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Identify and Manage Common Corn and Soybean Insect Pests

July 14, 2023
Japanese beetles on damaged soybean leaves

Japanese beetles on damaged soybean leaves

As the summer progresses and drought continues to be a concern across the Midwest, some common pests, like soybean aphids, may be a larger threat as they thrive under drought conditions.

Corn and soybean pests can damage plant stems, leaves and the crop itself, potentially killing plants and/or reducing yield potential. Many insects also act as vectors for disease, which can further impact your plant health. Being able to properly identify and understand how common insect pests threaten your yield potential is key to managing them.

Corn Insect Pests

Corn Earworm

Corn earworm larvae

Corn earworm larvae

Many experts consider corn earworm to be the most devastating insect pest in corn. The larvae sometimes feed on corn silks, causing pollination issues, but most of the damage is caused when they bore into and feed on corn kernels. Here’s how to identify and manage corn earworm:

  • Larvae have some variations in coloration, but generally the head is orange or light brown, and they have dark stripes along their sides.
  • Corn earworm larvae, unlike European corn borer larvae, have small black spines.
  • The adult moths’ forewings are usually light brown and have a distinct dark spot in the center.
  • The best way to monitor corn earworm population levels is with pheromone traps, as eggs and larvae are hard to detect with normal scouting.
  • Tillage in the fall can help prevent over-wintering of the pupae.
  • Insecticides like Besiege® insecticide can reduce populations once detected. Besiege is a broad-spectrum, foliar-applied insecticide providing both rapid knockdown and long-lasting residual control of lepidopteran, sucking and chewing insect pests.

Corn Leaf Aphids

Corn leaf aphid infestation

Corn leaf aphid infestation

Corn leaf aphids are migratory pests, travelling north as the weather warms. They secrete a sticky substance called honeydew, which can interfere with pollination. Excessive feeding in the whorl can lead to barren ears or incomplete kernel development. Here’s how to identify and manage them:

  • They tend to be blue-green or grey, and their shed skin can lead to an ashy, moldy appearance on the leaf.
  • They usually begin to appear in mid-June or July.
  • When scouting, begin sampling for aphids in the whorl a few weeks prior to tasseling.

If aphids are found, an insecticide like Besiege should be applied quickly, as they can be significantly less effective after tasseling.

European Corn Borer

European corn borer

European corn borer. Photo provided by Adam Sisson, Iowa State University, Bugwood.org

European corn borer larvae will bore into the stems of corn plants and the midrib of leaves, causing leaves to fall off and other damage to the crop. Later generations then bore into corn ears, feeding on their kernels. Here’s how to identify and manage them:

  • Mature larvae are around 1 inch in length and are creamy to greyish in color. Dark brown spots run down the length of their bodies.
  • Larvae are spineless, which distinguishes them from corn earworms.
  • They overwinter as pupae, with the first generation of adult moths emerging in late May through early June.
  • They are often found in the whorls of corn stalks. Look for “shot hole” style damage in the whorl.
  • Corn hybrids can protect against this pest and prevent the need for insecticide applications, but if you use a nonresistant hybrid, insecticides are an option for effective protection.

Soybean Insect Pests

Bean Leaf Beetles

Bean leaf beetle

Bean leaf beetle

Bean leaf beetles feed on soybean foliage and pods and can be vectors for bean pod mottle virus, so large infestations can cause significant yield loss. Here’s how to identify and manage them:

  • Adults are oval-shaped and come in a variety of colors (yellow, green, orange, red).
  • They frequently have black spots on their backs, but always have a black triangle at the top of the wing covers.
  • Their eggs and larvae are found in the soil. Larvae will feed on plants roots and stems but have minimal impact on yield.
  • Start scouting at the seedling stage and look for defoliation.
  • If there is significant defoliation, an application of a foliar insecticide like Endigo® ZCX insecticide can help reduce populations. Endigo ZCX provides both knockdown and residual control with the power of 2 complementary modes of action.

Soybean Aphids

Soybean aphid infestation on a soybean leaf

Soybean aphid infestation on a soybean leaf

Soybean aphids are typically found from late May through August and extract plant juices through their needle-like mouths. This can cause plants to weaken, making them susceptible to other stressors such as drought or disease. Here’s how to identify and manage them:

  • They are small and yellow with black cornicles near the rear and are typically found on the underside of soybean leaves.
  • If lady beetles or ants are present, that is often an indicator of early-season aphid colonization.
  • They pose the greatest threat to yield when present at the R1-R4 stages.
  • When scouting, it is important to note populations of their natural predators such as Asian ladybird beetles or minute pirate bugs.
  • If there are enough aphids present to reach the economic threshold, an insecticide like Endigo ZCX is essential for maximizing yield potential.
  • When considering treatment with an insecticide, take into account the growth stage of the plants, other plant stressors and the populations of their natural predators. Treatment may not be required if plants are in later growth stages, stressors are low and/or there are many natural predators present.

Japanese Beetles

Adult Japanese beetle on a soybean plant leaf

Adult Japanese beetle on a soybean plant leaf

Adult Japanese beetles can skeletonize plant leaves, stripping away leaf tissue while leaving larger veins untouched. This can cause serious defoliation and threaten yield potential in many crops beyond soybeans, including corn. Here’s how to identify and manage them:

  • Adults are metallic green with bronze wing covers. The 6 white tufts on their undersides distinguish them from other scarab beetles.
  • Larvae overwinter deep in the soil but do not pose a significant threat to yield potential. They typically feed on decaying plant matter rather than live plant stems.
  • When scouting, look for skeletonized leaves and measure defoliation to determine if treatment is needed.
  • Insecticides like Endigo ZCX can work to reduce populations; however, highly mobile adults can move into treated fields.
  • Keep the populations of their preferred weeds and cover crops down to prevent infestation.

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