Project : USDA ARS

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Research Project: Managing Insects in the Corn Agro-Ecosystem

Location: Corn Insects and Crop Genetics Research

2020 Annual Report

Objectives
Objective 1: Improve knowledge of the ecology, genetics, and behavior of key corn pests, especially corn rootworm and lepidopteran species, such as European corn borer, corn earworm and western bean cutworm, in relation to pest abundance and insect resistance to transgenic corn. Sub-objective 1.A. Correlate genetic markers with phenotypic traits of interest in European corn borer, western corn rootworm and western bean cutworm. Sub-objective 1.B. Determine how larval movement and adult dispersal influence insect resistance to transgenic corn. Objective 2: For corn agro-ecosystems, determine potential impacts of changing farming practices on the demographics and ecology of pest and non-pest arthropods, such as the monarch butterfly. Sub-objective 2.A. Assess the potential value of neonicotinoid insecticide seed treatments to growers of major row crops under different agronomic conditions. Sub-objective 2.B. Develop strategies for improving monarch butterfly habitat in modern farm landscapes. Objective 3: Characterize genetic and biochemical responses associated with corn defenses to rootworm and lepidopteran caterpillar pest injury to enhance conventional and transgenic crop protection strategies. Sub-objective 3.A. Develop genetic markers and genomic tools for western corn rootworm, European corn borer, western bean cutworm, and other pests of corn. Sub-objective 3.B. Characterize genetic regulation of surface lipids on corn silks and assess protective capacity of these lipids on corn earworm feeding.

Approach
Field-resistance in western corn rootworm (WCR) to Cry3Bb1 Bt toxins and in European corn borer (ECB) to Cry1Fa and Cry1Ab toxins will be mapped by using genotyping-by-sequencing (GBS) protocols and single nucleotide polymorphism (SNP) markers. Pedigrees will be constructed from Bt resistant and susceptible individuals of both species. GBS will be performed by constructing genomic DNA libraries from non-size-selected fragments, to which adapters with a unique barcode are ligated. A population mapping approach using a high density of SNP markers will be taken to identify loci that differ significantly between E- and Z-race ECB. The premise of population mapping is similar to quantitative trait loci (QTL) mapping, but SNPs with skewed frequencies between races are assumed to result from either selection for race-specific adaptations or genetic drift facilitated by restricted gene flow. GBS-derived SNP markers will be used to estimate gene flow among WCR populations. The strategy will be to estimate Wright's neighborhood area for WCR, the radius of which constitutes a measure of the typical distance genes move per generation. Dispersal and survival rates of fall armyworm (FAW) will be measured in field plot trials. FAW egg masses will be placed in the whorl of vegetative-stage corn plants surrounded by uninfested plants. Living and dead larvae on plants will be counted periodically using destructive sampling. Planting arrangements will include various combinations of Bt and non-Bt isoline plants. Flight behavior and capacity of WCR will be characterized and compared across three Cry3Bb1-resistant and two susceptible strains using flight mills to determine the degree to which resistance affects dispersal. In collaboration with other ARS laboratories and as a NP304 effort, a comprehensive review of the literature will be conducted to provide information on the usefulness of seed treatment with neonicotinoid insecticide in controlling target pests and protecting crops from yield loss. A series of studies will be conducted related to milkweed species selection (larval performance and oviposition preference), and milkweed plant establishment (determining patch sizes and maximizing sustainability of milkweeds). Initially, studies will focus on four Asclepias species found in Iowa: common milkweed, A. syriaca, swamp milkweed, A. incarnata, butterfly weed, A. tuberosa, and whorled milkweed, A. verticillata. Also, studies will be conducted to determine how to maintain these plants in the landscape while avoiding loss from plant competition. This will require identifying the best companion plants for the targeted milkweed species. Two sources of corn germplasm will be used to screen for silk activity against corn earworm (CEW). These corn lines and CEW resistant checks will be grown in the field. Emerged silks will be harvested, immediately frozen in liquid nitrogen, lyophilized, and ground to a powder using a knife mill. One cohort of powdered silks will be used for surface lipid metabolic analysis, while another will be used for CEW bioassays, allowing the metabolomic analysis and CEW feeding studies to be conducted in parallel.

