Location: Southern Insect Management Research
2020 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
This is the final report for this project which has been replaced by bridging project #6066-22000-091-00D pending completion of National Program 304 research review.
Substantial progress was made in research planned for the 60-month reporting period of this project. Assessments of insect resistance to conventional and transgenic insecticides in bollworm have expanded to include a newer vegetative-produced protein (Vip3Aa) and its combination with Cry toxins. Laboratory and field studies to evaluate the susceptibility of bollworm to various Bacillus thuringiensis (Bt) toxins are ongoing and adapting. Numerous heliothine populations were collected from wild and cultivated host plants (both Bt and non-Bt) across the MS Delta throughout the growing season. These assays continue to reveal variability across time and location and a trend for reduced susceptibility to Cry toxins. Data from these assessments are being used to document the spread of resistance across the southern U.S. and to allow time for mitigation efforts for the Vip3Aa gene if needed.
Wild populations of Helicoverpa species from New York, Pennsylvania, Texas, and Virginia were collected for the fifth consecutive season. Genomic DNA extractions were carried out to perform 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. Population genetic analyses of bollworm populations collected in 2002, 2005, 2016, and 2018 from two peripheral populations in Pennsylvania, indicated some genetic changes in 2018 populations compared to 2002 and 2005 populations.
Populations of Helicoverpa zea that develop on Bt or non-Bt corn are a major source of insects that infest cotton later in the growing season. Although insecticides are seldom used to control H. zea on field corn, insecticides are regularly used for their control on cotton in the southern U.S. To examine the impact of the development of H. zea on different corn types (Bt and non-Bt corn) and potential fitness cost of their progeny to synthetic insecticides, populations of H. zea were collected from MS and AR from Bt and non-Bt corn. Diet-incorporated bioassays were used to examine the susceptibilities of these insect populations to a diamide insecticide, which is commonly used for H. zea control on cotton. The response of populations collected from either Bt or non-Bt corn were similar. For Bt corn, the estimated insecticide concentration that controls half of the pest population (LC50 value) ranged from 17.7ng – 43.9 ng/ml of diet, while the estimated concentration for those collected from non-Bt corn ranged from 16.9ng – 39.3 ng/ml of diet.
Accomplishments
1. Temporal genetic variation in migratory bollworm populations. Genetic analyses of bollworm adults collected from two peripheral populations in Pennsylvania in 2002, 2005, 2016 with a 96 single nucleotide polymorphism (SNP) marker panel indicated high genetic diversity and low levels of genetic divergence in populations collected in 2018. ARS researchers in Stoneville, Mississippi, discovered 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. This drift may indicate either seasonal changes in migratory bollworm populations or beginning of a genetic drift in bollworm due to high selection pressure from Bt crops. The results were published in the journal Insects.
2. The bollworm is a major target of the vegetative-produced insecticidal protein Vip3Aa used in pyramided Bt corn and cotton in the U.S. Given the reduction in efficacy reported for the Cry proteins in cotton, the frequency of Vip3Aa resistance alleles in bollworm was evaluated and published. ARS researchers in Stoneville, Mississippi, exposed Bollworm isofamilies to F2 screens to estimate the resistance allele frequency in field populations in Texas. This study is the first to document a major resistance allele conferring high levels of Vip3Aa resistance in a field-derived strain of bollworm in the U.S. Data generated from this study will contribute to development of management strategies for the sustainable use of the Vip3Aa technology to control bollworm in the U.S.
Review Publications
Zhao, Z., Reddy, G.V., Chen, L., Qin, Y., Li, Z. 2020. The synergy between climate change and transportation activities drives the propagation of an invasive fruit fly in California. Journal of Pest Science. 93:615-625. https://doi.org/10.1007/s10340-019-01183-9.
Kuriwada, T., Kawasaki, R., Kuwano, A., Reddy, G.V. 2020. Mate choice behavior of female field crickets is not affected by exposure to heterospecific calling songs. Environmental Entomology. 49(3):561-565. https://doi.org/10.1093/ee/nvaa034.
Wang, L., Gao, F., Reddy, G.V., Zhao, Z. 2020. Optimization of nitrogen fertilizer application enhances biocontrol function and net income. Journal of Economic Entomology. https://doi.org/10.1093/jee/toaa112.
He, J., Reddy, G.V., Liu, M., Shi, P. 2020. A general formula for calculating surface area of the similarly shaped leaves: Evidence from six Magnoliaceae species. Global Ecology and Conservation. https://doi.org/10.1016/j.gecco.2020.e01129.
Allen, K.C., Luttrell, R.G., Little, N., Parys, K.A., Perera, O.P. 2019. Response of Bt and non-Bt cottons to high infestations of bollworm (Helicoverpa zea Boddie) and tobacco budworm (Heliothis virescens (F.)) under sprayed and unsprayed conditions. Agronomy. 9(11):759. https://doi.org/10.3390/agronomy9110759.
Yang, F., Santiago Gonzalez, J., Little, N., Reisig, D., Payne, G., Dos Santos, R., Jurat-Fuentes, J., Kurtz, R., Kerns, D. 2020. First documentation of major Vip3Aa resistance alleles in field populations of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Texas, USA. Scientific Reports. 10(1):5867. https://doi.org/10.1038/s41598-020-62748-8.
Perera, O.P., Fescemyer, H.W., Fleischer, S.J., Abel, C.A. 2020. Temporal variation in genetic composition of migratory Helicoverpa zea in peripheral populations. Insects. 2020(11):463. https://doi.org/10.3390/insects11080463.
