NC246: Ecology and Management of Arthropods in Corn

(Multistate Research Project)

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Over 80 million acres of field corn (Zea mays) and 600,000 acres of sweet corn, worth about $65 billion and $1 billion, respectively, are grown in the United States each year. The European corn borer (ECB) (Ostrinia nubilalis) and western corn rootworm (WCR) (Diabrotica virgifera virgifera) account for over $1 billion each in terms of total annual costs of control and yield loss. Rootworms are especially problematic due to their propensity to evolve resistance to crop rotation and other management tactics (Gray et al. 2009) including insecticides (Meinke et al. 1998; Pereira et al. 2015) and genetically-modified (GM) hybrids with transgenic expression of one or more Bacillus thuringiensis (Bt) toxins (Gassmann et al. 2011, 2014, 2016; Ludwick et al. 2017). Most recently, resistance to the Bt Cry1F protein was documented among ECB in Nova Scotia, Canada (Smith et al. 2019a).


Other insect pests also cause significant economic loss to U.S. corn producers. The corn earworm (CEW) (Helicoverpa zea) and fall armyworm (FAW) (Spodoptera frugiperda) cause significant foliage and ear feeding damage in the southeast (Buntin et al. 2004), even when Bt corn hybrids are used. Resistance to corn producing the Cry1F protein has been reported for FAW populations in Puerto Rico, Florida and North Carolina (Storer et al. 2010; Huang et al. 2014). Field evolved resistance to sweet corn producing the Cry1Ab protein was described for CEW in Maryland (Dively et al. 2016). The western bean cutworm (WBC) (Striacosta albicosta) is a perennial pest in the western Corn Belt (Archibald et al. 2017; Smith et al. 2019b), but due to range expansion is also now a serious corn pest in areas of the Great Lakes region of the United States and Canada (Smith et al. 2018), in part due to resistance to the Bt Cry1F toxin (Ostrem et al. 2016; Smith et al. 2017). The northern corn rootworm (NCR) (D. barberi) is a perennial pest in much of the Corn Belt, and has evolved resistance to multiple Bt toxins (Calles-Torrez et al. 2019). The southern corn rootworm (SCR) (D. undecimpunctata howardi) causes occasional significant economic damage in the southeast. Wireworms (Elateridae) and white grubs (Scarabaeidae) can cause serious local problems. The common prophylactic use of neonicotinoid seed treatments to control these soil pests is not sustainable due to inconsistent economic returns and increasing environmental concerns (Mullin et al. 2005; Krupke et al. 2012; Tooker et al. 2017).


The current NC246 committee was formed in 2016 by merging the former NC205 (Ecology and Management of European Corn Borer and Other Lepidopteran Pests of Corn) and NCCC46 (Development, Optimization and Delivery of Management Strategies for Corn Rootworms) committees. These parent committees have a long history of addressing critical pest issues. NC205 was established in 1953 to address issues relating to the control of ECB, but expanded to include a broader array of corn pests. NCCC46 was established in 1964 as WCR was spreading eastward from the Great Plains into the Central Corn Belt (Gray et al. 2009). These two committees began co-locating their meetings in the 1990’s and collaborations between the two became increasingly valuable with the rapid adoption of Bt transgenic corn, the stacking of Bt traits targeting both corn rootworm and lepidopteran pests in the same plant, and the discovery of WCR field resistance to Cry3Bb1 toxin (Gassmann et al. 2011). Due to these increasing interactions, NC205 and NCCC46 committees were merged following a majority vote by both and a joint proposal for renewal submitted in 2015. The newly combined NC246 project was initiated in 2016. The formal merging of the committees has been scientifically and logistically beneficial, allowing the committee to address the larger corn insect pest complex, encompassing both above- and below-ground pests.


Since commercialization of Bt-transgenic corn targeting ECB in 1996 and rootworms in 2004, Bt corn acreage in the U.S. has increased from 8% in 1997 to 83% in 2019 (USDA NASS 2019). This has sparked a fundamental change in the management of corn insect pests by stakeholders. Since then, seed companies continue to develop GM crops with transgenic expression of crop protectants. Most current commercial GM hybrids express multiple Bt toxins targeting both Lepidoptera and Coleoptera. Initially, the use of GM crops often eliminated the need to apply insecticides for pest control, improving insect management for growers (Rice 2004; Sappington 2014). However, the development of resistance by target pests threatens the sustainability of these transgenic technologies.


