This project will address the needs of soybean farmers who want to prevent current and future losses to insect pests. To prevent these losses we are conducting research and extension that focuses on the most important insect pests that farmers are facing- the soybean aphid and the soybean gall midge. The reason for focusing on these two is explained below.
The conventional approach to preventing yield loss from insects is using insecticides. Previous funding from the Iowa Soybean Association has revealed the repeated use of pyrethroids (e.g., lambda-cyhalothrin and bifenthrin) has resulted in resistance to them in the soybean aphid. Insecticide-resistant soybean aphids are found in southern Minnesota and northwestern Iowa.
Our prior ISA-funded research found four mutations in the voltage-gated sodium channel (vgsc) gene in aphids collected from southern Minnesota and northwestern Iowa that are resistant to pyrethroids. These mutations change the vgsc protein targeted by pyrethroids, reducing the insecticides ability to kill aphids. This resulted in genetic markers that detect aphids with the vgsc mutations. Using these markers, we found that 82.7% of field-collected aphids in Iowa were resistant in both homozygous and heterozygous forms.
Our prior ISA-funded research found four mutations in the voltage-gated sodium channel (vgsc) gene in aphids collected in southern Minnesota and northwestern Iowa that are resistant to pyrethroids (Valmorbida et al. 2022). These mutations change the vgsc protein targeted by pyrethroids, reducing the insecticides ability to kill aphids. This resulted in genetic markers that detect aphids with the vgsc mutations. Using these markers, we found that 82.7% of field-collected aphids in Iowa were resistant in both homozygous and heterozygous forms.
We have shared results from previous work in the following publications:
Hanson, A.A., J. Menger-Anderson, C. Silverstein, B.D. Potter, I.V. MacRae, E.W. Hodgson, and R.L. Koch. 2017. Evidence for soybean aphid (Hemiptera: Aphididae) resistance to pyrethroid insecticides in the upper midwestern United States. Journal of Economic Entomology. DOI: 10.1093/jee/tox235.
Menger, J., P. Beauzay, A. Chirumamilla, C. Dierks, J. Gavloski, P. Glogoza, K. Hamilton, E.W. Hodgson, J.J. Knodel, I.V. MacRae, D.T. Pezzini, B.D. Potter, A.J. Varenhorst, and R.L. Koch. 2020. Implementation of a diagnostic-concentration bioassay for detection of susceptibility to pyrethroids in soybean aphid (Hemiptera: Aphididae. Journal of Economic Entomology. DOI: 10.1093/jee/toz351.
Valmorbida, I., D. S. Muraro, E. W. Hodgson, and M. E. O’Neal. 2020. Soybean aphid (Hemiptera: Aphididae) response to lambda-cyhalothrin varies with its virulence status to aphid-resistant soybean. Pest Management Science. DOI. 10.1002/ps.5661.*
Valmorbida, I., J.D. Hohenstein, B.S. Coates, J.G. Bevilaqua, J. Menger, E.W. Hodgson, R.L. Koch, and M.E. O'Neal. 2022. Association of voltage-gated sodium channel mutations with field-evolved pyrethroid resistant phenotypes in soybean aphid and genetic markers for their detection. Scientific Reports 12, 12020. https://doi.org/10.1038/s41598-022-16366-1
Since pyrethroids can be less expensive than other insecticides, switching to a new insecticide will cost farmers more to protect yield. If farmers continue to use pyrethroids, there is the potential to lose 25-40% yield if an outbreak of insecticide-resistant aphids occurs.
In this project, we are exploring if silencing the mutations that allow the soybean aphid to be resistant can result in them being susceptible to insecticides again.
Our preliminary data shows RNA interference (RNAi) cures aphids of resistance, making them susceptible to pyrethroids again. Double stranded RNA (dsRNA) “silences” expression of a specific gene by degrading specific, the RNAi is a naturally-occurring mechanism that defends cells against viruses by degrading viral RNA. In the lab, RNAi in the form of designer dsRNA can be an insecticide to kill specific pests. We tested if applying dsRNA targeting one of four vgsc mutations (dsvgsc-h1) directly to aphids silenced the vgsc gene, and could selectively kill pyrethroid resistant aphids, by restoring pyrethroid susceptibility.
We determined that dsRNA sequences cause mortality to insecticide resistant aphids which independently target two of the four vgsc mutations. This caused high (nearly 40%) mortality when applied alone to resistant aphids at a rate of 0.5µg/µl. This mortality occurred without an insecticide and is 20-fold greater than when insecticide-resistant aphids are exposed to insecticide alone. Greater efficacy among resistant aphids is seen when these dsRNAs are applied with a pyrethroid; 2- and >30-fold increases compared to dsRNA alone and insecticide alone, respectively. This demonstrates that dsRNA sequences acted most synergistically with a pyrethroid at 0.29 ppm and capable of “rescuing” efficacy of pyrethroids against different resistant aphid genotypes.
By the completion of this project, we will determine the optimal concentration of dsRNAs and pyrethroid needed to cause mortality among resistant aphids, an optimal form of dsRNAs to kill a wide range of resistant mutants, and proof of concept tests to reveal if the combination of dsRNA and an insecticide can kill aphids when sprayed on a plant.
Insecticide use is likely to increase, as the frequency of outbreaks increases in the Midwest. Starting in 2018, several counties in western Iowa experienced yield losses due to a new soybean pest, the soybean gall midge.Larval feeding inside stems can lead to plant death. As of 2022, 44 Iowa counties have been confirmed, adding to the 155 counties in Iowa, Minnesota, Missouri, Nebraska, and South Dakota, with a range of infestation severity. Yield losses observed in commercial fields during 2019 and 2020 ranged from 1-50%, where dead plants were found mostly at field edges. Unlike the soybean aphid, little is known about the biology and how best to managing the gall midge. The recent loss of chlorpyrifos has increased farmers dependence on a few groups of insecticides. With fewer options, there is an increasing need to prevent the occurrence and spread of resistance. This proposal aims to share new management practices to minimize gall midge injury and to address resistance not only in the soybean aphid but potentially all insects that attack crops.

