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Stop These Early Season Pests

21:25PM Mar 08, 2018

They fly, crawl and jump through fields, stealing precious bushels with every bite. Catch these thieves before they affect your stands and reduce yield potential.

“At today’s prices, it will take a little more damage to justify treatment or replant,” says Eric Eller, crop consultant at ForeFront Ag Solutions. “There is a lot of math to it, but much of it boils down to getting in the field and determining how much damage you have.”

When you’re scouting, know what to look for. Pest threats change throughout the season, so it’s important to learn what each pest looks like, what damage they cause and what constitutes treatment.

Keep a close watch for wireworms, grubs, armyworms, seedcorn maggots, black cutworms, possibly slugs and bean leaf beetles. This time of year, most problematic pests are in the larvae phase, which means you’ll need a trowel and mud boots when you head out to the field.

“In corn, I focus on seed-attacking insects first,” says Brad Van Kooten, DuPont Pioneer technical services manager. “Wireworms, white grubs and seedcorn maggots are the primary insects of concern.”

Wireworms attack both corn and soybeans, so keep an eye on each crop. Click beetle larvae eat the seed as well as the seedling. Weedy fields, especially grass weeds, can be at a higher risk of larvae attack. Damage is less severe the first year wireworms appear in a field but it gets worse in subsequent years.
“Wireworm is a slender, brown, worm-like but hard-bodied insect that lives and feeds in the root zone,” says Christa Ellers-Kirk, BASF technical marketing manager. “It feeds on seeds leaving skips in the row or on roots leaving wilted or dead plants.”

Check for the pest in bait stations in at least five areas of the field; bury a mix of untreated corn and wheat seed (a handful of each) 6" deep and then cover the site with black plastic to heat the soil. The digging method requires you to dig 2'x1' wide by 6" deep (one cubic foot) in five areas of the field 10 days before planting. Inspect the soil on cloth or plastic, and count the number of live wireworms. There is no rescue treatment but seed treatments are common.

Look for higher populations of white grubs following years with high infestations of Japanese beetles. This beetle lays eggs in soybean fields, and hatchlings attack corn the following season.

“When you think about belowground insects, white grubs are one of the more widespread pests,” says Davie Wilson, Monsanto seed treatment product development manager. “It’s hard to model if it will be a bad grub year or not, but based on how cold winter was it probably won’t be a terrible year [for the pest].”

White grubs attack from April to mid-June (emergence to about V4). They eat plant roots causing wilting and death. They’re easy to identify: Look for white “C-shaped” larvae with brown heads, according to Purdue University. Two or more grubs per cubic foot could be cause for treatment, such as a seed treatment or a soil-applied insecticide. There are no rescue treatments.

Seedcorn maggots can be suppressed with preventative measures such as seed treatments, which are especially useful when fields are at a higher risk. “We see more seedcorn maggots when growers have residue or cover crops,” Van Kooten says. “We like those conservation practices, but it means farmers need to be more vigilant when scouting.”

If you see yellowish-white maggots that lack defined heads and legs they’re likely seedcorn maggots. They’ll attack crops from planting to mid-June (emergence to V4). When you see skips, dig up seeds and look for holes the seedcorn maggots create. There are no rescue treatments. Seed treatments are useful in fields with manure or residue. If infestation is severe, you might need to replant.

A jet stream from the South could mean you need to keep a closer eye out for black cutworms. “I pay attention in March and April to what way the jet stream is coming, and I watch what friends from the South are posting on social media,” Eller explains. “Some of the universities post information, too. Use this information to inform your scouting.”

Cutworm moths look for fields with vegetation when flying in, so fields that have cover crops or weeds are most at risk. Look for grayish-black worms near the base of seedlings. Consider an insecticide if 3% to 5% of plants have damage and two or more larvae are found per 100 plants.

Armyworm isn’t a typical consideration during emergence and early growth stages, but in some areas this pest can cause stand damage. They are bright green when small and change to a dull green or brown as they grow. The adult moth has light brown wings with white dots on each forewing.

“It will feed at night and hide in debris or the plant’s whorl during the day,” Ellers-Kirk says. “To scout, choose a minimum of five locations with 20 plants per location. If 50% of the field is infested a treatment might be necessary.”

Slugs are increasingly an issue to consider, from emergence through V8. They leave ragged holes in crop leaves, sometimes making them look shredded. While the pest is troublesome, there is no actual economic threshold. Scout for damage, and if it’s severe enough consider a replant. Tillage disturbs their habitat and can decrease your risk of replant.

Each of these pests surely attacks corn, and some will give soybeans a try as well. Bean leaf beetles, however, favor the legume for which they’re named.
“There are always pockets of bean leaf beetle damage—much of it depends on the winter we’ve had,” Wilson says. “They feed on soybeans and don’t like much else. If I planted early and have the only beans up in the country, I expect the beetles will go to my fields like a magnet.”

If your soybeans come up at the same time as your neighbors’ there is a good chance you won’t reach economic threshold for spraying: prebloom, 45% defoliation; blooming to pod fill, greater than 15% defoliation; full pod to harvest, greater than 25% defoliation; and pod feeding when 10% pod damage with 10 or more beetles per row foot. There are various insecticides available to control bean leaf beetles as well as some seed treatments to help defend against early feeding.

Scouting for insects starts at planting—when the seed hits the ground. Target your efforts based on weather patterns, field history, crop rotation and specific threats for time of season. Take action when warranted to reduce damage to your stand and harvest potential.