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White Grubs
White grubs that are destructive to field crops in North Dakota have a three year life cycle. In southeast North Dakota, the most common white grub pest occurs in continuous cropping situations at sites where willow and cottonwood trees are present. In other areas of the state, white grubs are most likely to be found when rotation from grassland, pasture, or grassy weed sites occur. Once a population builds up in a field, damage may reoccur in untreated fields at three year intervals.
White grubs are larvae of May or June beetles. They may be 1/4 inch to over 1 inch in length, and are typically curved into a "C" shape. Most root feeding occurs in the second year of the life cycle. In most cases, the number of second year grubs will only be great enough to justify control once every three years.
White grubs feed on roots of plants causing wilted and dead seedlings that result in reduced plant stand. When in corn, inspection of these areas will often reveal 2 or 3 grubs per plant. White grub populations are sometimes noticeable during spring tillage operations.
Thresholds for Corn:
Treatment is recommended when sampling indicates an average of one or more white grubs per square foot are found. The following sampling procedure provides treatment decisions based on this guideline.
Soil sampling . . . Sampling in late summer or early fall, before a freeze, provides a more reliable estimate of populations than spring sampling just before planting. Take soil samples, one square foot in size to a depth of 8 inches. Begin taking samples 45 yards from shelterbelts. A total of 30 samples per field, randomly spaced along the shelterbelts are necessary. If at least a single grub is found in less than 40% of the samples, treatment may be required, but only out 20 yards from the tree line. If 40 to 60% of the samples are infested, treatment is needed to this distance and maybe as far as 65 yards. If greater than 60% of the samples are infested, treatment may be needed out to 90 yards from the tree line.
For more information see the Field Crop Insect Management Guide at http://www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm
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