OBJECTIVE 1: Soybean insecticide field efficacy evaluations
Co-PI Erin Hodgson’s lab runs the nation’s largest soybean aphid efficacy evaluation trials. They have access to new products or improved formulations before they are commercially available. The program goes beyond foliar insecticides (host plant resistance) and for other soybean pests (e.g., Japanese beetle, bean leaf beetle, etc.). This research is novel in that it 1) uses replicated plots to compare seed- and foliar-applied insecticides, 2) collect intense data on soybean aphid seasonal exposure, and 3) presents unbiased data for products from multiple companies. During the summers of 2023-2025, experimental plots at high-risk locations in Iowa will be used to evaluate insect management tactics including seed treatments and foliar insecticides. These tactics will be evaluated alone and in combination to determine optimum yield protection.
Soybean aphid efficacy. At one location, 20-30 treatments will be replicated in small plots. Treatments will include a range of insecticidal groups and application sites. Aphid population dynamics will be monitored weekly in addition to secondary pest activity (e.g., beetles, mites, stink bugs, caterpillars, etc.). Drs. O’Neal and Coates will collect aphids from this trail to determine the frequency of pyrethroid-resistant mutations before and after insecticides are applied. Foliar applications will be based on our established treatment threshold for soybean aphid. At the end of each season, yield will be collected and compared to cumulative aphid days for each treatment.

OBJECTIVE 2: Using RNAi to restore pyrethroid susceptibility among resistant soybean aphids
We will accomplish this objective by employing a graduate student who has helped develop the assays for testing dsRNAi on soybean aphids, Bruna Wojahn. This assay will be used to complete to large experiments with aphid colonies generated from clones with a defined genotype and phenotype based on their susceptibility to pyrethroids. These colonies are maintained at ISU, generated from soybean fields in Iowa and Minnesota. These aphids will be tested with dsRNAi probes generated by Dr. Brad Coates of the USDA’s Corn Genetics Laboratory, based on the genetics of the colonies with varying forms of pyrethroid resistance.
Obj. 2a. Determine the optimal dsRNA sequence and concentration for aphid mortality.
This objective will include two separate experiments. The first includes two treatments; a dsRNAi probe applied alone or with lambda-cyhalothrin (i.e. Warrior). These two treatments will be applied to aphids that are resistant to lambda-cyhalothrin. We will select a colony with the greatest amount of resistance. The concentration of the dsRNAi probe will vary across 8 increasing concentrations. The mortality of aphids will be measured over a 48 hour period.
We predict that as the concentration of the dsRNAi increases, so too will the mortality of aphids, until a threshold is reached. We also predict that the mortality of aphids will be greatest when the dsRNAi probe is coupled with the insecticide, resulting in the insecticide-resistant aphids dying. Again, this mortality will increase with increasing dsRNAi concentration.
The second experiment will use the optimal concentration determined in the first experiment and varying the form of the dsRNAi probe. We will create probes that match the mutation of the various colonies we have collected in Iowa. We predict that the better the match, the more aphid mortality. We will also include a mix of all of the probes. We anticipate the probe mixture will work as well as a matched probe. If this is proven true, we can streamline an approach to managing soybean aphids with dsRNAi for managing susceptible and insecticide-resistant aphids.
Obj. 2b. Assess effects of optimized insecticidal dsRNA on non-target species.
We will use the optimal concentration and dsRNAi probe on a different species of aphid and a lady beetle. This simple test will determine if this approach results in a species-specific approach to improving the performance of an insecticide.

