Pyrethroid-resistant soybean aphids exist in Iowa and are so common that one of the most popular products (Warrior II) may be ineffective in the near future. If farmers continue to use pyrethroids in fields with resistant aphids, this bad situation could be made worse.
There are several reasons why farmers may not switch to alternative chemistries. First, the alternatives are more expensive. Second, some alternatives like those that include chlorpyrifos are restricted use products. Finally, there is genetic diversity among soybean aphids found in Iowa.
Even though we can find aphids with mutations, not all mutant aphids are sufficiently resistant to pyrethroids to cause a field failure. Depending upon the frequency of resistant aphids and their genotypes, farmers can still treat the low-resistant aphids (like those found in Nashua, IA) to provide adequate protection and protect profitability. But our modelling efforts indicate that when the occurrence of pyrethroid resistance is greater than 49%, a single pyrethroid application offered less revenue than no management (Dean et al. 2020).
The challenge for farmers is to understand where and when there are enough mutant aphids to require a different insecticide. Our proposed work will help farmers determine if they can continue to apply the cheapest product, or if they need to consider newer and often more expensive insecticides. Successfully managing aphids can save farmers hundreds of dollars per acre.
We want to build upon previously-funded soybean check-off sequencing of the soybean aphid genome, to provide protection against this pest in light of its resistance to insecticides. This project will continue a collaboration between the Soybean Entomology Laboratory at ISU, and USDA-ARS Research Geneticist, Dr. Brad Coates. By identifying the genotypes that produce the resistant phenotypes that cause field failures, we can prevent field failures. In the long-term, this work will lead to develop of a ‘dipstick’ test to confirm the presence of mutant aphids in real time. A ‘dipstick’ test would give farmers in the field quick results without specialized equipment or training (see milestones and deliverables for an example).

The Soybean Entomology Laboratory at ISU is able to achieve this long-term goal by building on these findings:
• We discovered four mutations in the voltage-gated sodium channel (VGSC) of insecticide-resistant soybean aphids. Colleagues at the University of Minnesota have also discovered a subset of these mutations.
• These mutations confer resistance to pyrethroids (e.g., Warrior II), an insecticide commonly used by Iowa farmers (Valmorbida et al., in review).
With Dr. Brad Coates, we developed and tested markers for these mutations. These markers revealed the presence of aphid resistance before an application and its subsequent increase 2-3 days after the application


References

Dean, A.N., Niemi, J.B., Tyndall, J.C., Hodgson, E.W. and O'Neal, M.E. (2021), Developing a decision-making framework for insect pest management: a case study using Aphis glycines (Hemiptera: Aphididae). Pest Manag Sci, 77: 886-894. https://doi.org/10.1002/ps.6093

Paula, D.P., Lozano, R.E., Menger J., Andow, D.A., Koch, R.L., 2021. Identification of point mutations related to pyrethroid resistance in voltage-gated sodium channel genes in Aphis glycines. Entomologia Generalis. https://10.1127/entomologia/2021/1226

Valmorbida, I., Hohenstein, J.D., Coates, B.S., Bevilaqua, J.G., Merger, J., Hodgson, E.W., Koch, R.L., O’Neal, M.E., 2021. Association of knockdown resistance mutations in the voltage-gated sodium channel with field-evolved pyrethroid resistance of soybean aphid, Aphis glycines. Insect Biochemistry and Molecular Biology. In review.

Valmorbida, I., Coates, B.S., Hodgson, E.W., O’Neal, M.E., 2021. Reproductive performance of pyrethroid-resistant soybean aphids. Pest Management Science. In preparation.

In 2022, we will demonstrate to farmers the value of our molecular markers for finding pyrethroid-resistant soybean aphids. We will answer the following questions:

I. how common are mutant aphids in farmers’ fields;
II. are insecticides still protecting yield when resistance is present; and
III. when and how should farmers change their approach to pest management?

With funding from the ISA and collaboration with key members of the ISA Research Center for Farming Innovation (RCFI), we can answer these questions and more. We are working with Teressa Middleton and Scott Nelson on a pilot project this summer (2021) to test the method outlined in Objective 1 on a smaller number of fields. We will expand upon this method in 2022 per the objectives below.

Objective 1: How common are mutant aphids in farmers’ fields?
We need to survey more locations in Iowa to determine how common these mutants are, and which of the genotypes increase as farmers use insecticide.

