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Ohio State University Extension

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Defoliation in Soybean and Corn, and Japanese Beetle Watch

Japanese beetles and feeding damage on soybean.  Photo by J. Obermeyer.

We have been receiving reports of higher-than-typical Japanese beetle activity in horticultural settings (e.g., ornamentals, home gardens) and this may be a warning sign to watch for them in field crops too.  Also, there are many weedy fields because of the difficulties with planting and weed management this season.  Weedy fields may be more attractive to Japanese beetle and other defoliators because certain weeds are preferred food.

Starting in July and through August, a number of different insect species (such as Japanese beetles, bean leaf beetles, and various caterpillars) can feed on soybean leaves and (for some species) corn.  Foliage feeding in corn is almost never economic, though economic damage from silk clipping by Japanese beetles is possible (though rare).  Consider a rescue treatment when silks are clipped to less than ½ inch and, fewer than 50% of the plants have been pollinated, and the beetles are still numerous and feeding in the field. 

In soybean, while the defoliation damage from various species might look startling, it is rather rare that this reaches economic levels.  Soybeans are master compensators.  OSU agronomist Dr. Laura Lindsey reports that she has even weed-whacked large portions of soybean foliage for an agronomic study and saw no difference in yield by the end of the season!  For defoliating insects in soybean, we usually use an overall defoliation measure as the threshold, regardless of what species is doing the feeding.  This helps account for situations where more than one species may be having its bite.  Entomologists have recently calculated new thresholds for soybean defoliation based on more modern crop values and input costs. A rescue treatment is advised when defoliation levels reach 30% in pre-bloom stages, 10% during pod development and fill, and 15% at R6 (full seed).  After R6, a spray will not pay.  These defoliation levels apply to the plant as a whole, not just certain leaves.  Damage is often worst at the top of the canopy but on closer examination most of the plant is relatively unharmed.  Make your decision based on the average condition of whole plants, not a scan of the top canopy.  Also, defoliation tends to be worse on field edges, so make your assessment based on the field as a whole, including interior.

defoliation visual guide

A visual guide to defoliation is useful because it is very easy to over-estimate defoliation in soybean.  For more information, visit our Soybean Defoliation scouting card at
https://aginsects.osu.edu/sites/aginsects/files/imce/Soybean%20defoliation%20Final.pdf

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.