There are many white grub species and a few of those can be found in fields across the Midwest every year. White grubs are the immature form of scarab beetles which include the masked chafer, Japanese beetle, and June beetle or true white grub. Some grubs rarely feed on row crops in the Midwest; however, others such as the true white grub are more likely to feed for portions of one to three seasons. While white grubs can be a nuisance, they are considered an occasional pest and generally do not have the same level of damage year to year.
White grubs are usually about ¼ inch to over 1 inch in length, white to cream color with a brown head, have 6 prominent legs, and usually maintain a “C” shape. It is common to see grub damage in fields that were sod, set-aside, cover crop, or just had poor weed control as June beetles and masked chaffers prefer to lay their eggs in grasses. It is also possible to see grub damage following a soybean crop as the Japanese beetle will lay eggs in grasses or soybean.1 The life cycle of the Japanese beetle and masked chafer is a one-year cycle with the larvae (grub) stage lasting from August thru the middle of the following June. June beetles have a 3-year cycle which includes a larval (grub) stage that begins in early July and lasts about 25 months before pupating into the adult. The larva of the different species can be differentiated by the formation of setae on the tail of the larva (Figure 1).1
When white grub feeding occurs, it is common to see stunted, wilted, discolored or dead/missing seedlings. This is because the grubs feed on the roots and stems of the plant, causing damage or death to the seedling. When scouting fields, if potential damage is noticed it is important to dig up the root zone of a few plants to check for the presence of white grubs and signs of root feeding damage. If grub damage is high enough to cause replant and feeding is still occurring, using a soil applied insecticide labeled to control white grubs may be recommended.
The best way to manage white grubs is to keep them out of your fields. One thing that greatly reduces the presence of white grubs is good control of vegetation during the offseason and into planting. This is because it reduces their ability to establish ahead of the crop and then jump to feeding on the row crop. This can be done through clean tillage or a robust herbicide program if in a no till/minimum till system.
Another option to control beetle populations is to reduce the amount of laid eggs. This can be done with the use of an in-season insecticide which is often sprayed with a fungicide application in corn or soybean. A Bayer option for controlling a broad spectrum of corn and soybean insects in-crop is Baythroid® XL insecticide (Restricted Use Pesticide). The insecticide is a pyrethroid which has fast knockdown and long residual control.
For more information about insect control options in corn and soybean, and Baythroid XL insecticide from Bayer, please contact your local Crop Science US representative.
1White grubs. 2009. Field Crops IPM. Purdue University. https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-whitegrubs.php#:~:text=White%20grubs%20are%20the%20immature,May%2FJune%20beetle/.
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