Methods: We will select fields identified with the help of RCFI and ISU Extension-Agronomists. We have budgeted for sampling at a minimum of 20 fields selected by participating farmers, but we will adjust if more express interest. Aphids typically arrive and establish in soybean fields in July. We will work with farmers to help us confirm which field is infested with aphids. We will travel to fields to collect a pre-sprayed population assessment and collect 20 randomly sampled individuals for testing at ISU. We will return to these fields after they have been sprayed, estimating the populations and collecting another 20 individuals. If the farmers participate in the second objective and leave an untreated check strip, we will also estimate the population and collect aphids from there as well.


Objective 2: Are insecticides still protecting yield when resistance is present?
We will conduct this objective with a subset of farmers to determine if they are losing yield to pyrethroid-resistant soybean aphids.

Methods: We will ask if cooperating farmer would leave a “check strip”, a portion of the field left untreated with insecticide. The check strip allows us to confirm the value of the insecticide by comparing yield at the end of the season. We are aware that farmers may be reluctant to participate in this objective, so in the spring of 2022, we will work with Teresa and Scott of the RCFI to identify these farmers. We will discuss the best location and configuration of these check strips. We will work with these farmers per the first objective, but collect even more data from their fields. After the insecticide is applied in these fields, we will return to them to estimate the population and occurrence of resistance in both the treated portion and the check strips. We will work with the farmers to collect yield data, compile these data from all participating farmer fields, and summarize the yield difference between treated and untreated soybeans.

Objective 3: When and how should farmers change their approach to pest management?
Pyrethroids are not the only insecticide farmers can use to manage soybean aphids. Although lambda-cyhalothrin (i.e. Warrior) is a popular pyrethroid, there are other modes of action available. These insecticides represent unique chemistries as defined by the Insecticide Resistance Action Committee (IRAC), as they kill insects in a unique way. Dr. Hodgson’s insecticide evaluation program has tested these in the field (Table 2), but not against aphids that are resistant to pyrethroids. We will test our field-collected, pyrethroid-resistant colonies in Table 1 if they are resistant to these insecticides.

Table 2. Insecticides to be tested in bioassays to determine cross-resistance patterns
Product name Active ingredient Notes – IRAC put pyrethroids in group 3
Sefina afidopyropen IRAC group 9D
Transform sulfoxaflor IRAC group 4C
Pyrifluquinazon* pyrifluquinazon IRAC group 9B (not yet registered on soybean yet)

Methods: Each insecticide listed in Table 2 will be tested in replicated bioassays conducted at the Soybean Entomology Laboratory at the ATRB laboratory on the campus of ISU. These test will include soybean aphids resistant to pyrethroids and one susceptible to pyrethroids, see Table 1. We will use new mutants if we discover them in objective 1 and 2.

Our ultimate goal is to develop the science for an in-field test that farmers could use without timely lab-based assays. This type of test is available for farmers to test if Roundup Ready plants are expressing the genes that provide protection against that herbicide (Figure 3). This type of test allows farmers to get results in real-time, to make decisions in the field. Through this project, we can get closer to just such a commercial product.

Specific things learned through each objective.

Objective 2:
What will we learn:
I. Does the field have pyrethroid-resistant aphids? This will be confirmed with the first visit (prior to foliar insecticide application).
II. Did the insecticide increase the frequency of any of the mutations (resistant aphids)? This will be measured with the second visit (after the foliar insecticide application).
III. Is the insecticide application making a bad situation worse? This will be assessed by comparing the proportion of resistant aphids between first and second visits.
These data will be shared with the farmer, ISU Extension and crop scouts so that they can make informed decisions about which insecticides to use during the field season and into the future.

Objective 3:
What will we learn:
I. Which alternatives to pyrethroids protect farmers from resistant soybean aphids?
II. Do aphids have cross-resistance to other insecticides? We will determine if these three insecticides that can be used to manage outbreaks.

Update:
We prepared a one-page summary of samples we collected in 2021. Check-off funding allowed us to process these samples and share them with ISA members that gave us access to farmers during 2021. We are building a list of collaborating farmers for 2022 based on these data.

View uploaded report

Update:
We were able to complete aspects of Objective 1 and 2. This is due in part to a change in personnel and very low populations of soybean aphids. This project began with a PhD student, Ivair Valmorbida, who completed his degree and took a job with ADAMA US (a crop protection company). After he left, Dr. O’Neal recruited a new MS student to conduct this work with a focus on the first two objectives.

