Soybean growers alone lost over 500,000 bushels due to Japanese beetles in 2024.¹ Though corn is less significantly affected by Japanese beetle defoliation damage, silk clipping can dangerously interfere with seed set. Dive into the details of how Japanese beetle damage impacts corn and soybean crops and how gauging infestations plays a critical role in your decision-making for effective management.
Signs of Japanese beetle damage
Japanese beetle infestations in corn and soybeans typically cause the most damage during July and August.
Japanese beetle symptoms:
Japanese beetle damage in soybeans: Japanese beetles feed on leaf tissue, causing lacy, skeletonized leaves with only the leaf veins intact. Large populations can cause heavy defoliation in patches of the field, but beetles usually feed only in the upper canopy and on a few trifoliates.
Japanese beetle damage in corn: Mature Japanese beetles feed on corn silks before pollination is complete, disrupting the process. Damage to silks may result in reduced seed set. Drought stress can exacerbate silk clipping damage from Japanese beetles.
Scouting for Japanese beetles
Japanese beetles are usually scattered in clusters or found on field borders or in weed patches. Scout the entire field; if you scout only one patch where the Japanese beetle population is high or low, the overall field population count may be skewed.
Japanese beetle versus corn root worm
Similar to corn rootworm, Japanese beetle damage includes scraping off the green surface tissue on corn leaves before silks emerge and subsequently feasting on silks (their food of choice in the corn acre).
However, mature rootworms are smaller and more random in their field distribution than the telltale clusters of Japanese beetles. Damage caused by these two pests looks very similar, so identify the beetles themselves.
Economic impact of Japanese beetle damage in corn and soybeans
Beetles typically prefer soybeans over corn, but both crops can experience yield reductions due to Japanese beetle damage.
Japanese beetle threshold in soybeans
Whether caused solely by Japanese beetles or in combination with other defoliating pests, economic thresholds are 30% in the vegetative stages and 20% in the reproductive stages.²
Japanese beetle threshold in corn
For corn, the economic threshold depends on how silk clipping has impacted pollination. Scout for beetles in at least five random locations within the field. If three or more beetles are found on each ear with silks clipped to ½" or less while pollen shed is less than 50% complete, you have hit the economic threshold.³
How to manage Japanese beetle damage
Both soybean and corn defoliation can look more catastrophic than it is and be difficult to accurately estimate to an untrained eye. Be careful to not overestimate Japanese beetle damage.
Avoid commercial Japanese beetle traps for scouting or control purposes since the attractant and pheromone used to collect beetles is highly enticing and the traps often overflow and spill over into adjacent plants, causing severe injury to plants surrounding the trap. Due to Japanese beetle congregation habits, it also may be hard to get an accurate count with traps.
Treat Japanese beetle damage based on the established thresholds and consider using Fastac® CS insecticide, labeled for use in both corn and soybeans to help control adult Japanese beetle populations.
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- “Soybean Invertebrate Loss Estimates from the United States — 2024.” Crop Protection Network, 3 Sept. 2025, cropprotectionnetwork.org/publications/soybean-invertebrate-loss-estimates-from-the-united-states-2024. Accessed 24 June 2026.
- Basol, Terry. “Keep an Eye out for Soybean Defoliators.” Integrated Crop Management, Iowa State University, 12 July 2023, crops.extension.iastate.edu/cropnews/2023/07/keep-eye-out-soybean-defoliators. Accessed 24 June 2026.farmdoc.illinois.edu/field-crop-production/uncategorized/managing-japanese-beetles-in-corn-and-soybean.html. Accessed 13 June 2024.
- Bick, Emily, et al. “Managing Japanese Beetles in Wisconsin Corn and Soybean Fields.” Crops and Soils, University of Wisconsin-Madison, cropsandsoils.extension.wisc.edu/articles/managing-japanese-beetles-in-wisconsin-corn-and-soybean-fields/. Accessed 24 June 2026.
