Location: Southern Insect Management Research
2019 Annual Report
Objectives
Objective 1: Determine impacts of Bt toxins on pest insect biology, assess population dynamics, pest behavior, and host-plant relationships that enhance resistance, and develop management strategies to mitigate evolution of insect resistance to host plant expressed insecticidal genes.
Sub-objective 1A: Determine the impacts of transgenic crops producing two or more Bt toxins on population ecology and phenology of heliothines in cotton.
Sub-objective 1.B: Evaluate optimal management strategies to delay resistance of heliothines to transgenic cotton.
Objective 2: Determine genetic diversity of bollworm populations and impacts of changes in allele frequencies of loci known to be associated with resistance to Bt toxins and insecticides.
Sub-objective 2.A: Determine genetic diversity of bollworm populations and allele frequencies of loci known to be associated with resistance to Bt toxins and insecticides.
Sub-objective 2.B: Evaluate the allele frequency changes during selection with Bt toxins and insecticides.
Objective 3: Determine impacts of insecticide resistance on management of lepidopteran pests and develop environmentally sound strategies to manage pest complexes in transgenic cropping systems.
Sub-objective 3.A: Determine impacts of insecticide resistance on management of bollworm.
Sub-objective 3.B: Evaluate IPM tactics for optimal management of pests in transgenic cotton.
Approach
The impacts of transgenic crops producing two or more Bacillus thuringiensis (Bt) toxins on population ecology and phenology of bollworm (BW) will be studied using replicated field experiments structured to examine multi-generational effects of selection by different sequences of transgenic crops (Bt-crops) and non-Bt crops. Experiments will be conducted using 1/16th acre field cages during the first three years of the project followed by five-acre field plots during the remainder of the project. Paired treatments will compare Bt-crop varieties with non-Bt counterparts (near isolines). Experimental crops inside cages will be infested with pupae reared from early season larval collections. Insect densities, species composition, survival on a given host, and crop damage data will be used to predict relationships between within-season selection of Bt-crop hosts and the effects of selection on population dynamics of BW. Sentinel plots of cotton and corn will be established on a spatial gradient representative of the range of latitudes within the Mississippi Delta and used to evaluate the effects of supplementary insecticide control of BW on primary Bt and non-Bt crop hosts. Different Bt crop varieties will be paired and planted with a non-Bt isoline. One replication of the Bt variety and its non-Bt isoline will be sprayed with chlorantraniliprole if and when recommended threshold for BW is reached. Other plots will receive no sprays for BW throughout the growing season. Non-target pests on the experimental plots will be controlled as needed with blanket applications of insecticides with no or low lepidopteran activity. Larval collections will be used to determine species composition infesting plots. Crop damage, species composition, and survival from each crop will be analyzed using each location as a replicate in a split plot design to determine the effects of supplementary control of BW in Bt and non-Bt crops on yield.Molecular markers will be used to evaluate genetic diversity of BW populations and impacts of changes in allele frequencies of loci associated with resistance to Bt toxins and insecticides. Allele frequencies in insects collected during the first three years of the project period will be compared with data from insects collected from 2002-2006. Identification of loci under selection will help us evaluate the impacts of field selection on BW over time. In addition, we will be able to estimate the mutation rates of the genes associated with Bt resistance and use those estimates in Bt resistance prediction models. A BW strain tolerant to Bt toxin Cry1Ac will be used to identify genomic regions responding to selection. Impacts of insecticide resistance on management of lepidopteran pests will be determined by mutating target receptor genes to generate insecticide resistance in BW lines with high tolerance to Bt toxins. Fitness costs of dual resistance will be evaluated using controlled experiments. Integrated pest management tactics utilizing various combinations of chemical and microbial agents will be evaluated to develop environmentally sound strategies to the management pest complexes in transgenic cropping systems.
Progress Report
Substantial progress was made in research planned for the 48-month reporting period of this project. We have continued our assessments of insect resistance to conventional and transgenic insecticides in numerous populations of key caterpillar pests of cotton. Laboratory studies examining the susceptibility of bollworm populations to Bacillus thuringiensis (Bt) toxins were conducted. Populations were collected from both wild and cultivated host plants (both Bt and non-Bt) across the MS Delta throughout the growing season. Responses of populations to these assays continue to show variability across time and location. Data from these assessments are being used to document the potential spread of resistance across the study area and to allow time for circumvention when necessary.
