What's in Your Fields?

Insects and Weeds Ready to Steal Yield

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Scouting early and often will help you catch weeds under 3 inches tall and spy early season pests like this bean leaf beetle damage on a soybean seedling. (Photos by Pamela Smith)

ROCKVILLE, Md. (DTN) -- This week, Dayton, Iowa, farmer AJ Blair has been cruising his soybean fields on a 4-wheeler, hunting for escaped weed patches to target with his post-emergence herbicide pass.

In central Kansas, Justin Knopf and his family have logged miles through their young cornfields looking for cutworms and grubs. When their soybeans are up, they'll add bean leaf beetles to the list.

Growers might be ready for a break after the frantic pace of springtime spraying, tilling and planting. But summertime weed and insect scouting starts immediately after plants emerge, entomologists and weed scientists warn.

"It's hard to scout too early," said University of Illinois weed scientist Aaron Hager. "I don't think folks realize how fast and how soon they can begin to lose yield to weeds."

Scouting for weed escapes from pre-emergence and post-emergence herbicide applications will be critical this year, as herbicide resistance expands its reach nearly every year, Hager noted. Depending on your local weather and field conditions, early season pests like bean leaf beetle, cutworms, armyworms, grubs, wireworms, and slugs should also be on the scouting checklist, University of Michigan entomologist Chris DiFonzo said.

SCOUT OFTEN, SPRAY EARLY

In Iowa, Blair fights glyphosate-resistant waterhemp populations. Down in the Sunflower State, Knopf has an even uglier foe: glyphosate-resistant Palmer amaranth. Both rely on burndowns and timely applications of pre-emergence residual herbicides to get control early in the season.

But spraying is only half of the equation, Hager added. Residual herbicides -- and their application rates -- are affected by a range of factors including moisture, soil texture, organic matter, and ph levels.

Never assume your application has done the trick, Hager said. Scout soon after spraying and look for signs of resistance: A lone weed species left standing, unaffected or injured weeds that have resumed growth, and live plants side by side dead ones.

Both Blair and Knopf were able to get into their fields for timely herbicide applications this year, and scouting trips have shown clean fields so far. But last year was a different story, Knopf noted. Heavy rains in May and June kept many farmers out of the field and allowed weeds to flourish. Those growers will face dense seedbanks this year.

New types of herbicide resistance are cropping up to add to the problem, Hager added. Last year, PPO-resistant Palmer amaranth populations were confirmed in the southern states of Arkansas and Tennessee.

This type of resistance hasn't been reported in the Midwest yet, Hager noted. "But just because we haven't found PPO-resistant Palmer doesn't mean it isn't here," he added. "It could very well be here."

For now, Midwestern growers will find plenty of problems with glyphosate-resistant waterhemp and marestail, as well as PPO-resistant waterhemp, Hager said. As always, try to get into fields to spray before weeds top 3 to 4 inches -- or the size of a soda can.

DON'T BET ON BT TOO HEAVILY

Neonicotinoid seed treatments have put a dent in early season insect damage, and Bt crops that protect against worms and caterpillars are widespread.

That doesn't mean you can let down your guard entirely, warned DiFonzo. "Even Bt corn can suffer damage under heavy pressure by cutworms and armyworms, for three reasons," she told DTN in an email.

First, while most Bt proteins target black cutworm, not all will work against armyworm. You can use DiFonzo's Handy Bt Trait table to determine which traits are in your hybrids: http://www.msuent.com.

"Under heavy pressure, many larvae taking many small bites can add up to damage" before the insects die from the Bt they ingested, DiFonzo added. Finally, larvae that started out on weeds or cover crops may be large enough to survive bites of Bt corn plants they moved onto.

Growers who opted out of Bt traits this year will need to be especially vigilant. Rootworm hatch is underway and earworm moths have been reported as far north as Illinois.

Because he rotates corn and soybeans faithfully, Blair opts out of rootworm Bt proteins in his corn. He also opts out of above-ground Bt traits in some fields. To bolster his below-ground insect protection, he uses soil insecticides at planting; for above-ground pests, he relies on regular scouting and Iowa State Extension reports on which pests are emerging across the state.

"It seems like all of June is just out looking at fields every couple days," Blair said. "We're just always looking for problems."

EVALUATE YOUR INSECT RISKS

Certain conditions -- cool temperatures, abundant moisture, delayed planting, and slow germination --- can leave young crops more vulnerable to seedcorn maggots, grubs, wireworms and slugs, both before and after emergence, DiFonzo said.

"These pests all feed on seeds germinating underground, creating gaps in the stand. In worst cases, entire portions of fields may be lost," she wrote. "After emergence, grubs and wireworms feed on new roots and slugs feed on leaf tissue."

DiFonzo has created a checklist table for farmers to easily and quickly evaluate their fields' risk of damage from these early season pests:

Early-season insect infestation risk checklist army-worm black cut-worm Seed-corn maggot slugs white grubs Wire-worm springtails (beets)
*Factor
= how does it favor a pest
*no- or reduced-till x x x x x x
= heavier crop residue; low soil disturbance
*heavy crop residue x x
= shelter; food source; poor closure of seed
Furrows; delayed emergence
*dense low-growing winter annuals x x x
= egg-laying sites; becomes green decaying
organic matter after herbicide application
*grassy weeds or rye cover crop x x x
= egg-laying sites
*herbicide application at or just after planting x x x
= forces insects to move to the crop
*recently incorporated fresh manure x x
= attractive for egg-laying to flies and
one scarab species (Aphodius dung beetles)
* green decaying organic matter, i.e. a x
Recently-incorporated cover crop
or weed mat
=attractive to flies for egg-laying
*cool conditions x x x x x x x
= delayed emergence
*moist conditions or wet areas x x x x x x
= poor closure of seed furrows; slow
emergence; keeps critters moist and happy

DiFonzo's insect infestation risk checklist table can also be found here: http://bit.ly/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/GH/CZ)

Emily Unglesbee