Scout for Insects Now

Top Insect Threats to Newly Planted Corn and Soybeans

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Black cutworms are among the many early season pests that can cause stand and emergence issues at this time of year, especially for corn or soybeans planted in marginal conditions. (Photo courtesy John Obermeyer, Purdue University)

ROCKVILLE, Md. (DTN) -- Once again, farmers are busy laying out a large dinner buffet of freshly planted corn and soybeans -- but they don't get much control over the guest list.

The most likely party crashers of #plant22?

Expect the usual lurking soil-borne insects, such as white grubs, seed corn maggots and wireworms, as well as other early season troublemakers like bean leaf beetle, black cutworm and slugs, said University of Illinois entomologist Nick Seiter. And don't forget more sporadic pests such as grape colaspis, which can do great damage when they gather in large numbers.

Fields most at risk from insect damage this spring include early planted fields that are the only crops up, no-till or weedy fields, which can attract insects and slugs, but also fields of any kind planted into marginal conditions, Seiter noted.

"It's not that cool temperatures or wet soils are necessarily good for these insects, but the same amount of injury will hurt the plants a lot more if they are emerging slowly in poor conditions, rather than emerging rapidly in warmer conditions," he explained. A warmer, drier trend in parts of the Farm Belt might allow plants to rapidly outrun much damage, but slower emerging stands will need to be scouted carefully, he added.


Black cutworm moths have made their annual migration from wintering grounds in the southern U.S. and Mexico, and their hungry little larvae will soon be active in emerging corn and soybean fields.

University Extension scientists track the cutworms' American roadtrip each year via moth traps, which give growers a healthy heads up, given that the eggs the moths lay will hatch and start eating a few weeks after traps are filling up.

So far, entomologists from Ohio, Indiana, Iowa, Kentucky, Pennsylvania and Minnesota are all reporting significant catches in April and May, with some reporting above-average numbers. Big numbers of cutworm hitting small, late-planted corn stands could create some issues, Seiter noted.

He urged growers to carefully scout new cornfields, especially those that are no-till, or had heavy weed pressures or cover crops earlier this year. "One of the scenarios that favor them is to have a lot of winter annual weeds, especially broadleaves like chickweed and shepherd's purse," he explained. "That draws moths in to lay eggs, and once that growth gets burnt down, the larvae have to move over to something and it tends to be corn, although it can be soybeans as well."

See more on cutworm management and risks here:….


In no-till fields and lush, weedy acres, slugs are also a risk to growers in areas with plentiful moisture this spring, Seiter noted. As with cutworms, slow growth of a seedling will greatly enhance slug damage, so a turn towards dry, warm weather will be a farmer's best defense against this slimy pest.

Penn State University, housed in a state that sees high slug activity due to the high adoption of no-till agriculture, is a good source on slug management tactics. The university's PA Slug Project monitors populations and produces research on integrated pest management for this hard-to-treat pest, which -- as a mollusk -- doesn't respond to insecticides.

See more here:….


The bean leaf beetle is a special threat, as it can cause yield loss on both ends of a season, Seiter noted. For now, the first soybeans coming up will be most at risk from overwintering adults, which can feed on the emerging soybean plants and cause yield loss. Later in the season, their larvae will feed on soybean roots, and adult beetles will target leaves and pods.

See more from Illinois here:….

Likewise, grape colaspis has some unique quirks. Seiter estimates it reaches economic levels of population and injury only once every 10 to 15 years, so it can be a shocking nuisance when it happens, such as it did in 2018. And this pest poses the most stand damage to first-year corn after soybeans -- a rotation which normally protects crops from certain insect threats, Seiter noted.

First, grape colaspis adults are drawn to legumes, such as alfalfa and soybeans, where they lay their eggs in mid-summer. The newly hatched larvae feed on the legume roots that same season, before overwintering as partially grown larvae.

The following spring, they wake up -- usually in a rotated corn field -- hungry and far less picky about their diet, Seiter explained. "So in Illinois, we see the most problems in rotated corn, although if you have continuous soybeans, they can be really hard on the second year of soybeans in the spring, as well," he said.

See more from Illinois here:….


Last, but never least, is the annual gang of soil-borne insects that love to target young seedlings or seeds. The most common of these are white grubs -- the larval stage of various beetles such as Japanese beetles and May/June bugs -- seed corn maggots and wireworms.

White grubs and wireworms tend to be at their highest levels in cropfields that have recently been converted from pasture or sod production or housed cover crops regularly, Seiter noted. Seed corn maggots favor fields that had recent manure applications or lots of decaying crop residue.

See more on white grubs and seed corn maggot here:… and here:….

Wireworms are especially tricky to control or predict outbreaks for, given their multi-generational habits in the soil, he added. They are also not killed by the most common seed treatment insecticides, neonicotinoids.

For more on wireworm challenges and a newer seed treatment option from BASF, see this DTN article:….

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Emily Unglesbee