Growers and agricultural consultants have reported increasing incidences of bollworm survival and subsequent injury to Bt corn and cotton across the southern U.S. Although there have been numerous reports of bollworm resistance to Cry1 and Cry2 Bt proteins, the extent of the problem is not known. Therefore, we have partnered with Texas A&M University, North Carolina State University, and the University of West Georgia to conduct surveys to detect resistance in bollworm to Bt proteins expressed in both Bt corn and cottons planted in the southern U.S. F2 screens are also being conducted to measure Bt resistance allele frequencies. Results generated from this study will provide essential data to Bt resistance modelers and Federal regulators for making sound science-based decisions regarding Bt resistance management strategies.
Wild populations of Helicoverpa species from New York, Pennsylvania, Texas, and Virginia were collected for the fourth consecutive season. Genomic DNA extractions were carried out from moth collections for use in genotyping assays and for detecting invasive old-world bollworm (OWB), Helicoverpa armigera using a high throughput detection system developed at the USDA-ARS Southern Insect Management Research Unit in Stoneville, Mississippi. Bollworm populations from Pennsylvania collected in 2002 and 2005 were compared with those collected in 2016 and 2018 using 96 single nucleotide polymorphism (SNP) markers to assess changes in population characteristics over time. Although, changes in allele frequencies at some loci, no substantial genetic drift was observed in bollworm populations during last decade.
Accomplishments
1. Linkage between laboratory and field Bt resistance in bollworm. Resistance in caterpillar pests to insecticidal toxins expressed in tissues of Bt cotton plants threatens to increase usage of more dangerous foliar insecticides for their control on this crop. Early detection of resistant populations is needed to prevent widescale control failures, but laboratory assessments of resistance levels of these pests don’t provide a clear picture of insect survival in the field. Populations of bollworms with different levels of resistance were tested in both the lab and field. A linkage between bollworm survival on Bt cotton and survival on Bt proteins in the lab was determined and published. This information will be used to more accurately predict bollworm survival in Bt cottons based on laboratory measurements, and therefore, increase the sensitivity of these tests for detection of “true” field resistance of these insects.
2. Comparative activity among different isolates of baculoviruses for heliothine management. Microbial insecticides, such as baculoviruses, are naturally occurring and a relatively safe means of providing control of targeted insects in production agriculture. The comparative activity of commercial baculovirus formulations for control of bollworms and tobacco budworms in cotton was completed and published. Current commercially available formulations were evaluated and compared to the first product registered for heliothine control in the United States in the 1970s. The study demonstrated that there has been little functional change among wild-type nuclear polyhedrosis virus insecticides for control of heliothines over the past 40 years. This study will positively impact the use of baculoviruses for heliothine management in cotton by providing growers and pest managers information regarding the potential ecological benefits of microbial insecticides compared to those synthetic in nature.
3. Temporal genetic variation in bollworm. Bollworm adults collected from two locations in Pennsylvania in 2002, 2005, 2016, and 2018 were genotyped with 96 single nucleotide polymorphism (SNP) markers. Estimations of population genetic parameters indicated moderate levels of allele frequency changes in some SNP loci and no significant genetic drifts during past 11-13 years.
Review Publications
Walsh, T.K., Perera, O.P., Anderson, C.J., Gordon, K.H., Czepak, C., McGaughran, A., Zwick, A., Hacket, D., Tay, W. 2019. Mitochondrial DNA genome resources of five major Helicoverpa pest species from the Old and New Worlds (Lepidoptera: Noctuidae). Ecology and Evolution. 9(5):1-12. https://doi.org/10.1002/ece3.4971.
Little, N., Elkins, B.H., Mullen, R.M., Perera, O.P., Parys, K.A., Allen, K.C., Boykin, D.L. 2019. Differences between two populations of bollworm, Helicoverpa zea (Lepidoptera: Noctuidae), with variable measurements of laboratory susceptibilities to Bt toxins exposed to Non-Bt and Bt cottons in large field cages. PLoS One. 14(3). https://doi.org/10.1371/journal.pone.0212567.
Little, N., Luttrell, R.G., Mullen, R.M., Allen, K.C., Parys, K.A. 2019. Comparative activity of commercial baculovirus formulations against heliothine pests of cotton. Southwestern Entomologist. 44(1):105-115.
Pothula, R., Shirley, D., Perera, O.P., Klingeman, W.E., Oppert, C., Abd-Elgaffar, H., Johnson, B.R., Jurat-Fuentes, J. 2019. The digestive system in Zygentoma as a model for high cellulase activity. PLoS One. 14(2):e0212505.
