While assessing corn stands for black cutworm and armyworm feeding, you should also watch for wireworm activity. If you do find wireworms injuring your young corn plants and reducing your stand, you can’t do anything about it this year. Unfortunately, there are no rescue treatments available for wireworm. But if you do find wireworms injuring your young corn stand, such knowledge will come in handy the next time you plant corn in that field.
Prior to planting corn is when you should check to see if wireworm is present in the field, says Iowa State University Extension entomologist Erin Hodgson. If the pest is present, you can apply an insecticide at planting time to control wireworm, or you can use an insecticide treatment on the seed you plant.
Wilted and stunted plants are a sign of wireworm injury
Injury from wireworms can leave young corn plants looking wilted or stunted, sometimes even leaving gaps in stands, says Hodgson. She, along with Rebecca Ahlers, an Iowa State University Extension field agronomist in south-central Iowa, provide the following wireworm management information and scouting tips.
Last week (May 25-29, 2015) some wireworm activity was noted in south-central Iowa. Ideally, scouting for wireworms should occur prior to planting (click here and read the article on ISU’s ICM website at imp.iastate.edu. That’s because there are no effective rescue treatments for wireworm. However, most people don’t see the impact this pest can have on a corn stand until corn plants emerge. While assessing corn stands for black cutworm and armyworm feeding, you should also look for wireworm activity.
How to tell: what is a wireworm, and what isn’t?
Wireworms are slender larva and range from ½ to 1½ inches in length (see Photo 1). There are soft- and hard-bodied wireworm species that are pale white or shiny yellow to brown in color. Wireworms have three pairs of legs near the head and lack any fleshy, abdominal prolegs. There is an obvious pair of appendages on the last abdominal segment that form a “keyhole.”
Photo 1: This hard-bodied wireworm was found in a field near Ottumwa, Iowa, on May 22, 2015. Photo by Rebecca Ahlers.
Wireworms are the larvae of click beetles. Female click beetles lay their eggs around the roots of grasses, which is why wireworms can be problematic in fields with a history of sod. Wireworms can also be found in land that has transitioned from CRP or pasture. It is unclear why wireworms are present in fields without a history of grasses growing in them. Wireworms can have an extended development and may live in the soil for up to six or seven years where they feed on the roots of plants, particularly grasses.
This pest likes to feed on early-planted corn, in chilly weather
Wireworm feeding commonly occurs when corn is planted early and the weather turns cold, slowing germination. They are more likely to be found in well-drained soils on ridgetops or hillsides. Wireworms feed on the germ of corn kernels, hollowing out seeds sometimes only leaving the seed coat. This will result in gaps in the rows.
Wireworms will also feed on the underground portion of the root or stem of young corn plants by tunneling into them (see Photo 2). Plants with this type of feeding usually appear stunted or wilted compared to surrounding plants. As soils warm up, wireworms will move deeper in the soil profile, posing less of a threat to corn seedlings.
Good reasons to sample fields for wireworm prior to planting
Fields with a history of wireworm injury should be sampled prior to planting corn. Unfortunately, there is no rescue treatment available for fields with significant stand loss from wireworms. Instead, insecticide treatment needs to occur before or at planting time. Treatment options include using a seed treatment insecticide or using a soil-applied insecticide.
If a field has significant stand loss, replanting the corn is an option. Table 1 may help in making a replant decision. Consider the seed and insecticide costs associated with replanting as well as the extended weather forecasts and the hybrid maturity before making a replant decision.
Table 1: Relative yield potential of corn by planting date and population
Rebecca Ahlers is an ISU Extension field agronomist in southeast/south central Iowa. She can be reached at [email protected] or by calling 319-643-811. Erin Hodgson is an associate professor of entomology with ISU Extension and research responsibilities; contact her at [email protected] or by calling 515-294-2847.