Investigate the ecology, biology, evolution, genetics, genomics, and behavior of corn arthropods.


Dispersal of adults and gene flow within a species are critical factors for modeling the development and spread of resistance to Bt corn, as well as for implementing and monitoring the efficacy of IRM strategies. Regardless, these factors remain poorly understood for most corn pest species. Adult dispersal is high within ECB (Showers et al. 2001). Gene flow was also shown to be high between ECB pheromone strains, except at loci under sexual selection for strain differentiation (Coates et al. 2019). Genes involved in diapause differences were identified (Kozak et al. 2019), and their allele frequencies associated with increasing number of generations across latitudes (Levy et al. 2015). An inversion on the Z chromosome housing genes determining diapause variation is at highest frequency in populations differing in both pheromone and voltinism traits (Kozak et al. 2017), wherein allelic combinations within the inversion were suggested to be “optimized” for local conditions (Coates et al. 2018). A genetic marker was developed to differentiate pheromone strains based on specific alleles at the female pheromone production locus, the pheromone gland fatty acyl-reductase gene (pgfar; Coates et al. 2013a), but analogous markers are needed to determine and estimate gene flow between voltinism ecotypes. A region of the Z chromosome containing four genes was linked to differences in male pheromone response (Koutroumpa et al. 2016), but the specific gene needs to be determined and genetic markers developed to track this trait in field populations.


There is a high beetle dispersal rate in WCR (Spencer et al. 2009), which is density dependent (Yu et al. 2019a). Correspondingly, no genetic differentiation has been detected within WCR populations (Kim and Sappington 2005; Coates et al. 2009). A recent study corroborated the homogenization of WCR across the Corn Belt to the Atlantic coast, but also determined the ancestral source of the United States population was located in Mexico (Lombaert et al. 2018). Within this genetically homogenized United States population, no variation was predicted between WCR soybean variants compared to non-variants (Miller et al. 2006). Analogously, high levels of gene flow were reported between NCR populations with 1-year compared to 2-year extended diapause (Krafsur 1995). It remains a challenge to detect variation among variant phenotypes in corn rootworm populations. Although a relatively large number of SNP markers were developed for WCR (Coates et al. 2016), there still is a need to apply these or other markers within population genetic analyses or genome-wide association studies. Spatial and temporal aspects of adult movement have proven difficult to characterize. Additional population genetics studies and the development of molecular genetic tools are critically needed to more accurately estimate dispersal distances of corn pest insects.


The high dose refuge strategy has been implemented within IRM plans to delay the onset of Bt resistance among WCR and ECB, wherein non-Bt refuges were planted in a structured block design. This scenario changed in 2010 when regulatory decisions allowed the use of blended refuges consisting of either 5 or 10% of non-Bt seed to be planted in the Corn Belt, referred to as “refuge in a bag” (RIB) products. RIB had the advantage of facilitating grower refuge compliance. Structured refugia did not facilitate the expected movement of mate-seeking adults from refuge and Bt corn blocks; WCR abundance was distributed uniformly across fields planted with RIB products (Hughson and Spencer 2015). Mating between WCR from refuge and Bt corn was more likely in seed blends than in block refuges; however, the low abundance of refuge-produced WCR in seed blends likely limits their contribution to delaying resistance (Taylor and Krupke 2018). There has been a relatively recent evolution of Bt resistance among several ear-feeding lepidopteran pest species including CEW, FAW, and WBC (Tabashnik and Carrière, 2017), but the cause of some of these events remains uncertain. Resistance in FAW seems to be linked to alterations in receptor genes for the Cry1F and Cry1Ab proteins, and current evidence suggests the frequency of these resistance alleles may be high in some locations (Banerjee et al. 2017). Larvae may be exposed to varying levels of one or more Bt toxin when feeding on kernels of cross-pollinated refuge plants, which is exacerbated via intra-plant movement (Mallet and Porter 1992; Tabashnik 1994). Field studies show higher larval densities on non-Bt refuge corn plants within structured compared to blended refuges for ECB (Oyediran et al. 2016) and CEW (Burkness et al. 2015), but not for the sugarcane borer (Wangila et al. 2013). Larval behaviors influencing differences in exposure and survival observed between species remain unknown, but research is needed to investigate timing of larval movements, tissues fed upon, and Bt toxin exposures across instars. Moreover, while exposure to sublethal levels of Bt proteins in lepidopteran larvae alters their migratory behaviour as adults (Jiang et al. 2013), there is no available data on this process or how it may affect resistance management tactics in CEW, FAW or ECB populations. Understanding these factors is crucial for evaluating the efficacy of current IRM strategies and product durability for ear-feeding pest species of Lepidoptera.


