Assessing and reporting potential cases of soybean aphid resistance to pyrethroids

Farm Forum

History has shown us many times that over-reliance on pesticides often results in development of pesticide resistance in pests. Last summer in parts of southern Minnesota, some pyrethroid insecticides (bifenthrin [e.g., Brigade, Tundra , Hero and others] and lambda-cyhalothrin [e.g., Warrior and others]) failed to provide adequate control of soybean aphid. Historically, these have been among the top performing pyrethroid insecticides for soybean aphid management. Follow-up laboratory bioassays confirmed resistance (10- to 44-fold resistance) to these insecticides in a soybean aphid population collected near Lamberton, MN. The majority of these performance issues in 2015 appeared centered in and near Brown, Redwood and Renville counties. In most other areas, pyrethroid insecticides preformed as expected and effectively suppressed aphid populations. Here, we provide updates on resistance monitoring for 2016, and recommendations for resistance management and how to report potential cases of resistance.

Updates on resistance monitoring

Our efforts to monitor for resistance in soybean aphid populations have begun this year as aphid populations are increasing in fields. Significant resistance was not detected in soybean aphid populations collected from near Lamberton and Chandler in 2016. However, we have received a couple reports of performance issues with field applications of pyrethroids for soybean aphid over the last couple weeks. We will continue collecting and assaying aphids from locations around Minnesota.

Insecticide resistance management

As soybean aphid populations increase, we ask that you use insecticides wisely to prevent or postpone further development of insecticide resistance in the soybean aphid. A detailed explanation of insecticide resistance and resistance management strategies can be found in “Insecticide Resistance Management in Soybean.”

Some key strategies to keep in mind for insecticide resistance management (IRM) are:

• Alternate insecticide groups (modes of action). For example, if more than one foliar application of insecticide is required, alternate between pyrethroid and organophosphate. Or, if a neonicotinoid seed treatment was used, avoid a foliar application with a neonicotinoid-containing product.

• Alternation is preferred over tank mixes (or formulated mixtures) of insecticides for resistance management.

• Follow the label instructions to use insecticide properly. Use full recommend rates (i.e., avoid using reduced rates in tank mixes or alone)

• Apply insecticides only when necessary (i.e., scout fields and use the economic threshold of 250 aphids per plant with majority of plants infested and population increasing)

• Use integrated pest management (IPM) to reduce likelihood of needing to apply insecticides

Insecticide mixtures generally not for resistance management

The topic of mixtures of insecticides (tank mixes or formulated mixtures) deserves further discussion, because the recommendations for insecticide resistance management differ from recommendations for herbicide resistance management. The best fit for insecticide mixtures is to broaden the spectrum of pest control for when multiple insect species are economically threatening; however, this rarely is the case in Minnesota soybeans, where soybean aphid continues to be the primary insect threat. The Insecticide Resistance Action Committee (IRAC), which is comprised of scientists from many of the agrochemical companies, released an official statement explaining why insecticide mixtures are generally not a good fit for insecticide resistance management. Instead of using mixtures they recommend alternation (rotation) of individual insecticide groups (modes of action). The challenges with using mixtures for insecticide resistance management are:

• The individual insecticides within the mixture should be used at the rates at which they are individually registered for the target pest

• Mixtures of insecticides from the same insecticide group (mode of action) are not recommended

• Cross resistance between the individual insecticides must be considered

• Mixtures are less effective for resistance management if resistance is developing to one or both of the insecticides in the mixture

• The individual insecticides within the mixture should have similar durations of residual activity (longevity of toxicity after application)

Insects are not weeds. Due to obvious differences in their biologies (e.g., ability to move to new locations of own choosing), recommendations for control and pesticide resistance management differ between these pest types. For example, when one insecticide of a mixture is less persistent (e.g., organophosphates versus pyrethroids) or is present in different locations of the plant (e.g., absorption and translocation or volatilization), the pest insect population may essentially be exposed to a single compound spatially or temporally.

Resistance to a persistent broad-spectrum insecticide, like a pyrethroid, creates another problem. When mixed with a less persistent insecticide, like an organophosphate (e.g., chlorpyrifos) the spectrum of insects controlled and perhaps speed of kill increases. Unfortunately, this includes killing beneficial insects (e.g., predators and parasitic wasps). The inclusion of the persistent insecticide will eliminate these beneficial insects even after the less persistent compound has dissipated. This allows any surviving or immigrating insecticide-resistant aphids to colonize and reproduce very quickly.

Insecticide resistance in spider mites

Another important thing to remember is that applications of insecticides targeted at soybean aphid also affect other pests, such as twospotted spider mites. In 2012, a chlorpyrifos-resistant population of twospotted spider mites was identified in southwestern Minnesota. This pest is rarely the primary target of insecticide applications, so this case of resistance was likely the result of non-target exposure of the mites to insecticides applied for soybean aphid. Unnecessary insecticide applications of insecticides (i.e., those made below economic threshold for aphids) and use of mixtures of insecticides may expose these other pests to one or more modes of action and potentially increase pressure of resistance development.

How to report potential cases of resistance

Several factors can cause an insecticide application not to perform as inspected. Before assuming resistance, you must rule out factors such as misapplication of the insecticide, unfavorable weather, and recolonization by the pest. The preferred method to report potential cases of resistance is to use “Report Insecticide Failures for Soybean Aphid.”