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Sweet corn pests: the ones you'll notice versus the one to worry about

Author: Marissa Schuh, Integrated Pest Management Extension Educator, University of Minnesota Extension. Reviewed by Natalie Hoidal.

Some of the state’s earliest sweet corn is making its way to markets. Plantings will continue to tassel, silk, and size up ears for the next two months, meaning we are entering the key window for sweet corn pest control.  

The bugs you may see but shouldn’t worry about

Adult corn rootworm beetles feeding on silks. Photo: Natalie Hoidal.

Field crops extension educators are starting to detect the emergence of corn rootworm.  You might see these yellow and black beetles clustered on silks or even feeding inside pumpkins flowers.  These beetles can reach high numbers, but tend not to be an issue in sweet corn.  They do the most damage as larvae, feeding on corn roots (hence the name). Beetles lay eggs as the base of corn stalks, and larvae hatch the next year and feed on corn roots.  This means that rotation can effectively take care of corn rootworm in sweet corn. The key to rotation is this sense is just getting sweet corn in different ground, as larvae aren’t capable of travelling very far.

So, when you see adult corn rootworm beetles, don’t panic! While they may clip silks, it takes a high density of beetles throughout the field to do economic damage. There are other pests worth saving your sprays for, but if you are seeing extreme amounts of silk clipping, the Midwest Vegetable Production Guide has detailed management information.

Another pest that has historically been an issue, but now causes issues much less often, is European corn borer.  The widespread adoption of Bt corn has suppressed corn borer populations at a landscape level.  You don’t need to grow Bt corn to feel the positive effects.  A threshold for spraying for corn borer is when 10 moths are being caught in traps per night, which has only been reached once in the last three years trapping at multiple Minnesota locations.  

Current populations of European corn borer detected in UMN traps around the state. Via Veg Edge.

Corn borer larvae are small, with pale bodies and darkly colored heads. Photo: Adam Sisson, Iowa State University,

If you have had recent issues with European corn borer, keep an eye for foliar feeding starting the pre-tassel stage.  If 20% of the plants are showing corn borer feeding, spraying the late whorl stage may be justified.  See the Midwest Vegetable Production Guide for details.

European corn borer leaf feeding appears as a series of elongated, windowpane feeding. Photo: John C. French Sr., Retired, Universities:Auburn, GA, Clemson and U of MO,

The bug you won’t see (until a customer complains) is the biggest threat

Corn earworm can vary in size and color, and can be found feeding in the tip of an ear. Photo: Whitney Cranshaw, Colorado State University,

Corn earworm is the biggest sweet corn pest in Minnesota, with many factors in its biology making it hard to monitor and control.  Corn earworm doesn’t live in Minnesota year round, instead coming up from the southern states on weather fronts.  When moths arrive in an area, they are laser-focused on finding green corn silks.  Females settle and release sex pheromones, which attracts male moths.  Females lay tiny eggs (read:too small to scout for) on green silks.  Eggs hatch and caterpillars climb up the silks and into the developing ear, where they feed (and are totally protected from any insecticide populations.  

Let’s simplify how this biology makes corn earworm hard to manage.

  1. Adult moth populations can go from zero moths to hundreds of moths literally overnight.

  2. Eggs are not detectable, and caterpillars protect themselves from sprays shortly after hatching.

When taken together, this means our control window is literally the few inches of silk freshly hatched caterpillars will climb to enter the ear.

Figuring out when moths are laying eggs

You can get rough estimates of when these fronts might occur using Insect Forecast.  This online tool monitors weather conditions and overlays pest biology, giving us a rough idea of how a weather front might move corn earworm (and other moth pests) around.

The corn earworm forecast for July 14, with a low pressure system keeping moth flights from reaching Minnesota. Photo via Insect Forecast.

Models can only get us so far, and having more localized information will lead to better corn earworm management. The way we monitor corn earworm on the ground is with traps.  These traps are baited with the sex pheromon of female moths, which draws male moths into the trap.  The goal of the trap isn’t to reduce corn earworm populations, but to get a rough estimate of how many moths are present.  

University of Minnesota monitors corn earworm populations with traps around the state; the data they collect can be found on the VegEdge webpage.  

Corn earworm populations, as detected by pheromone traps. Via Veg Edge.

Just like rainfall can be highly localized, so can corn earworm moth drops.  A trap on your own farm is the best bet for getting an accurate idea of corn earworm populations, and is worth trying if you’ve had losses from corn earworm.  This series of videos from Michigan State University Extension covers the how and why of trapping, including how to translate trap counts into spray timing.

Making Spray Decisions 

Corn earworm biology limits the Integrated Pest Management strategies which are effective against corn earworm.

  • Cultural: Plant early maturing varieties and try to wrap up sweet corn production before corn earworm pressure ramps up later in the season. Genetically engineered sweet corn varieties provide good corn earworm control.

  • Physical: Remove harvested wormy ears before the corn reaches the customer.

  • Biological: There are species of parasitoid wasps that parasitize corn earworm eggs, but their commercial applications aren’t well-studied.

This means that chemical control is going to be the main tool for managing corn earworm.

Some growers just spray weekly for corn earworm, which may be far too many sprays, or not enough sprays, depending on what corn earworm populations are doing in a given year.  Corn earworm traps have been so studied that traps catches have fairly prescriptive spray recs attached to them.


Corn earworm spray decisions, diagram via Michigan State University Extension.

Pyrethroids are the most commonly used chemistry.  There is some level of pyrethroid resistance in corn earworm populations, so use a high rate, and consider using an alternate chemistry when populations are high.  For more information on chemical control, see the Midwest Vegetable Production Guide.

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