Preplant Wireworm & White Grub Assessments for Corn & Soybean

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Figure 1. Wireworms persist in the soil for several years and are most problematic when soils are cool and moist.

• Wireworms and white grubs are occasional early season pests of corn and soybeans
• Preplant assessments of these pests can be used to determine overall field risk for stand and yield loss
• Assessments should be made approximately 1-2 weeks before planting 
• Several seed treatment, soil, and hopper box treatments can be used to help control these pests in high-risk fields

Wireworms and white grubs  can occasionally be of economic significance in corn and soybean fields. These insect larvae feed predominantly on the roots of emerging seedlings.  In situations where feeding is severe, reduced stands, crop growth, and yield can occur.  Luckily, we can determine our risk for related yield losses by conducting simple preplant assessments.  Making preplant assessments can help strategize our management plan for particular fields, maximizing return on investment. This article will discuss wireworms, scouting, and potential management strategies.

What are wireworms and grubs and what do they look like?
Wireworms are the larvae of click beetles, whereas white grubs are the larvae of different scarab beetles including the Asiatic Garden beetle, May / June beetle, masked chafer, and Japanese beetle. Both wireworms and grubs are found in the soil and become active when soils start to warm around April.  Wireworms are thin, yellow to brown, hard-bodied larvae ranging from ½ to 1 ½ inches in length and can have a larval stage that lasts 4-7 years (Figure 1).  White grubs can range from ¼ to over 1 inch in length, are white with a brown head, often are “C” shaped, and have six distinct legs (Figure 2).  Species can be identified by the pattern of hairs on the rear of their abdomen, or raster.  Most grubs only last a single year, but if grubs contain a “zipper” raster pattern, they will persist in the field for up to 3 years (Figure 3).

What does damage by wireworms and white grubs look like in the field?
Wireworms and white grubs  tend to be most problematic when fields of sod or pasture are being put back into corn/soybean production or in systems where small grains or grass cover crops are rotated with corn or soybeans. Wireworms are most active in cool, moist environments; therefore, early plantings may be at greater risk for damage.  Conversely, under warm, dry conditions, the larvae move deeper into the soil and do not damage germinating seedlings and developing plants.  Plants with wireworm or grub issues may die rapidly after emergence, or appear stunted, wilted, or discolored.  

How do I determine my risk for wireworm or white grub related crop damage?
High risk fields will be those where corn is  being planted early, behind sod, fallow, or a grassy cover crop or in fields where high numbers of Japanese beetles were present.  Fields where wireworms were detected the previous year, or 3-year grubs were detected the previous year, are also at elevated risk for related crop damage.  These fields should be a top priority for sampling.

How do I assess fields?
The easiest way to do a pre plant assessment for these pests is to pack a hand lens, shovel, and black trash bags and drive out to the field.  Sample fields about 1-2 weeks before planting by assessing at least 5-6 locations in a zig-zag pattern across the field.  At each location, dig up a 2 ft x 1 ft x 6 in deep hole and place the soil onto the black plastic bag.  Break up the soil and record the number of wireworms or grubs at each location.  If you do find grubs, examine the raster to look for the “zipper” pattern characteristic of the 3-year white grub.  If most of the grubs observed contain a zipper pattern, make sure to note that this field should be a priority for grub sampling the following season.  An average of 1 or more wireworms and 2 or more grubs per location would indicate a potential wireworm/grub issue.

For example, if I assess 5 locations in a field and find 1, 6, 3, 0, and 0 grubs, then my average grub count per location is:

(1+6+3+0+0) / 5 = 2 grubs = potential grub issue

Management options
Seed treatment insecticides (soybean) or soil-applied  insecticides may be considered when pre plant assessments indicate a potential problem or In situations where replanting is needed, and these pests remain active.  There are several soil insecticides, seed treatments, and planter box treatments labeled for wireworm and white grub control in corn and soybean.  Make sure that the rate used is sufficient for wireworm and white grub suppression, and of course that wireworm control is listed on the label.  Always follow label guidelines.  Contact your local FS crop specialist for assistance.     
 

Figure 2.  White grubs are larvae of scarab beetles with brown heads and six distinct legs.

Figure 3.  The presence of a “zipper” raster pattern indicates that this is a May/June beetle larva, which can persist in soils for 2-3 years.