Progress Report
This is the final report for the project 5030-22000-018-00D, which terminates September 2020. Several important results were generated over the past five years. The most important pest of corn is the western corn rootworm (WCR), usually managed by genetically engineered (GE) corn expressing Bacillus thuringiensis (Bt) insecticidal proteins. However, resistance to Bt corn has developed in many locations, threatening major economic losses. Knowledge of insect dispersal is critical to models of resistance evolution, spread, and mitigation strategies, but major knowledge gaps exist in understanding variation in adult WCR dispersal behavior. ARS researchers at Ames, Iowa, in collaboration with Iowa State University (ISU) and Mississippi State University (MSU) conducted studies [funded in part by Monsanto Company and the National Science Foundation (NSF) sponsored Center for Arthropod Management Technology at ISU and University of Kentucky] to address these gaps. Estimates of gene exchange, and therefore dispersal rates, between locations indicated average lifetime dispersal of 0.2 km for about 85% of a WCR population, but the remaining 15% disperse farther with some dispersing at least 275 km. Laboratory flight experiments demonstrated that crowded larval conditions lead to increased female flight activity. This implies females emerging from high density Bt-resistant populations in Bt-cornfields disperse farther than those from low density Bt-susceptible populations. Knowledge of different dispersal rates will improve models used by regulatory agencies, industry, and public-sector scientists and resistance management strategies for WCR. Studies [funded in part by USDA National Institute of Food and Agriculture (NIFA)] by ARS researchers at Ames, Iowa, and Brookings, South Dakota, in collaboration with North Carolina State University, University of Nebraska, University of Illinois, University of Maryland, and ISU elucidated genomic mechanisms underlying Bt resistance in WCR, important for predicting and avoiding resistance development. Disabling genes thought to contribute to Bt resistance showed seven of eight play critical roles in survival of larvae, pupae, and adults, and that two partially controlled egg production. Discovery of the genome region responsible for resistance to an older chemical insecticide is a major step in identifying the genetic mechanism involved. These advances for both Bt and conventional insecticide resistance mechanisms are important for development of gene-silencing and gene editing technologies for pest control. European corn borer (ECB) is a major corn pest currently controlled in North America with Bt corn, but ECB resistance to Bt corn was recently reported in Canada and is of great concern in the United States. Strains of ECB are defined by different pheromone (sex-attractant) communication systems and different numbers of generations per year (voltinism). Although individuals are most likely to mate with others of their own strain, they can inter-mate if co-located. Gene exchange between strains and locally adapted populations impacts how fast and how far Bt resistance spreads once it develops. The pheromone strains differ ecologically in host plant preferences and possibly adult dispersal behavior, which could discourage inter-strain mating. ARS researchers in Ames, Iowa, in collaboration with University of Massachusetts-Dartmouth, Tufts University, ISU, Jilin Academy of Agricultural Sciences (JAAS), and Pennsylvania State University, examined genome variation between pheromone strains and found differences only in regions containing the genes responsible for pheromone production (females) and pheromone response (males). This information is important for modeling ECB population dynamics and gene flow between pheromone strains in different areas, and thus the rate of spread of adaptations like Bt resistance across a landscape or region. In a series of NSF-supported studies, ARS researchers in Ames, Iowa, in collaboration with Tufts University, Cornell University, and Montana State University, identified genes that determine voltinism, which will be crucial for future development of genetic markers to differentiate voltinism strains and estimating levels of gene exchange among field populations. ARS researchers in Ames, Iowa, in collaboration with ISU and Chinese collaborators from JAAS and Xinjiang Academies of Agricultural Sciences, examined how ECB genes entered the genome of the related Asian corn borer (ACB) through hybridization as ACB invaded a region hosting only ECB. This research determined the potential for the spread of ecologically adaptive and resistance traits which impact the efficacy of insect resistance management programs. Together, information from these studies also can be used to predict how moth pests in general may adapt to changing climates. Understanding the effects of pest management tactics on beneficial and other non-target organisms is important. ARS researchers in collaboration with the International Life Sciences Institute, DuPont Pioneer, Agroscope (Switzerland), and the Biotechnology Directorate of Argentina reviewed studies using surrogate non-target species to test effects of pesticides and insecticidal proteins in GE crops. The results show that current surrogate species have worked well to predict effects of existing GE crops, and should work well to predict effects of future GE crops based on similar technologies. A series of studies (funded in part by USDA NIFA) by ARS researchers in Ames, Iowa, in collaboration with Cornell University and Agroscope addressed the evolution of pest resistance to Bt plants and the effects of Bt plants on important natural enemies that suppress moth pest populations. The studies indicate that important natural enemies are not harmed by the commonly used insecticidal proteins in Bt plants and that these natural enemies can, in turn, reduce the evolution of resistance in the pest species. A good illustration of this principle is a study by ARS researchers in Ames, Iowa, in collaboration with the Chinese Academy of Agricultural Sciences, which showed a fly parasite provides more effective biocontrol of the oriental armyworm (a sister species of the armyworm found in the United States) on Bt corn than on non-Bt corn. Alone, the parasite and the Bt corn provide only imperfect control of the pest. This finding is important in demonstrating the safety and cooperative potential of traditional biocontrol organisms and transgenic Bt crops in controlling a difficult pest insect. RNA interference (RNAi), an innovative pest management technology, is initiated by double-stranded RNAs (dsRNA) ingested by the target pest and disable the molecular target. A study (funded in part by Monsanto Company) by ARS researchers in Ames, Iowa, in collaboration with ISU determined the environmental fate of a representative dsRNA in three aquatic microcosms. dsRNA doesn’t persist in aquatic environments and its use in agricultural settings should have little environmental impact on aquatic organisms. This information will be used by scientists worldwide to improve management of WCR and related beetle pest species, and by regulatory agencies tasked with assessing the environmental risks of GE corn to non-target and beneficial insects. Most corn planted in the United States is protected from several sporadic early-season pests by neonicotinoid insecticides applied to seed. However, these insecticides are usually used without knowledge of the threat posed by the pests in a given field, and there is growing concern that their indiscriminant use has negative environmental impacts. ARS scientists in Ames, Iowa; Brookings, South Dakota; and Stoneville, Mississippi, reviewed the prevalence and risk factors associated with all these sporadic pests to help farmers and consultants better assess the value of preventative protection of seedling corn under local conditions, and provide regulators and others with a better understanding of the complex issues involved in decision making at the farm level. Monarch butterfly populations have significantly declined over the past two decades, due in part to the loss of milkweed in agricultural landscapes. Restoration efforts call for re-establishing milkweed populations in the important summer breeding areas in the Midwest. ARS researchers in Ames, Iowa, in collaboration with ISU examined the attractiveness of nine common milkweed species. The results indicated that while females do make choices, egg-laying preferences can in response to environmental conditions. Thus, conservationists should supplement plantings of multiple species—especially in habitat areas subject to variable climates or soil types. A second study revealed that milkweed patches containing at least two to four closely-spaced plants increases the likelihood that searching larvae will encounter enough biomass to support development. Movement behavior and biomass requirements are critical aspects of monarch larval biology that should be considered in habitat restoration and maintenance plans, survey designs and protocols, and population modeling. Corn earworm (CEW) is a major pest of corn causing yield and quality losses through herbivory and disease vectoring. New sources of native plant resistance are vital to protect corn hybrids from future economic losses to this pest. In research (co-funded by NSF) by ARS scientists in Ames, Iowa, in collaboration with ISU developed a method for measuring the effects and strength of plant resistance against CEW, and tested it on four corn populations derived from Piura 208 (CEW resistant) and GT119 (CEW susceptible). The results demonstrated the utility of the new testing method, while providing useful information about the Piura 208 derived resistance factor to enable corn breeders to incorporate the factor into elite germplasm to protect the crop from future damage by this pest.