OBJECTIVE 3: Extension communication strategies
Co-PI Erin Hodgson has a 70% field crops extension appointment and collaborator Ashley Dean has a 50% extension appointment at Iowa State University (ISU). Throughout the project, they will actively participate in extension programs that summarize efficacy data and promote sustainable soybean pest management, drawing directly from Obj. 1 and 2. Field day demonstrations and winter programs will be coordinated with ISU Extension and Outreach, Iowa Soybean Association, industry partners, and other organizations. Potential ISU Extension and Outreach programs include: Integrated Crop Management Conference, Crop Advantage Series, and field days with ISU. Participants will largely be farmers, but will also include crop consultants, agricultural professionals and other ISU personnel.

This project will generate data on insecticide performance for naturally occurring aphid outbreaks and share these findings in a Yellow Book (www.ent.iastate.edu/soybeanresearch/content/extension). As a way to deliver real-time updates on soybean pests in the summer, Dr Hodgson and her staff will continue posting informal articles to ICM Blog and contributing articles to ICM News.
At the completion of this project we anticipate having a novel method for managing both insecticide-susceptible and resistant soybean aphids. This will be developed for soybean aphids that have evolved resistance to pyrethroids.
We have a larger goal of demonstrating a novel method for addressing insecticide resistance in insects. Although the focus is on the soybean aphid, there is a larger issue of resistance for insecticides across multiple insects and several insecticides. There is the potential to conduct both pest management and insecticide resistance management simultaneously. Many insects become resistant to pyrethroids through mutations to the vgsc channels, suggesting that this method could be translated to other insects that have developed resistance, or could develop resistance in the future

Update:
OBJECTIVE 1: Soybean insecticide field efficacy evaluations
Erin Hodgson (Professor), Greg VanNostrand (Research Scientist), and Ashley Dean (Education Extension Specialist)

Soybean aphid. We established plots at the ISU Northwest Research Farm in 2023. In total, we evaluated 14 treatments with four insecticidal group/subgroups. The plots were initially colonized by soybean aphid in late June, and populations remained very low throughout the summer. There were a few other soybean pests present (e.g., spider mites, stink bugs, grasshoppers) but economic populations were not evident. Natural enemies, such as beetles, flies, lacewings, and wasps, were also present throughout the reproductive stages. The threshold was never reached, so plots were sprayed on 15 August when plants were at the R5 (beginning seed set) growth stage. Soybean aphid populations in the untreated control peaked in August with 60 aphids per plant. Since soybean aphid populations were well below the economic threshold, no yield losses were expected or observed. Average treatment yields ranged from 79.10 to 84.83 bushels per acre.
What does it mean for farmers: Soybean aphid is a sporadic pest, and populations are unpredictable
annually and field-to-field. Our recommendation for soybean aphid management in Iowa is to:
• Plant early if the field is in an area with persistent soybean aphid populations.
• Scout for soybean aphid, especially during R1–R5, and use a foliar insecticide if aphids exceed the economic threshold of 250 per plant.
• Use a product labeled for soybean aphid; there are some aphid populations that do not respond to pyrethroid insecticides anymore and considered resistant. If possible, select another insecticidal group for better yield protection.
• Evaluate foliar insecticide efficacy three days after application to ensure soybean aphid populations were sufficiently reduced.
• Understand that late-season accumulation of aphids (i.e., after R5) may not impact yield like it does in early reproductive growth; a foliar insecticide applied after seed set may not be an economically profitable choice.
Bean leaf beetle. We established plots at the ISU Johnson Research Farm in 2023. In total, we evaluated 31 treatments with at least six insecticidal group/subgroups. The plots were initially colonized by bean leaf beetle in June and populations remained active throughout the summer. There were a few other soybean pests present (e.g., spider mites, stink bugs, grasshoppers) but economic populations were not evident. Natural enemies, such as beetles, flies, lacewings, and wasps, were also present throughout the reproductive stages. We continued to sample through seed set where direct injury to pods was extensive in September. Most treatments had injury that exceeded 10%, which would be considered economically important.
What does it mean for farmers: Bean leaf beetle injury is increasing in intensity throughout Iowa. Direct injury to pods and seeds is. Our recommendation for bean leaf beetle aphid management in Iowa is to:
• Delay planting in fields with persistent populations, especially late-season injury.
• Continue to scout through seed set and use a foliar insecticide if pod injury exceeds 5-10%. Evaluate foliar insecticide efficacy three days after application to ensure soybean aphid populations were sufficiently reduced.
Soybean gall midge. We contributed to several regional research projects in 2023, including some commercial farm and ISU Research Farm locations. Summary of findings:
• Germplasm screening for host plant resistance remains the most hopeful suppression tactic. We screened 118 accession lines over three maturity groups (12 highly susceptible and 106 with suspected resistance from previous screenings) from University of Nebraska-Lincoln. Larvae are found in all the plots but at the suspected resistant lines have significantly reduced larval injury.
• Our lab identified six new counties with first detections (Hamilton, Hardin, Story, Marshall, Jasper, and Marion). They were located in the central part of Iowa, with now 49 counties now in total. See a current infestation map for the U.S. here: https://soybeangallmidge.org/soybean-gall-midge-distribution

