Early-season insect risk checklist for field crops

Certain conditions may result in an increased risk of early-season pest insect infestation in some fields.

In early May 2016, I warned of the risk for black cutworms and armyworms, both of which were moving north. Since then, bucket traps on the Michigan State University campus captured a “significant” number of black cutworms (more than nine over a two-night period) and quite a few armyworms as well. Given the amount of weed pressure, especially low-growing annuals, moths that make it up to Michigan will have no trouble finding egglaying sites in corn and soybean fields.

Initial infestations will be hard to see, amounting to just small holes in leaves. As larvae grow, damage is much more apparent, especially after weeds are killed in corn and soybean fields. The important thing is to scout fields to find wilting, cutting or ragged leaf feeding early enough that something can be done. Given limited time, scout fields that had the heaviest weed pressure, and look at conventional corn first.

Although not bullet-proof, most Bt hybrids have at least one toxin for cutworm and armyworm management. If an infestation is found, there are many insecticides that will do the trick as a rescue treatment with good coverage and proper timing. Armyworms also infest small grains, so don’t forget to check wheat fields too and apply a rescue treatment if needed. Armyworm outbreaks in wheat tend to be on a wider scale with most fields in a neighborhood infested.

Most Bt trait packages claim black cutworm control. Look for hybrids with the VIP protein (Agrisure Viptera, Duracade) or Cry1F Bt (Herculex, Optimum packages, SmartStax, Powercore). Some Bt trait packages claim true armyworm control. Look for hybrids with the VIP protein (Agrisure Viptera). In other words, know which Bts you planted. You can determine this with the Handy Bt Trait Table. MSU Extension recommends saving a representative sample of bag tags to help troubleshoot field problems later.

Despite the wonders of Bt, even Bt corn can suffer damage under heavy pressure by cutworms and armyworms for two reasons. First, larvae have to eat Bt to die. Many larvae taking many small bites can add up to damage. Second, larvae usually start out on weeds and cover crops and move to Bt corn later in life. Older larvae tend to be less susceptible to Bt toxins, thus harder to kill. In heavily-infested field, even a Bt hybrid can sustain damage and need a rescue treatment to save the stand.

More than just massive weed pressure this year, there are the additional negative conditions of cool weather, wet soil, delays in planting and herbicide applications, and slow germination in fields that did get planted. All of these factors combine to increase the risk of damage from seedcorn maggot, grubs, wireworms and slugs. These pests all feed on seeds germinating underground, creating gaps in the stand. In worst cases, entire portions of fields may be lost. After emergence, grubs and wireworms feed on new roots and slugs feed on leaf tissue (also, springtails feed on tiny sugarbeets), delaying seedling growth. In worst cases, plants may be killed, leaving more gaps in the stand.

There is little that can be done to manage these pests after planting. One key is to do a good job at planting, placing seed at the right depth and closing furrows. This is to prevent slug feeding and movement, and to get plants out of the ground fast. Then, favorable weather patterns have to hold so plants can emerge and grow quickly out of insect damage.

The checklist below reiterates conditions that increase risk for early-season insect infestation. Fields with many of these factors are the ones to keep an eye on in the next month for poor emergence, cut or wilted plants, leaf feeding and other damage.

Early-season insect infestation risk checklist.

Factor and how it favors a pest

Army-
worms

Black cutworms

Seedcorn
maggots

Slugs

White grubs

Wire-
worms

Springtails
(beets)

No or reduced-till. Heavier crop residue; low soil disturbance.

Risk

Risk

No risk

Risk

Risk

Risk

Risk

Heavy crop residue. Shelter; food source; poor closure of seed furrows; delayed emergence.

No risk

No risk

No risk

Risk

No risk

No risk

Risk

Dense, low-growing winter annuals. Egglaying sites; becomes green, decaying organic matter after herbicide application.

No risk

Risk

Risk

No risk

Risk

No risk

No risk

Grassy weeds or rye cover crop. Egglaying sites.

Risk

No risk

No risk

No risk

Risk

Risk

No risk

Herbicide application at or just after planting. Forces insects to move to the crop.

Risk

Risk

Risk

No risk

No risk

No risk

No risk

Recently incorporated fresh manure. Attractive for egglaying to flies and one scarab species (Aphodius dung beetles).

No risk

No risk

Risk

No risk

Risk

No risk

No risk

Green, decaying organic matter, i.e., a recently-incorporated cover crop or weed mat. Attractive to flies for egglaying.

No risk

No risk

Risk

No risk

No risk

No risk

No risk

Cool conditions. Delayed emergence.

Risk

Risk

Risk

Risk

Risk

Risk

Risk

Moist conditions or wet areas. Poor closure of seed furrows; slow emergence; keeps critters moist and happy.

Risk

Risk

Risk

Risk

No risk

Risk

Risk

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