Insecticide resistance management in soybean
photo credit: John Obermeyer
Resistance to the insecticide chlorpyrifos was confirmed in a two-spotted spider mite population from southwestern Minnesota in 2012
Insecticide (and miticide) resistance is a heritable decrease in a pest population's susceptibility to a pesticide, meaning the genetics are passed from generation to generation. As pest populations become less susceptible (i.e., more resistant) to a pesticide, the utility of that pesticide becomes less and less effective to a point where the pesticide and potentially other related pesticides may become effectively lost as tools for management of that pest.
Since the invasion of soybean aphid in 2000, use of foliar insecticides on soybean has greatly increased. In addition, increased crop values are stimulating increased adoption of seed treatments and prophylactic foliar treatments. If not well managed, this overall increased use of insecticides in soybean favors the development of insecticide resistance in pests.
Insecticide resistance management
Insecticide resistance management (IRM) is a collection of strategies used to prevent or slow the development of resistance to insecticides in order to prolong the utility of pesticides as management tools.
IRM strategies recommended for insect and mite pests in Minnesota soybean include the following:
1. Reduce likelihood of needing to use insecticides
Integrate multiple management tactics (e.g., pest-resistant varieties, biological control, crop rotation) to discourage pest outbreaks.
2. Use insecticides only when necessary
Scout regularly and base insecticide applications on economic (action) thresholds.
Economic threshold for soybean aphid: Through R5 soybean, treat when populations are increasing, the majority (at least 80%) of plants are infested, and average aphid counts exceed 250 aphids/plant.
High crop values can drive increased prophylactic (insurance) use of insecticides. Such use can lead to resurgence of the target pest, replacement by secondary pests, adverse impacts on natural enemies and pollinators, development of pest resistance, and increased costs.
3. Use insecticides appropriately
Follow the product label directions and use labeled rates of insecticides.
4. Don't repeat use of the same insecticide mode of action.
Alternate insecticide modes of action (Groups) to delay resistance. For newer products, the insecticide group is listed on the label. Table 1 shows insecticide groups to assist in selection of insecticides. Further information can be found at www.irac-online.org.
For pests with more than one generation per year, alternate insecticide modes of action being used within the year. For less-mobile pests with one generation per year, alternate modes of action between years. In terms of exposure to insecticides, think of the pest complex as a whole. An insecticide application targeted at one pest will also expose other pests present to the insecticide. Twospotted spider mite resistance to chlorpyrifos in soybean in southwestern Minnesota likely resulted from non-target exposures. Years of insecticide applications targeted at soybean aphid were also exposing populations of twospotted spider mite that were always present in the fields, but usually at low levels.
Example: If a foliar application of an organophosphate (group 1B) insecticide is applied to a field that has exceeded the economic threshold for soybean aphid (250 aphids per plant), the insecticide used for a subsequent treatment for soybean aphids or any other pest should not contain an insecticide from group 1.
Though insecticide mixtures (formulated mixtures or tank mixtures) may be effective for pest suppression, alternation (i.e., the use one after another) of insecticide modes of action is generally more effective for insecticide resistance management.
The widespread, prophylactic use of systemic insecticidal seed treatments (e.g., neonicotinoids) across many acres regardless of pest risk is counter to the philosophy of integrated pest management. It may pose a risk for development of pest resistance to these insecticides, as well as other environmental concerns. The use of systemic insecticidal seed treatments is an effective pest management tool for some pest situations. In particular, these treatments should be used in situations with high risk for economically damaging populations of seed- and seedling-feeding insects.
Table 1: Representative insecticide groups and active ingredients used in soybean (always follow label directions)
|1A: Carbamates||Carbaryl, Methomyl, Thiodicarb|
|1B: Organophosphates||Acephate, Chlorpyrifos, Dimethoate, Methyl parathion, phorate|
|3A: Pyrethroids||Beta-cyfluthrin, Bifenthrin, Cyfluthrin, Deltamethrin, Esfenvalerate, Gamma-cyhalothrin, Lambda-cyhalothrin, Permethrin, Zetacypermethrin|
|4A: Neonicotinoids||Clothianidin, Imidacloprid, Thiamethoxam|
|5: Spynosins||Spinetoram, Spinosad|
|11A: Bacillus Thuringiensis||Bacillus Thuringiensis|
Identifying cases of potential resistance
Before assuming resistance, rule out these factors:
- Misapplication of insecticide (incorrect insecticide or rate, poor coverage)
- Unfavorable weather (wind, rain, temperature) (for example, some pyrethroids can be less effective at high temperatures)
- Improper timing of application (susceptibility of life stage present)
- Recolonization by the pest
If you still suspect insecticide resistance, contact Robert Koch at firstname.lastname@example.org.