OBJECTIVE 2: : Using RNAi to restore pyrethroid susceptibility among resistant soybean aphids

We have employed a PhD Student from the Federal University of Santa Maria as a hourly technician to conduct a series of laboratory-based experiments as part of this objective. From these experiments we have learned the optimal concentration for aphid mortality through a topical application of dsRNA. This experiment has revealed that when the dsRNA is delivered at this concentration with a sub-lethal dose of insecticide (lambda-cyhalothrin), ~90% of the soybean aphids died.

We are currently conducting an experiment to determine the optimal construction of dsRNA to kill all genotypes with mutations in their voltage-gated sodium channels. These mutations provide resistance to pyrethroids. We predict that a dsRNA probe that can silence multiple mutations will be the most efficacious. We anticipate this experiment will be completed in May.

Finally, we are preparing for an experiment that will employ the optimal dsRNA probe at a concentration that can kill 90% of the insecticide-resistant aphids when mixed with the insecticide. This experiment will be conducted in a spray chamber that mimics the application of insecticides in a field setting. Unlike previous experiments that involved applying dsRNA directly to aphids on small sections of soybean leaves, the spray chamber will apply treatments to aphid-infested plants. This experimetn will be completed by August.

We are unable to conduct any further experiments at this time as the cost of producing dsRNA is beyond what we had budgeted. A commercial source is available but is beyond our current resources to purchase.

OBJECTIVE 3: Extension communication strategies
Erin Hodgson (Professor) and Ashley Dean (Education Extension Specialist)
Summary: Our extension program is led by Dr. Erin Hodgson; however, many lab members also participate in programs throughout the year. Ashley Dean is also highly involved in presentations, publications, and social media. We are focused on delivering research-based information to a variety of clientele. Our group serves farmers, crop consultants, agricultural professionals and other university/extension personnel. Most of our programs are within the state of Iowa, but we travel throughout the Midwest to deliver content. Our program topics are mostly focused on promoting IPM tactics, such as host plant resistance, sampling, economic thresholds, crop rotation and buffer strips. We strive to provide sustainable recommendations so that farmers can reduce production costs but still protect yield. Although most of our extension is soybean insect-related, we are also involved with corn insect management, too.
Extension Outputs and Metrics for Soybean (January – April 2024):
Iowa Pest Alert Network: During our third year of this real-time messaging system, we have increased our subscribers to 371!
Meeting Proceedings: 1
Field Guides and Technical Reports: 4
Newsletter Articles, Blog Posts, etc.: 8
Lectures and Field Days: 6
Face-to-face contacts: 525
Erin Hodgson (@erinwhodgson) Twitter Followers: 3,331[167,000 impressions]
Ashley Dean (@ashleyn_dean) Twitter Followers: 457 [23,489 impressions]

Farmers continue to face damage from insect pests to soybeans. Complicating this issue is the frequent use of pesticides is resulting in pests becoming resistant to these tools. Keeping farmers informed of these changes and suggesting alternative active ingredients and pest management methods will help farmers avoid unnecessary loss of yield or increasing cost of production. Objectives 1 and 3 will both generate data and extension programing to inform farmers and agribusiness of these issues.
Benefits associated with Objective 2 will not be as immediate, but will address managing pests and their resistance to pesticides. Investments by Soybean Checkoff programs and the USDA in sequencing the genomes of insect pests, like the soybean aphid, pay off in revealing the specific forms these mutations take. By identifying these mutations, we can exploit them. We will learn if a novel use of RNAi technology can help specifically kill aphids resistant to pyrethroids. We will share this information with farmers and agribusiness, to preserve the utility of inexpensive insecticides that provide protection to multiple insect pests. By coupling RNAi with this pesticide, we can protect farmers from both the insecticide-susceptible and resistant aphids. By targeting the most common form of resistance (mutations in the vgsc gene), we can limit future increasing occurrence of resistance. And finally, if this works for soybean aphids, this can be a model for other insects that have developed resistance in this manner.