Date of Award

May 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Division of Agriculture (SAFES)

Committee Member

Jeremy K Greene

Committee Member

Matthew W Turnbull

Committee Member

Dominic D Reisig

Abstract

The corn earworm, Helicoverpa zea, is a highly polyphagous insect that is a pest of corn, cotton, sorghum, and soybeans in the southern United States. It is currently managed primarily using transgenic cultivars producing insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt) in corn and cotton, and foliar insecticides in sorghum and soybeans. However, the development of resistance to several Bt toxins has been reported and is the primary threat to the success and longevity of Bt crops. We performed a number of field and laboratory experiments over three years to investigate the development, survival, and feeding behavior of H. zea in Bt corn with implications for resistance. H. zea feeding on Bt corn significantly reduced pupal weight but this did not result in detectable effects on fecundity or egg viability. We characterized the age- and tissue-specific feeding behavior of H. zea larvae in silk, tip, and kernel ear tissues in relation to tissue-specific concentrations of Cry1F and Cry2Ab2 using ELISA. Bt protein concentration varied in space and time, but we did not detect changes in larval feeding behavior between Bt and non-Bt hybrids, although development was delayed on several Bt hybrids. Resistance monitoring of 22 field-collected populations from North and South Carolina was conducted using purified protein diet-overlay bioassays to determine susceptibility to Cry1A.105 and Cry2Ab2. Compared with a susceptible laboratory colony, resistance ratios indicated that H. zea resistance to Cry1A.105 is widespread across North and South Carolina and resistance to Cry2Ab2 was detected in several populations but others remain susceptible. Susceptibility to Cry proteins did not correlate with H. zea developmental parameters. We evaluated the use of Bt and non-Bt corn hybrids in combination with foliar applications of chlorantraniliprole at varying infestation thresholds to protect field corn from infestation and damage and determine effects on grain yield. All Bt hybrids were more effective at reducing fall armyworm, Spodoptera frugiperda, infestation rates and leaf injury than multiple insecticide sprays, and no Bt hybrid required supplemental insecticide treatments. Significant protection of yield was detected only in the Bt hybrids producing Cry1A.105 + Cry2Ab2. These data fill knowledge gaps that can be incorporated into insect resistance management (IRM) models, which can aid in the implementation of IRM strategies that can improve risk management decisions regarding H. zea in Bt crops in the complex landscapes of the southern United States.

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