Corn Mid-Season Insect Identification

KEY POINTS

  • Mid-season insects that can be a concern to farmers in North America in corn include armyworms, European corn borer, corn rootworm, stalk borers, and grasshoppers.
  • Many mid-season insects feed on corn leaves, thereby reducing the amount of leaf surface area available for photosynthesis.

Descriptions of plant damage caused by common mid-season corn insect pests and the insects responsible appear below. If crop damage or the number of insects reach economic thresholds, the appropriate control measures should be applied to protect yield potential.

Armyworms

Armyworms are named for the way they move as a group across the ground in an army-like fashion. True armyworm (Pseudaletia unipuncta Haworth) tends to be a problem in fields that are grassy and weedy. When herbicides begin to control weeds, the larvae move to host plants such as corn. Fall armyworm (Spodoptera frugiperda Smith) tends to be more of a problem in late-planted corn.

True armyworm remove leaf tissue from the edges, often eating everything but the midrib. Feeding begins on lower leaves and progresses upwards, with the whorl leaves being eaten last. Fall armyworm larvae feed deep inside the whorls, leaving behind large, ragged-edged holes in whorl leaves. Severe feeding may resemble hail damage. For more information on armyworms, click here.


armyworm

Figure 1. (Left) True armyworm vary in color from dark greenish-brown to black, with long white, orange, and brown stripes running the length of each side of the abdomen. The head is yellowish-brown with a mottled appearance. (Photo courtesy of Roger Schmidt, University of Wisconsin-Madison, Bugwood.org.) (Right) Fall armyworm vary in color from light tan or green to almost black. The larvae have a distinguishing white inverted Y between the eyes on their head.

armyworm

Figure 2. (Left) True armyworm feeding and (Right) Fall armyworm feeding.


European corn borer

European corn borer (ECB) (Ostrinia nubilalis Huber) larvae injure corn by boring into stalks and vascular tissues, disrupting the flow of water, sugars, and nutrients.

Newly hatched larvae usually feed deep within the whorl. As they grow, ECB larvae can chew completely through leaves and emerging leaves from the whorl may exhibit ‘shothole’ patterns. The ECB larvae can also tunnel into leaf midribs, leaf collars, and later into ear shanks. Their presence is indicated by sawdust-like frass accumulated at the entrance of a hole. For more information on European corn borer, click here.

armyworm

Figure 3. (Left) European corn borer eggs, larva, pupa, and moth. Larva are creamy to grayish in color with subtle rows of small brown spots. (Right) Shothole injury in a corn leaf.


Corn rootworm

Corn rootworms (CRW) can cause damage as both larvae and adult beetles. The western (Diabrotica virgifera virgifera Leconte) and northern (Diabrotica barberi Smith & Lawrence) are the two prominent species. In the Midwest and farther north, the southern corn rootworm (Diabrotica undecimpunctata howardi Barber) causes damage only as an adult as they cannot overwinter in these areas. Newly hatched larvae begin feeding on root hairs, but tunnel into roots as they grow and can chew them down to the base of the plant, resulting in extreme damage to plant vascular and structural systems. Beetle feeding results in long, narrow stripes on corn leaves that turn light gray. Leaves that experience heavy feeding may split or fray. For more information on corn rootworm, click here.

armyworm

Figure 4. (From left to right) corn rootworm (CRW) larvae, northern CRW adult, western CRW adult, and southern CRW adult. Larvae are white and slender with a brown head and a dark plate on the top of the ‘tail’ end. Northern CRW beetles are tan to pale green and males and females are similarly colored, but females are longer than males. Western CRW beetles are yellow to green colored with a black stripe along the sides of the wing covers, and the male wing covers are typically darker colored than those of the females. Southern CRW beetles, also called spotted cucumber beetles, are yellow to green and have 11 dark spots on their back. 

Common stalk borer

Common stalk borer (Papaipema nebris) feed inside the whorl causing large, ragged holes in leaves. When holes are several inches long, the leaves can break or be cut from the plant. Affected plants may grow abnormally, being twisted or bent over, and may not produce an ear.

armyworm

Figure 5. Common stalk borer larvae have distinct longitudinal white stripes interrupted by a purple midsection and an orange head with a black stripe. Photo courtesy of James Kalisch, University of Nebraska, Bugwood.org.


armyworm

Figure 6. Grasshopper feeding on corn. Grasshoppers are brown to grayish-green. Nymphs appear very similar to adults, except for a smaller size and the absence of fully-developed wings. Photo courtesy of University of Georgia Archive, University of Georgia, Bugwood.org.


Grasshoppers

Grasshoppers (Melanoplus spp.) feed on corn leaves, and large populations may consume all leaf material except for the tough midrib. Grasshopper feeding may look similar to armyworm feeding, except that armyworm feeding usually begins on lower leaves and progresses upwards, whereas grasshopper feeding shows no pattern. Damage from grasshoppers is most likely to occur during dry years when grasshopper populations are high. For more information on grasshoppers, click here​.

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