What our results mean for farmers-
The low populations of aphids suggest that there was a limited risk to farmers from this pest in 2023 for the region we studied (northern third of Iowa). However, we observed some disturbing evidence that soybean aphids were not affected by an insecticide spray. Our observations suggest aphid populations continued to grow after a foliage application of an insecticide. Although this increase occurred, the populations were very low to begin with and did not rise above the economic threshold. Going forward, if farmers continue to spray insecticides preventatively, there is the risk of the frequency of insecticide-resistant aphids spreading despite their low populations.

We will share these data in podcasts, newsletters and other extension materials during the 2023 growing season.

Update:
We surveyed commercial soybean fields in the northern third of Iowa and found strong evidence of insecticide resistant aphids. This evidence included populations spreading and increasing beyond the what was measured before an insecticide was sprayed. This is remarkable because insecticides have performed very well, reducing populations and preventing yield loss after they are applied. We are using molecular markers to determine if the aphids collected both before and after the use of an insecticide have mutations associated with insecticide resistance. We anticipate that the frequency of these mutations will increase after the application. This is very worrisome as this will increase the likelihood of these mutations spreading.

We also learned that despite the increase in aphid spread and population after the insecticides were applied, the populations were very low, at least an order of magnitude below what is considered a threshold to spray insecticide. This is somewhat comforting, but highlights a larger issue. Farmers may be using insecticide unnecessarily, applying them when they are not needed to prevent yield loss from soybean aphids. By using insecticides in this way, they are eroding the value of insecticides. Even though the populations are too low to cause yield loss, they are still responding to the insecticide, with the resistance evolving and spreading amongst the survivors.

These data will be used in the years to come to demonstrate how insecticide use in soybeans can erode their value. This will be done across multiple platforms, including twitter, podcasts and the many farmer meetings conducted through ISU extension and outreach.

View uploaded report

We surveyed commercial soybean fields in the northern third of Iowa and found evidence of insecticide resistant aphids. This result is important for two reasons. The first is that the soybean aphid populations across Iowa were very low, including the fields we visited. Some of the 22 fields we visited had no aphids, even the fields with the most aphids had populations below what is required to reduce soybean yield (< 250 aphids per plant on average). A subset of these fields were sprayed with insecticide. Either the farmer was very risk-adverse or was practicing a preventative approach (i.e. a 'prevent-defense') to avoid possible outbreaks later in the season. This brings up the second reason our result is important is that even with this low density of aphids, we found evidence of insecticide resistance in the population. This was most noticeable after the insecticides were applied. When we returned to these fields at least 7 days after an insecticide was applied, not only had the percentage of plants infested with aphids increased, but so too had the average number of aphids on them. This is the response we would see if the aphids were resistant to the insecticide.We are using molecular markers to determine if the aphids collected both before and after the use of an insecticide have mutations associated with insecticide resistance. We anticipate that the frequency of these mutations will increase after the application. This is very worrisome as this will increase the likelihood of these mutations spreading.


Even though the populations of aphids increased after insecticides were applied, they were too low to reduce soybean yield. So the farmers did not suffer immediately from these resistant aphids. But this is very troubling. Insect susceptibility to insecticide is a natural resources that can be eroded with continued insecticide use. Farmers may be using insecticide unnecessarily, applying them when they are not needed to prevent yield loss from soybean aphids. By using insecticides in this way, they are eroding the value of insecticides. Even though the populations are too low to cause yield loss, they are still responding to the insecticide, with the resistance evolving and spreading amongst the survivors.

Going forward, we will share these results with farmers. We hope that these results will show farmers the potential consequences of insecticide use that is not immediately necessary. Furthermore, we will couple our molecular data to determine if that data can more rapidly determine the presence of aphid resistant soybeans than bioassays with living aphids.

We will integrate data on the frequency of resistance into economic models that account for the sporadic nature of this pest, helping farmers avoid a $67 per acre loss that can occur if an outbreak is treated with an ineffective insecticide. If only 10% of the acres in this region are at risk for damage by insecticide-resistant soybean aphids, there is the potential to lose $154,100,000 in revenue per year. Given the trends in insecticide use described below, this risk will increase if farmers are not better informed.