Accomplishments
1. Identified genes determining differences in number of European corn borer generations. The European corn borer (ECB) is a major pest of corn, which most U.S. farmers control by planting corn varieties that produce an insecticidal protein with a gene introduced from the bacterium Bacillus thuringiensis (Bt). Populations of univoltine (one generation per year) and multivoltine (two or more generations per year) ECB occur together in many parts of the United States and are reproductively compatible, but adults of these voltinism types often do not mate freely with each other because they are short-lived and are not always present at the same time during the season. Development of resistance to Bt corn by ECB is a great concern, and levels of gene exchange between locally adapted voltinism types can impact how fast and how far Bt resistance spreads once it develops in a population of one of them. ARS researchers in Ames, Iowa, in collaboration with Tufts University and the University Massachusetts-Dartmouth (funded in part by National Science Foundation) have discovered the genes responsible for controlling the number of generations, the first time this has been accomplished for any species of moth. The results have opened the door to developing genetic markers to identify the voltinism type of any field-collected ECB, something that cannot be accomplished visually because the types look identical. This capability will be crucial in estimating rates of gene flow between the voltinism types in different areas, and thus the rate of spread of adaptations like Bt resistance across a landscape or region.

2. Pheromone system, not ecological adaptations, responsible for maintaining strain differences in European corn borer. European corn borer (ECB) is a major pest of corn, which most U.S. farmers control by planting corn varieties that produce an insecticidal protein from a gene introduced from the bacterium Bacillus thuringiensis (Bt). ECB females produce a volatile chemical called a pheromone to attract males for mating, and there are two strains of ECB, which use either a "Z" or "E" pheromone communication system. Although mating between individuals of different strains is possible in regions where both the Z and E strains co-occur, hybridization is partly curtailed by preference of males for females of their own pheromone type. However, the strains differ ecologically in other ways that might also discourage inter-strain mating, such as host plant preferences and possibly adult dispersal behavior. Development of resistance to Bt corn by ECB is a great concern, and levels of gene exchange between locally adapted pheromone strains can impact how fast and how far Bt resistance spreads once it develops in a population of one of the strains. ARS researchers in Ames, Iowa, in collaboration with the University Massachusetts-Dartmouth, Tufts University, Iowa State University, Jilin Academy of Agricultural Sciences, and Pennsylvania State University (partly funded by NSF and USDA National Research Initiative) examined variation in the genomes of both Z and E pheromone strains and concluded that only the genomic regions containing the genes responsible for pheromone production by females and pheromone response by males are involved in maintaining differences between the strains. This information is important for modeling ECB population dynamics and gene flow between pheromone strains in different areas, and thus the rate of spread of adaptations like Bt resistance across a landscape or region.

U.S. DEPARTMENT OF AGRICULTURE

Corn Insects and Crop Genetics Research: Ames, IA