Agronomy Blog

Japanese Beetles

by Mark Seem | Jul 14, 2017

Japanese beetles are appearing in great numbers in cornfields throughout portions of the Corn Belt. Normally these pesky insects are not an economic threat to the corn or soybean crops. When winters are somewhat milder than normal, a large number of white grubs survive allowing for a substantial population of Japanese beetles. Very early planted, and very late-planted, corn crops will act as “trap” crops for beetles. These are the fields that need to be monitored closely for beetle feeding. Japanese beetles are considered the “king” of the soybean defoliators. Grasshoppers take second place.

These hard-shelled insects with the shiny metallic green/bronze-covered wing covers start their life cycle as larvae that hatch from eggs laid in the soil during July and August. After passing thru several instars and feeding on decaying vegetation and grass roots in the soil, these white grubs (the larval stage) will over winter deep in the soil, pupate in the early summer, and emerge as adults in late June and early July. As planting progressed this spring, producers may have noticed a number of white grubs in their fields during tillage and planting operations. A larger than normal over wintering population of white grubs would lead one to assume that populations of the adult Japanese beetle would be higher than normal as a consequence.
Jap Beetles Corn Silks 1 citedJap Beetles Corn Silks 2 cited

Jap Beetles Corn Tassels 1 citedJap Beetles Corn Leaves 1 cited


Japanese beetle adults are a threat to corn during pollination. Beetles may clip back the corn silks and prevent proper pollination of corn ears. Such silk clipping by Japanese beetles is usually minimal and spotty, and does not occur much beyond the field borders, 10-12 rows deep into the field. However, this summer in parts of the Corn Belt, most noticeably in Illinois, the silk clipping is severe. Leaf feeding is occurring, and leaves appear “lacy”, or skeletonized. The leaf feeding is hardly ever of economic importance. Thresholds for Japanese beetle control in corn are established:

  1. If beetles are feeding on silks, an insecticide should only be applied if the silks are being clipped to less than ½ inch before 50% pollination has taken place, and Japanese beetles are still present and actively feeding.

  2. University of Illinois additionally states that if three of more adults are found per ear shoot, control measures are warranted.

Sampling cornfields for Japanese beetle feeding is somewhat straightforward. Check 5 randomly selected plants in 5 random selected areas of the field for Japanese beetle adults. Record the number of beetles found per plant. Also estimate and record the length of the silks remaining on each plant (1/4, 3/8, 1/2, 5/8 inch, etc.), as well as the maturity of the silks (no silks, green silks, some brown silks, all brown silks). Finally determine if pollen is still being shed, by shaking the tassel of all 25 plants being checked. Beetles are often attracted to dead or dying silks to feed. Such silks have already pollinated the ovule. Do not mistake feeding on already pollinated silks for silk clipping of emerging silks.

REMEMBER: if silk clipping is occurring, there may be more than on insect pest involved. One should scout for corn rootworm beetle feeding at the same time. If the scouting results in a determination that control is warranted, a number of insecticides are available. Please read and follow all label directions if applying an insecticide.

Here is a link to Purdue University’s Corn Insect Control Recommendations for 2016


Damage from Japanese beetles is primarily to the leaves. Defoliation can appear extensive. The soybean plant’s amazing capacity to compensate for leaf defoliation seldom allows this defoliation to affect yield. There are few guidelines established for soybeans:

  1. 40% defoliation prior to bloom can be tolerated without affecting yield
  2. 15-20% defoliation after bloom and during pod fill will affect yield

 LG Seeds agronomist Ryan Dunsbergen wrote about Japanese beetles and soybeans in August of 2016:

If you are seeing any issues in your fields, please do not hesitate to contact your LG Seeds District Sales Manager or Agronomist.  We would love to help!


Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
LG Seeds and design are trademarks of SCA Limagrain.

Download a copy of this Technical Bulletin: Tech_343 - Japanese Beetles