larval development. It is expected that continental hydrogen isotope ratios (delta-D) in surface water at moth natal sites will be significantly correlated with delta-D values in moth wings. We examined wings of H. zea moths that developed as larvae on corn from known origins across the mid- and eastern US to determine the potential use of hydrogen isotope ratios in H. zea wing tissue as geographical indicators of H. zea natal origins. However, the delta-D values in wing tissue did not reflect the continental delta-D gradient in rainfall. Therefore, without further investigation, pheromone trapped moths cannot be used to determine moth natal origins via stable hydrogen isotope analysis.
Date: 2003-02-10
Degree: MS
Discipline: Entomology

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