Plants and insects have proven their ability to overcome a variety of herbicides, pesticides and/or insect tolerant traits in crop production. These events of resistance that occur in a population are often a result of over-reliance on particular practices over time. Growers are familiar with the weed resistance issues impacting multiple herbicide modes of action, and the issues with certain corn insect traits failing to adequately control pests such as the western corn rootworm in continuous corn systems.
A new development that’s gaining attention is the failure of pyrethroid insecticide to control soybean aphids. This issue has been documented in Minnesota and Iowa over the past two growing seasons. University of Minnesota entomologists were able to confirm insecticide resistance to an aphid population near Lamberton, MN via lab bioassays in 2015. Insecticide failures were also reported in a few Southern Minnesota fields during the 2016 growing season. Bob Koch from the University of Minnesota reported that so far confirmed aphid populations have been resistant to bifentrin (Brigadeâ, Tundraâ, Heroâ, etc.) and lambda-cyhalthrin (Warriorâ, etc.) products. In 2016, Iowa State University reported a pesticide failure in Northwest Iowa where a field was sprayed twice within two weeks with a bifentrin product and had little control. Vial assays showed increased resistance ratios for the aphid population collected from this field.
Since they were introduced to the Midwest in 2000 from Asia, the Soybean Aphid has become the most yield damaging pest to soybeans through parts of the Corn Belt. Aphids are very small (less than 1/16” length), yellow to green in color with distinct black cornicles at the end of the abdomen. They may be winged or wingless. Found primarily on underside of newest growth. Early season, they are found near upper canopy while later in plant development they will move down the plant to pods, stems and lower leaves.
Applying insecticides only when necessary:
Aphids are known for their asexual reproduction that allows rapid duplication throughout the growing season. Those who have scouted soybean fields for aphids can attest to how rapidly these populations can grow from one day to the next. On the other hand, there are also situations where aphid counts fail to grow due to timing in development or environmental conditions. While there are several predictors to help guide scouting efforts, no two fields are alike- so just because a neighbor is spraying for aphids doesn’t necessarily mean your fields need sprayed. Scouting each field separately is warranted to avoid unneeded applications that can potentially reduce farm profitability and build the development of resistance to insecticides.
Conditions that favor aphid population growth and damage are cooler temperatures; moderately dry soil conditions and plant stresses present -particularly drought stress. High temperatures have shown to slow aphid population growth, especially temperatures above 90 F. Primary timing of infestation and damage is considered to be at stages R1-R4. Early infestation can be found in fields near buckthorn and are more abundant near field edges. Also, later maturing fields can have more infestation so planting early in fields with a history of persistent aphid issues is a management option.
Fortunately, in most areas pyrethroid insecticides are still doing an adequate job at controlling soybean aphids but it’s critical growers are aware of these developing issues and take action to delay further development; and preserve these chemistries for as long as possible. The table below provides an overview of available insecticides for soybean aphid control currently on the market. This table highlights that there are only three different modes of action currently available: Organophosphates, Pyrethroids and Neonicotinoids. Pyrethroid insecticides (Group 3A), have typically been some of the most widely used and best performing insecticides used- which should be concerning that there is now evidence of resistance developing.
Below are a few guidelines that can help improve aphid management and slow resistance development:
University research has maintained that the economic threshold for soybean aphids is 250 aphids per plant with 80% of the plants infested with populations increasing through full seed. While this value has been debated through the years by way of changing commodity prices or differing opinions by local advisers, researchers set 250 aphids as a conservative threshold with actual economic injury level being at a much higher level of over 600 aphids per plant. A detailed report justifying this threshold can be found here
There are also several beneficial insects that are killed with insecticide applications; lady beetles, parasitic wasps, insidious flower bug and green lacewing larva. These insects have the ability to lower aphid populations. Additionally, there is the risk of potentially increasing pressure of resistance development in other soybean pests. The University of Minnesota discovered a chlorpyrifos-resistant population of two spotted spider mites in Southwestern Minnesota in 2012. They attributed this to non-target exposure of these spider mites to insecticides applied for soybean aphid control.
Insecticide Stewardship: It is recommended to alternate insecticide mode of actions and avoid relying on one product if treatment is justified. If a pyrethroid insecticide is used for the initial application, rotate to an organophosphate when an additional application is needed. While herbicide resistance management has incorporated the use of tank mixes or formulated mixtures of multiple modes of action as a recommended practice - this is not recommended with insecticide resistance management due to differences in plant and insect biology. Rotation of individual insecticide group is preferred. More on this topic can be found here.
Following the chemical label and using full rates is important. Reducing rates for lower insect populations or as an insurance application will only increase the rate of population resistance and is not good practice. High volume and pressure is recommended so droplets make contact with aphids on bottom side of leaves.
The bottom line of this information is not to discourage insecticide use. There are plenty of situations that warrant a spray treatment to prevent yield loss. However, these are guidelines to prevent overuse or unneeded applications.
In recent years there have been aphid-resistant soybean varieties available on the market. These varieties contain Rag genes, a single gene (Rag 1, Rag2) or pyramided (Rag1+Rag2). However available varieties have been limited across the industry. The pyramided products have shown greater protection but the University of Minnesota Bob Koch reports that they have already identified aphid populations that can overcome single gene products. This is an area that companies continue to evaluate for future product development.
The goal of reporting these cases of insecticide failure is to build awareness and improve future management to preserve chemicals. However not every insecticide application issue is necessarily resistances as there are several factors such as weather, misapplication or aphid movement than can lead to performance issues. If you observe a potential case of resistance, the University of Minnesota has an online submission website.
Feel free to contact your LG Seeds sales agronomist with questions.
Sources and Additional Information
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
Brigade and Hero are registered trademarks of FMC Corporation. Tundra is a registered trademark of Winfield Solutions, LLC. Warrior is a registered trademark of a Syngenta Group Company.
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Download a copy of this technical Bulletin: Tech_323 - Soybean Aphid Resistance