Genomic tools have been developed and applied. For instance, WCR, ECB and FAW genome and transcriptome sequence assemblies have been produced through international and multi-institutional efforts involving several NC246 members. Application of these genomic resources will facilitate probing the genetic basis of many important pest traits, including resistance to insecticides and Bt toxins (Coates 2016; Banerjee et al. 2017; Abdelgaffar et al. 2019), behaviors relevant to pest status, and insect-plant interactions. To date, applications of large numbers of genetic markers have resulted in identification of genes or genomic regions controlling organophosphate resistance in WCR (Coates et al. 2016) and Cry1F resistance in ECB (Coates and Siegfried 2015). Differential gene expression studies suggest candidate genes involved in Bt resistance in WCR (Rault et al. 2018) and ECB (Vellichirammal et al. 2015; Yao et al. 2017), WCR rotation resistance (Chu et al. 2015), and insecticide resistance (Wang et al. 2013; Coates et al. 2016). Transcriptome profiling has allowed the identification of field-evolved alleles for resistance to Cry1F and Cry1Ab in FAW (Banerjee et al. 2017). The effect of these resistance alleles on the metabolism of FAW larvae have been studied to identify resistance markers (Abdelgaffar et al. 2019). Additionally, genomic research has led to the identification of genes controlling differences in diapause duration between univoltine and bivoltine ECB (Kozak et al. 2019), and estimation of gene flow among ECB populations (Coates et al. 2019).


Develop and assess Integrated Pest Management systems for the arthropod complex in corn. Promotion of IPM-based approaches has reached a critical cross-road, as resistance to some toxins has led to recommendations from various quarters to layer multiple control tactics on Bt corn, including conventional insecticides, without assessing actual need. Since 2004, the percentage of U.S. corn acreage with seed-applied neonicotinoids has increased dramatically, and they are now used on virtually all non-organic corn acres (Douglas and Tooker 2015; Tooker et al. 2017; Papiernik et al. 2018). Changes in pest insect biology, pest management technologies, and regulatory issues have illustrated the fact that much of our past information and research about the corn cropping system must be re-evaluated. However, across most of the Corn Belt the distributions of targeted early-season pest species are sporadic and patchy, and most crop fields do not experience annual economic infestations. Furthermore, neonicotinoids have potential roles in pollinator declines and contamination of surface waters (Goulson 2013; van der Sluijs et al. 2014). Seed-applied active ingredients can persist in soil for up to two years and move into aquatic systems where they can reduce aquatic invertebrate communities (Laurent and Rathahao 2003; Krupke et al. 2012; Van Dijk et al. 2013). A recent Canadian study revealed substantial ground water contamination with thiamethoxam (Main et al. 2014). Clearly, an evaluation of the effects of seed treatments upon a range of non-target insects in and around cornfields is warranted.


Characterize and monitor pest resistance and assess Insect Resistance Management and mitigation strategies. More information is needed regarding sensitivity to Bt, and the influence of host plants. Furthermore, the impact of Bt corn on the ecology and behavior of lepidopteran target pests, such as CEW, FAW, and WBC is important, especially in relation to Bt resistance of these pests and the worldwide range expansion of FAW. The Committee has an interest in the use of Bt and other insect control tactics in an economically viable and sustainable manner that will prevent or delay the onset of insect resistance. Ensuring long-term durability of these tools requires IRM strategies that are compatible with production practices.


