|Head, G. - MONSANTO COMPANY|
|Bradley, J -|
|Van Duyn, J -|
|Leonard, B. -|
|Luttrell, Randall -|
|Ruberson, J -|
|Mullins, Walt -|
|Orth, Robert -|
|Sivasupramaniam, Sakuntala -|
|Voth, R -|
Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 10, 2009
Publication Date: March 30, 2010
Citation: Head, G., Jackson, R.E., Adamczyk Jr, J.J., Bradley, J.R., Van Duyn, J.W., Gore, J., Hardee, D.D., Leonard, B.R., Luttrell, R.G., Ruberson, J., Mullins, W., Orth, R.G., Sivasupramaniam, S., Voth, R. 2010. Spatial and Temporal Variability in Host Use by Helicoverpa zea as Measured by Analyses of Stable Carbon Isotope Ratios and Gossypol Residues. Journal of Applied Ecology. 47(3):583-592. Interpretive Summary: Since the 1996 introduction of Bt insecticidal cottons, cotton producers have been required to plant a portion of their cotton acreage to non-Bt cotton. The purpose of this refuge was to produce non-selected insects that would mate with resistant individuals that may develop in Bt cotton, thus diluting Bt resistance genes in the general insect population. Refuge requirements have been set at either 5 or 20 percent plantings of non-Bt cotton to the overall acreage of a given farm, depending on how whether or not the refuge was managed for caterpillar pests. Information from this study demonstrated that structured non-Bt cotton refuges had minimal impact on Bt resistance management in bollworm because of its preference for non-cotton crop hosts. Because of this information and that from studies with tobacco budworm host use, the structured non-Bt cotton refuge requirement has been eliminated for Bollgrd II (Bt) cottons. This decision will increase the sustainability of cotton production and will make cotton production more profitable for farmers.
Technical Abstract: Concerns with resistance evolution in tobacco budworm, Heliothis virescens (F.) and bollworm, Helicoverpa zea (Boddie), to transgenic insecticidal crops that produce Bacillus thuringiensis proteins led to the adoption of the high dose/refuge strategy for managing cotton insect resistance in the United States. In order to manage resistance development to Bt cottons that produce a high dose of toxin, structured refuges consisting of non-Bt cotton must be planted in order to produce non-selected insects that are temporally and spatially synchronous with insects that develop in the Bt crop so that Bt resistance alleles can be diluted through mating. These mandated refuge requirements only consider non-Bt cotton as a source of refuge insects. However, both of these insect species are highly polyphagous, exploiting a number of crop and weedy hosts that are present when these insects utilize Bt cotton. A regional study was established in five cotton-producing states to estimate the proportion of the bollworm population that develops on cotton and non-cotton hosts and can serve as a refuge from transgenic cottons that produce one or two proteins from Bt. Because insects acquire the carbon isotopic signature of their natal host and plants that utilize different photosynthetic pathways (C3 versus C4) possess distinct ratios of 13C to 12C, a stable carbon isotope technique was used to estimate proportions of bollworm populations originating from C3 or C4 plant hosts. We determined that C4 hosts, primarily corn, sorghum and grassy weeds, serve as the principal source of bollworm moths (>50% and as high as 100%) through mid-to-late August and into early September, depending upon the state. Because Bt cotton produces bollworm moths during this period, corn, sorghum and grassy weeds function as a vast refuge from Bt selection pressure. Beginning late August to early September and lasting for 1 to 4 weeks, the majority of moths collected retained isotopic compositions characteristic of C3 hosts. During this period, however, the minimum percentage of moths that developed as larvae on C4 hosts ranged from 10-41%, depending upon the state and year. Typically, greater than 25% of moths collected during this period possessed C4 isotopic compositions. By mid-September and through October to November, the majority of the bollworm population was comprised of individuals with C4 isotopic compositions. The proportion of the population originating from C4 hosts during this period typically exceeded 50%. These findings contrast with the accepted hypothesis that the majority of late season bollworm moths develop as larvae on cotton. The results presented here support other research, which demonstrated that non-cotton crop hosts contribute more to Bt resistance management of bollworm than does the structured cotton refuge. These findings, along with information developed on contributions of non-cotton hosts to the tobacco budworm population, indicate that consideration should be given to non-cotton host contributions to insect populations when formulating resistance management strategies.