How Does Late-planted Corn Change the Way You Address Pest and Disease Pressures?

LeRoy Toohey

May 31, 2019

As if planting late wasn’t enough to stress farmers out, now we need to consider how outside factors might be magnified by the delayed start to the 2019 crop.

Let’s take a look at how the impact of insects and diseases may be different in later planted corn fields.


Insect pressure on  late-planted crops is always a concern. Insects generally enjoy more succulent plants on which to feed and lay their eggs. Typically, late-planted crops will be smaller when destructive insects (such as the armyworm or stalk borer) move in. In a normal year, plants are larger and can handle some feeding injury without reaching economic thresholds. Smaller plants, however, can be greatly impacted by larval feeding. Since small plants can’t handle as much foliar loss, this can quickly impact yields.


Wireworm infestations can greatly be impacted - positively and negatively - by planting date. Delayed corn planting can cause wireworm larvae to move deeper into the soil as temperatures warm, taking them deeper than the seedling root zone. However, if corn was planted early and development is slowed due to cool, wet conditions, you may expect to see increased injury - potentially requiring an insecticide treatment.

White Grubs

Annual white grubs, known as Japanese beetles, typically complete pupation by late May or early June. The potential injury to late-planted corn is minimized the later the planting season goes. However, corn planted back to these fields in 2020 will be at greater risk for Japanese Beetle injury, because more adults will be present to lay eggs - which will hatch to attack corn roots in 2020. The true white grubs, known as June beetles, have a three-year life cycle, so they may feed on the root system all summer long in the second year of their life.

Pro Tip: Make a mental note to not skimp on seed treatments that control root feeding insects in 2020.

Corn Rootworms

Western corn rootworm larvae typically begin to hatch in the Corn Belt between Memorial Day and early June. Cooler temperatures generally slow down larval development, which can impact  late-planted corn. However with normal or slightly above normal temperatures, we may see a slight reduction in overall root feeding, if larvae hatch prior to planting corn. Keep in mind that corn planted back to these fields in 2020 will be at greater risk for rootworm injury, as there may be more adults present in late-planted corn in 2019 – similar to the concern mentioned above with white grubs.

Adult rootworms are attracted to  late-planted corn, due to fresh silk and pollen on which to feed. Adults will move from earlier planted corn into more immature fields, possibly causing elevated injury levels. To protect pollination, apply insecticide if silks are clipped to 1/2-inch or less from the ear and pollination is less than 50% complete. This usually requires approximately five beetles per plant. For corn-on-corn rotations, apply insecticides as a seed treatment or in-furrow application.

Black Cutworm

Late-planted fields which had tillage are more susceptible to black cutworm injury this year than in normal years. Winter annual weeds elevate concerns for a higher potential for black cutworm injury. Look for early signs of leaf feeding to assess the potential injury thresholds. A basic threshold for black cutworm is 2-3% of the plants cut or wilted when the larvae are less than 0.75 inch long.

And the other usual suspects…

Corn earworms, corn leaf aphids and fall armyworms migrate into the Midwest each year from the warmer climates in the South. With late-planted corn, pollination will be later, possibly landing during periods of hot and dry conditions - putting more stress on the plants from insect feeding. We don’t know for sure how the late planting may impact populations of these insects, but we may reach economic insect densities due to the late silking and pollination.

  • The threshold of corn earworms in field corn is not well understood, but within sweet corn fields, it is an average of five corn earworm moths per night in pheromone traps when green silks are present. Stop applications when 90% of the silks turn brown.

  • Corn leaf aphids generally show up in mid-June and run through August. With  late-planted corn, infestations may run into September.

  • Insecticide treatments for fall armyworms is usually not economical for control. However, treatment may be advisable if 75% of plants exhibit whorl feeding damage when larvae are less than 1.25 inches long. Feeding occurs from mid-July to mid-October.


We could see a greater impact from leaf diseases like gray leaf spot, northern corn leaf blight and common rust, as well as a whole host of others. Typically, these types of fungi occur in the mid-to late-stages of corn development. Many years these pathogens do not meet thresholds for applying fungicides, but with late plantings the impact will appear much earlier on the plants. We will want to keep a close eye and apply fungicide in a timely manner to ensure we have enough leaf area to fill the ear.

Gray Leaf Spot

Warm and humid conditions are favorable for gray leaf spot development. Genetic resistance is helpful, but may not be enough to protect the plant. Scout fields just prior to tasseling in order to determine pressure. At early dent stage, plants with 6-25% leaf area affected can potentially cause a 2-10% reduction in yield, so prevention is key.

Northern Corn Leaf Blight

Northern corn leaf blight develops during periods of high humidity and moderate temperatures. Its classic cigar-shaped lesions show up late in the growing season. When summers are cooler and wetter than normal, expect earlier development.

Common Rust

Common rust likes cooler weather during infection. Common rust can be seen early in the season, but this is dependent on how quickly it blows in from the South. Immature leaves are more susceptible to infection, so late-planted corn may be at greater risk than early planted corn. In most years, hybrid field corn does not require treatment, but keep an eye on late-planted, immature corn.

Just Keep Scouting

It already feels like it has been a long growing season, but we have a long way to go to maximize our remaining yield potential. Remember to scout your fields a bit later into the growing season than in previous years, because  late-planted crops will be susceptible at later stages than normal. Lastly, all overwintering insects may be at an elevated population in 2020, so keep this in mind when making your plans for next season.

ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. It is a violation of federal and state/provincial law to use any ag chemical or crop protection product other than in accordance with its label. It is your responsibility to confirm prior to purchase and use that a product is labeled for your specific purposes, including, but not limited to, your target crop or pest and its compatibility with other products in a tank mix.


LeRoy Toohey

May 31, 2019