Managing the Corn Earworm Pest in Soybean
The corn earworm is one of the more devastating insect pest to soybean production within Eastern NC. Typically, a major moth flight occurs within Craven County, NC between late-July and Early August. In some years, an additional flight may occur in late-August or early-September. Eggs laid during these peak flights develop into caterpillars that eat the tiny, developing soybean pods to greatly reduce yield if not controlled. Proper management mandates an understanding of potential resistance of the CEW to pyrethroid products and slight changes in the economic threshold.
NC Cooperative Extension in Craven County provides black light moth catches to monitor the intensity and duration of these moth flights. A volunteer checks the light trap several times a week and results are immediately recorded for viewing at https://cotton.ces.ncsu.edu/trap-data/. An increase in CEW moths does not warrant an insecticide treatment. Rather it simply provides evidence for need to increase scouting intensity.
Scouting methods should be based upon row width. Typically, rows widths 36 inches or larger requires a beat sheet (3 feet in length). Soybean production on 21-30 inch rows can be checked with a rigid screen (3 feet standard length). Soybean production on rows 20 inches or less are usually monitored using a sweep net. The method chosen will impact the economic threshold.
Begin monitoring for pests when fields are at early bloom since the CEW prefers such fields to lay eggs. Scout several random areas and record the number, type and size of caterpillar pests. It is critical to properly identify the types of caterpillars since velvetbean caterpillars, soybean loopers, green cloverworms, and fall armyworms may also be present. Pest identification of major soybean pest can be found at https://soybeans.ces.ncsu.edu/soybean-insect-pests/. Should multiple caterpillar pest be present, a non-pyrethroid product is preferred (Pyrehtroid products will not likely control armyworms and loopers.)In addition to the types of caterpillar pest, size of these pest should be recorded. A dominance of larger caterpillars will indicate a greater urgency for treatment and a mixture of size of caterpillars indicates a continued moth flight that may result in several days to weeks of continued caterpillar feeding.
Once scouting is complete, use a simple mathematical average to determine the average number and size of CEW. Specific soybean scouting methods and additional information can be found at https://soybeans.ces.ncsu.edu/scouting-for-insects/. Once the average CEW is determined, evaluate the economic threshold. This threshold is dynamic and will vary by selling price of soybean and control cost (product and application cost) so do not use the same threshold each year unless selling price is the same. As example, the threshold for soybeans on 7 inch rows sold at $9.20/bushel and a treatment cost of $12/ac is 2.5 caterpillars per 15 sweeps. Changing only the treatment cost to $8 lowers the threshold to 1.7 caterpillar per 15 sweeps. To determine the actual threshold based on individual circumstances, visit https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.
Typically, to prevent unwarranted treatment that will only reduce profit, CEW are only treated with an insecticide when soybeans are setting small pods and a threshold limit has been exceeded. Until both of these conditions are met, no economic loss will have occurred from CEW feeding. Historically, pyrethroid products have been used because these products were inexpensive and highly effective. However, recent research data and grower testimony reveals that an increasing number of the CEW population has become resistant to pyrethroid materials. Within Craven County growers relate to NC Cooperative Extension remarkably good results with almost 100% control as well as only 50-70% control when using a pyrethroid or premix of a pyrethroid with a neonicitinoid product. Thus, both science and experience shows an increasing number of pest control failure when using pyrethroid products. Read more on the data and NCSU recommendations in an article posted by Dr. Dominic Reisig, NCSU Entomology HERE )
In addition, one last consideration should be to protect our pollinating insects. If bee hives are nearby, law, pesticide labels and common-sense dictates that the applicator notify the beekeeper. Typically, 48-72 hours of advanced warning meets legal requirements and allows the beekeeper ample time to move or close the hive. Simply aim to provide updates should weather, mechanical breakdown or other management decision alter original plans for treatment. Lastly, consider the product chosen. Pyrethroid products, especially when tank mix or purchases as a premix with other products, tend to control a broad-spectrum of insects and may be more harmful to bees than other products that only target caterpillar pests. Two resources that may help to evaluate the relative toxicity of products to bees are below. The first is a section from the 2017 North Carolina Agricultural Chemicals Manual titled, Relative Toxicity of Pesticides to Honey Bees. The second, title, RT25 Data Table is an EPA document with select pesticide toxicity ratings.
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