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Title:   Author  item
                                            BABU, ARUN - Mississippi State Universityitem
                                            COOK, DONALD - Mississippi State Universityitem
                                            CAPRIO, MICHAEL - Mississippi State Universityitem
                                            Allen, Clintitem
                                            MUSSER, FRED - Mississippi State University
  Submitted to: Crop Protection
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 6/10/2014
                                    Publication  Date: 10/1/2014
                                    Citation: Babu, A., Cook, D.R., Caprio, M.A., Allen, K.C., Musser, F.R. 2014. Prevalence of Helicoverpa zea (Lepidoptera: Noctuidae) on late season volunteer corn in Mississippi: implications on Bt resistance management. Crop Protection Journal. 64:207-214.Interpretive Summary: The corn earworm is an important insect pest of multiple field crops in the southern United States.  Corn is the preferred host of this insect and, in Mississippi, the corn crop is harvested in mid-summer.  Kernels of corn left in a field after harvest can germinate and produce new corn plants which remain in the field until the field is cultivated or until the first killing frost.  This late-season  "volunteer" corn can serve as a host for the development of corn earworms which hibernate in the soil during the winter months.  A study was conducted to examine the suitability of late-season volunteer corn for caterpillar development and the potential contribution of these individuals to the overwintering population.  Based on historical weather data, corn earworm eggs which are laid on volunteer corn plants by September 9th in Mississippi have enough time to develop and enter hibernation before the first frost, with the probability of successful development decreasing rapidly thereafter.  However, numbers of  corn earworm caterpillars and their chance of survival is much lower on whorl-stage corn compared to development on corn ears.  The proportion of the late-season volunteer plants which reach the ear stage of development before cultivation or first frost is fairly limited.  Thus, the number of corn earworm that develop and successfully contribute to the overwintering population on late-season volunteer corn which express an insecticidal protein within the plant appears to be minimal.
                                Technical Abstract: The southern United States has a long growing period between corn harvest and first winter frost, so volunteer corn which germinates after corn harvest has a growing period sufficient for corn earworm, Helicoverpa zea (Boddie) to feed on these plants. However, lower air temperatures can limit larval development on late season volunteer corn and thereby successful pupation. Here we explore the suitability of late season volunteer corn for H. zea larval development and the potential contribution of these larvae to the overwintering population. Our survey revealed the occurrence of volunteer corn in high densities, with monthly mean densities ranging from 56,000 to 143,000 plants ha-1. H. zea larvae were found feeding on both vegetative and reproductive stage plants. An analysis of H. zea growing degree day (GDD) accumulations based on Mississippi weather data from 1980-2010 revealed a 100% probability that sufficient GDD would be accumulated to reach prepupation before first frost if oviposition occurred by 9 September, with the probability of successful pupation decreasing rapidly thereafter. This limits the proportion of the overwintering population that develops on late season volunteer corn to those larvae that established on volunteer corn by mid-September. However, most of the larvae started developing after this period, and could not reach pupation. Low suitability of whorl stage  corn for larval development coupled with low larval densities during this stage can effectively diminish the number of larvae that complete development on late season volunteer transgenic corn expressing genes from the soil bacterium, Bacillus thurnigiensis (Bt). This limits the Bt resistance risk posed by larvae developing on late season volunteer corn in all but the most southern locations in the US.


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