Research HighlightsConsiderations for Insecticides in a Post-Chlorpyrifos World
By Nathan Kleczewski
Chlorpyrifos is an insecticide in the organophosphate (OP) (IRAC-1B) group that has been in use since 1965. To put that in perspective, in 1965 Mike Ditka, Dick Butkus, Doug Atkins, and Gale Sayers were all starters for the Chicago Bears. This insecticide interferes with nerve signaling by preventing the breakdown of acetylcholine, an important neurotransmitter molecule, in insects. Products with this chemistry (e.g., Cobalt, Lorsban, Stallion, numerous others) were used widely in food and grain crops due to the broad-spectrum insecticidal activity and highly translaminar activity of the chemical in plants.
Unfortunately, some of the properties that made this chemical an effective pesticide also made it very dangerous. OP’s also impact acetylcholine in animals and humans, making them highly toxic. After a lengthy discussion, the U.S. Environmental Protection Agency announced that tolerances of chlorpyrifos on food crops will be removed, preventing its use soon (likely in 2022). Corteva, the largest producer of this chemical, has also drastically reduced production of the chemical. Although non-food uses of the chemical will still be permitted (e.g., termite control), we need to be prepared for field crop production in a post-chlorpyrifos world.
In soybeans, products containing chlorpyrifos were cheap and effective at controlling a wide range of insect pests of varying feeding habits. In general, products used in soybeans were good options for suppression of pests such as aphids and spider mites. Systemic insecticides are the most effective at suppressing these pests, as they feed on tissues in addition to largely residing in protected portions of the plant, such as leaf undersides and flowers.
With soybean aphids, there have been instances of insecticide resistance to some active ingredients (e.g., pyrethroids) in several states, and caution must be taken to avoid mis-/over application in situations where control is not warranted. Many pesticides will have off-target impacts on natural insect enemies that keep aphid populations in check, such as the recognizable Ladybug larvae, lacewings, and parasitic wasps. Applications made when not needed are known to negatively impact the populations of these predators, resulting in flare-ups of aphids that can reproduce unimpaired. Best results are achieved when aphids reach threshold amounts of at least 250 aphids per plant with more than 80 percent plants infested between late vegetative stages through R5. University research does not indicate that populations less this result in a yield response and may be held in check at lower levels by natural predators. Once an insecticide has been applied, the field should be checked to ensure that it worked, and that additional issues such as insecticide failure or application issues are not of concern.
Similar to aphids, two spotted spider mites can be impactful, typically during very hot and dry conditions. Mites are frequently held at bay by a fungal parasite. As is true with other fungi, humid environments favor the growth and development of these parasites. When conditions are hot and dry, mites are not impacted by the fungus and can flare up in fields. Mites typically move into soybeans from field edges and progress inwards. Scouting should occur at field edges, especially during droughty periods, focusing on edges where plants appear stressed, or near wood lots or ditches. Assess 23 plants at each of 20 locations across the field edge, following a “U” pattern that moves 100-150 ft. into the field. If you observe mites and characteristic foliar stippling on an average of 20 percent of plants assessed by R3, or more than 10 percent stippling from R3-R6, a chemical treatment may be beneficial.
There are several products of varying efficacy for two spotted spider mites. Some are only listed for suppression, so it is important to read labels carefully. Examples of products that have labels for spider mites include Agri-Mek, dimethoate products, bifenthrin products (Brigade; Hero) and a newer product, Zeal. Some of these chemistries are very specific to mites and therefore do not impact natural predators or other insects. Others have a broader spectrum. Regardless, ensuring adequate coverage and canopy penetration is essential. Edge treatments are sometimes considered if populations are highly localized at specific field edges, but this may not be appropriate in small fields, and opinions of entomologists and agronomists on this subject vary. Regardless of your decision, make sure to check your fields 5-7 days afterwards to make sure that application had the expected effect.
The last group of insects that are often of concern are defoliators/pod/seed feeders. There are numerous insects in this group that can impact fields at different crop growth stages. Japanese beetles, grasshoppers, bean leaf beetles, Western bean cutworms, stinkbugs, cloverworms, and loopers, are a few that we see in Illinois. Data on individual thresholds for these are limited, but in general if you see 30 percent defoliation before R1, 20 percent defoliation from R1–R5 or 5–10 percent pod feeding, a treatment would be beneficial and likely to improve yields.
We frequently overestimate defoliation, so if you are scouting, make sure you do some training with standard area diagrams or make yourself a sheet with reference images to help guide you. The Crop Protection Network has a very useful tool to do this at this link: https://cropprotectionnetwork.org/news/new-disease-severity-and-insect-defoliation-training-tool.
When you are estimating defoliation, an easy method is to sample a trifoliate from the lower, middle, and upper canopy. Then for each trifoliate, remove the leaflet with the most and then the least amount of feeding. Rate the remaining trifoliate for defoliation. Average your defoliation ratings for each part of the canopy to arrive at a plant-level defoliation estimate. Do this for at least 20 locations throughout the field. For more information, check out this article by Dr. Nick Seiter from the University of Illinois. The best results and highest probability for a return on investment come when insecticide sprays are made based on scouting.
Although chlorpyrifos will no longer be available, we still have numerous options for insect suppression and management in our soybeans. Paying attention to our fields and selecting the right product at the right point in time will continue to add value to your production acres.
Note: The preceding information is for educational purposes only. Any products listed above are not endorsements by Dr. Kleczewski and are listed as examples of a broader range of products. Consult your providers for specific product recommendations for your soybean acres.
Dr. Nathan Kleczewski is the Plant Disease and Entomology Specialist for GROWMARK Agronomy Services and their technical agronomist for the eastern and southern territories of Illinois. In his role, he assists member companies with troubleshooting day to day issues encountered in fields, assists crop specialists with management recommendations, and works to train and educate crop specialists in practical, sustainable, and economical management of diseases and pests in crops grown in the United States and Ontario. He has worked extensively in applied field crop and vegetable pathology since 2013, serving Illinois (2017-2021) and Delaware (2013-2017) as their extension field crop plant pathologist. He worked at FMC as a research plant pathologist working to evaluate and develop new fungicide, nematicide, and biological products, and has post-doctoral experience from both Purdue University and Indiana University. He earned his doctorate in plant pathology from The Ohio State University in 2009 and his bachelor’s degree in plant ecology and evolutionary biology from the University of Wisconsin-Oshkosh in 2004.
EPA takes action to address risk from chlorpyrifos and protect children’s health. www.epa.gov/newsreleases/ Accessed 13 December, 2021
Hanson AA, Koch RL. 2018. Interactions of host-plant resistance and foliar insecticides for soybean aphid management. Crop Protection. Oct 1;112:232-8.
Hodgson, E. Why use the economic threshold for soybean aphid? crops.extension.iastate.edu/blog/erin-hodgson/ Accessed 14 December 2021.
Koch, R. L., Hodgson, E. W., Knodel, J. J., Varenhorst, A. J., & Potter, B. D. 2018. Management of insecticide-resistant soybean aphids in the Upper Midwest of the United States. Journal of Integrated Pest Management, 9(1), 23.
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