Monitoring for resistance among pest insects targeted by GM corn-incorporated insecticidal toxins remains a United States Environmental Protection Agency (US EPA)-mandated component of IRM plans (Siegfried and Spencer, 2012). Being able to detect insecticide resistance is necessary to determine if control failures occur because of resistant insects or other factors, determine the extent of the resistance, and design resistance mitigation programs. This is particularly important for WCR field populations, which have confirmed resistance to, and cross-resistance among Cry3Bb1, eCry3A, mCry3A (Gassmann et al. 2011, 2014), as well as resistance to Cry34/35Ab1 (Gassmann et al. 2016, Ludwick et al. 2017). Analogous resistance has been shown in populations of NCR (Calles-Torrez et al. 2019). Recently, field-evolved resistance to Cry1F for WBC (Smith et al. 2017) and FAW (Storer et al. 2010; Huang et al. 2014), as well as field-evolved resistance in CEW to Cry1A.105 and Cry2Ab2 in sweet corn (Dively et al. 2016) and field corn (Bilbo et al. 2019), have been reported. Although no field failures are documented, alleles conferring FAW resistance to Vip3A corn were identified in FAW field populations (Yang et al. 2019). Identification of resistance alleles to Cry1F in FAW has allowed preliminary efforts at developing DNA-based resistance monitoring tools with increased sensitivity and amenability to high throughput (Banerjee et al. 2017). This research is opening the door to the development of more sensitive, cheaper and high throughput resistance monitoring methods. However, to realize this vision, there is an urgent need to characterize the mechanism(s) of resistance among corn pests, and develop and deliver IPM recommendations utilizing multiple pest control strategies to mitigate the development of resistance. There is also need to assess the geographic distribution of Bt susceptibilies, as well as a proactive plan to address the risk of resistance to Vip3A and other insecticides. As additional pests become primary targets of transgenic technology, monitoring methodologies and programs will have to accommodate these new target pests.


Damage by ear-feeding Lepidoptera has increased during the last decade partly due to a range expansion of the WBC (e.g., Archibald et al. 2017; Smith et al. 2019b), and the development of resistance to Bt hybrids among CEW, FAW, and WBC that is associated with increased aflatoxin levels in the grain. The corn pest complex includes several Lepidoptera, but also coleopteran and hemipteran pests, which presents significant management and research complications. Basic issues such as accurate quantification of injury and the effects of sequential feeding require characterization, as well as the broader issues relating to IPM and IRM.


Employ traditional and innovative delivery methods to disseminate information. This includes information and educational materials related to sustainable management of corn arthropod pests delivered to farmers, industry, academic colleagues, agricultural professionals, educators, and policy makers. Packaging unbiased results for agricultural and public sector stakeholders remains critical. Although traditional methods for information delivery are still needed, the use of timely, interactive and mobile accessible digital formats is in increased demand.


A Multi-State Approach. Collectively, a multi-state approach to researching the knowledge gaps described above, developing IPM tools and programs, assessing IRM strategies, and implementing technology transfer is appropriate and necessary. Geography plays an important role in how pests interact with other organisms in their environment, and how IPM and IRM strategies are designed and employed. This significant spatial effect of population and community dynamics makes a regional project necessary. Lack of knowledge has led to fears by the general public about the potential environmental and health risks associated with adoption of new technologies. Controversy about the effect of GM and seed treatment technology on non-target organisms and human health has fueled public concerns. These fears have the potential of forcing legislation to ban or slow the introduction of these technologies. Answers to questions regarding new technologies should help focus the public's perception of them and allow growers to gain the pest control advantages provided by future technologies. Timely, directed, and relevant national research efforts that span multiple climates and cropping systems is facilitated through coordination of the NC246 multistate research committee.


The proposed multi-state plans will continue to be a model for the development of science-based resistance management programs and risk assessment for other pests, other crops, and future crop protection technologies. Our efforts will provide fundamental advances in the knowledge of pest ecology, genetics, behavior, and evolution. Our work will continue to provide science-based assessments essential to the policy decision-making process, which should help to increase the public's acceptance of environmentally friendly technologies while identifying potential negative impacts that need further investigation. Our work also will continue to lead to more sustainable pest management systems for corn pest complexes. There is ample evidence that the NC246 research group has the skills, collaborative working relationships, and commitment to provide the missing biological information and to incorporate this new information into evolving IPM programs and IRM models.

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