White grubs and corn stand reductions

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

We have had at least one instance this spring in St. Joseph County where white grubs have caused enough stand reduction to cause re-planting of a corn field. The grower’s seeding population was around 26,000 seeds per acre and the stand in areas affected in the field was reduced to around 9,000 plants per acre. The previous crop was a grass-alfalfa mixed hay.

While many areas in the state have had high populations of European chafer in the past, areas in St. Joseph County are seeing the pest in significant numbers for the first time. We helped identify the adult stage of this pest for several homeowners last summer. Also, true white grubs, the larvae of May and June beetles have long life cycles, up to two to three years, and are found in higher numbers in undisturbed areas such as sod, hay or pasture. Fields planted after these uses are often thought to be at higher risk for true white grub damage in addition to challenges from the annual grubs of Japanese beetle and European chafer.

Grub damage often tends to be patchy. European chafer damage is often found in sandier parts of fields. Delayed emergence can also increase damage, especially from annual grubs. There is no rescue treatment for grubs in field crops. Corn planted into grub-infested fields should be treated with a soil insecticide to protect plants until grub feeding stops before pupation, or until sufficient root mass is developed to withstand damage. Poncho seed treatment has also worked well under low to moderate grub pressure.

There is no threshold for determining when it is important to control grubs in corn. However, a history of grub damage in an area, plus seeing large numbers of grubs during field preparation, is a good indicator to use a soil insecticide to protect your crops from these pests. 

Reference: Some of the information in this article is extracted from articles by Entomologist Christina DiFonzo.

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