Subscribe and stay up-to-date with the latest news and great offers from DEKALB, Asgrow and Deltapine.
Don't miss out on the latest agronomic news.
Local agronomic alerts.Delivered straight to your inbox.
There are several mid-season insects that can infest a corn crop. Assessing mid-season corn insect damage should include observation of plant symptoms and insects present. Many mid-season corn insects feed on leaf material, which can reduce the surface area available for photosynthesis.
Figure 1. Severe armyworm feeding on corn.13;10;
When inspecting corn for mid-season insect damage, look for both plant symptoms and insects present. Included below are descriptions of plant damage and the insects responsible. If specific damage or insects are found, the appropriate insect thresholds and control measures should be applied.
Armyworms are named for the way they move as a group across the ground in an army-like fashion. They migrate together in search of food sources, consuming available host plants in their path. True armyworm (Pseudaletia unipuncta Haworth) tend to be a problem in grassy, weedy fields. When herbicides begin to control weeds, the larvae move to host plants such as corn. Fall armyworm (Spodoptera frugiperda Smith) tend to be more of a problem in late-planted corn.
Figure 2. True armyworms.13;10;
True armyworm (TAW) 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. The larvae are easily viewed on plants and many may feed upon the same plant. Larvae are about 1½ inches (38 mm) long, vary in color from dark greenish-brown to black, and have long white, orange, and brown stripes running the length of each side of the abdomen. The head is a yellowish-brown color with a mottled appearance (Figure 2).
Fall armyworm (FAW) larvae feed deep inside the whorls, leaving behind large, ragged-edged holes in whorl leaves. Holes may be 1 or 2 inches across and often mirror each other on opposite leaves that have emerged from the whorl. Severe feeding may actually resemble hail damage. Larvae grow to about 1½ inches (38 mm), have smooth skin, and vary in color from light tan or green to almost black (Figure 3).
The larvae have a predominant white, inverted ‘Y’ between the eyes on their head, thus distinguishing them from other armyworms.
Figure 3. Fall armyworms.13;10;
The European corn borer (Ostrinia nubilalis Hübner) is a significant economic pest of corn. Plant injury from European corn borer (ECB) occurs when their boring damages vascular tissues, disrupting the flow of water, sugars, and nutrients.
Figure 4. Figure 4. ‘Shothole’ pattern on emerging corn leaves from European corn borer feeding within the whorl.13;10;
Figure 5. European corn borer larva.13;10;
Newly hatched larvae usually feed deep within the whorl, removing shallow layers of leaf tissue creating thin ‘windows’ in leaf tissue. As they grow, larvae can chew completely through leaves and emerging leaves may exhibit ‘shothole’ patterns (Figure 4). The ECB larvae can also tunnel into leaf midribs, leaf collars, and later into ear shanks. Sawdust-like frass accumulated at the entrance of a hole indicates ECB presence. Larvae are creamy to grayish in color, about 1 inch (25 mm) long, and exhibit subtle rows of small brown spots running the length of their bodies (Figure 5).
Western corn rootworm (Diabrotica virgifera virgifera LeConte), northern corn rootworm (Diabrotica barberi Smith & Lawrence), and southern corn rootworm (Diabrotica undecimpunctata howardi Barber) are important corn pests.
Figure 6. Corn rootworm larvae.13;10;
Corn rootworms (CRW) can cause damage as both larvae and adult beetles. 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 strips on corn leaves that turn light gray. Leaves that experience heavy feeding may split or fray.
Figure 7. Western corn rootworm beetle.13;10;
Larvae are white and slender, ½ inch (13 mm) long, have a brown head, and have a dark plate on the top side of the ‘tail’ end (Figure 6). The adult western CRW beetles are yellow to green colored with a black stripe along the sides of the wing covers (Figure 7), 5/16 inch (7.5 mm) long, and the male wing covers are typically darker colored than those of the females. Northern CRW beetles are tan to pale green, ¼ inch (6 mm) long, and males and females are similarly colored, but females are longer and have a larger abdomen than males. Southern CRW beetles, also called spotted cucumber beetles, are 3/8 inch (9 mm) long, yellow to green, and have 11 spots on their back.
Common stalk borer (Papaipema nebris Guenee) damage to corn appears as large, ragged holes in leaves from their feeding inside the whorl. 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.
Figure 8. Common stalk borer. James Kalisch, University of Nebraska, Bugwood.org.13;10;13;10;
Common stalk borer larvae exhibit distinct longitudinal white stripes interrupted by a purple midsection and an orange head with a black stripe. Mature larvae are around 1¼ inches (32 mm) long (Figure 8).
Grasshoppers (Melanoplus spp.) feed on corn leaves, and large populations may consume all leaf material except for the tough midrib (Figure 9). Grasshopper feeding may look similar to armyworm feeding, except that armyworm feeding will begin on the bottom leaves and progress upwards, whereas grasshoppers rarely begin feeding on the bottom leaves and show no pattern to feeding. Damage from grasshoppers is most likely to occur during dry years when grasshopper populations are high.
Figure 9. Grasshopper feeding on corn.13;10;13;10;
Grasshoppers can be up to 1¾ inches (44 mm) long when mature and are brown to grayish-green. Nymphs appear very similar to adults, except for a smaller size and the absence of fully developed wings.
Damage caused by spider mites can result in substantial economic losses due to decreased yield and increased costs of control. Spider mites can regularly infect up to 50 percent of corn produced on the Texas High Plains, which can lead to a 20% or more decrease in yield potential. With chemical control expenses being as high as $25 per acre it is important to regularly scout for infestations.
Figure 10. Spider Mite damage in corn. Daren Mueller, Iowa State University, Bugwood.org.13;10;13;10;
Spider mites feed on the undersides of leaves, causing a burned appearance (Figure 10) and ultimately killing the leaf. Infestations are the most significant after tasseling and during grain fill, but due to difficulty with application and coverage, it may be best to scout and make an application prior to tasseling.