Corn Leaf Aphid
Fields in the seedling stage rarely need to be treated for corn leaf aphid. Yield losses have occurred only where the pest has reduced stands of seedling plants. Pre-tassel and later growth stages can tolerate large numbers of aphids without economic damage.
Cutworms are dingy, grayish-black, smooth “worms” that are the larval stages of several moth spe- cies. Most cutworms are active at night, when they damage seedling corn by cutting the stalk just above ground level. Grassy or weedy areas may harbor large numbers of cutworms. Most species hide in the soil during the day and are not visible on the plants.
When cutworms are dam- aging plant stands, applying insecticide by ground will usually give adequate control
(Table A8). For best results, apply the insecticides in the late afternoon. If the soil is dry, cloddy, or crusty at the time of treatment, control may not be as effective as in moist soil.
Southwestern corn borer
All types of Bt corn with toxins targeted at cater- pillars will control southwestern corn borer effectively; they do not need an insecticide for southwestern corn borer control (Table A9). However, as of this writing in late 2015, there is evidence that southwestern corn borer may be resistant to Cry1F in Arizona and New Mexico, so the use of a two-toxin (pyramid) Bt corn is recom- mended in case that resistance has spread to Texas.
Southwestern corn borer moths emerge from corn stubble in the spring to lay eggs on whorl-stage corn. More larvae typically feed in the whorl on the corn that is near unplowed stubble.
Eggs are laid on the upper and lower surfaces of expanded leaves in the whorl. Freshly laid eggs are creamy white; after about 24 hours, three red bands appear on each egg.
Small larvae hatch from the eggs in about 5 days and begin feeding in the whorl. As the leaves unfold, they will reveal the typical rows of holes across the leaf surface associated with whorl feeders.
Another leaf symptom commonly associated with southwestern corn borer feed- ing in the whorl is long, trans- parent areas (the windowpane effect) on the leaf where the young larvae feed only par- tially through the leaf tissue.
After the larva has fed in the whorl, it crawls down the plant and bores into the stalk. Corn borer larvae reach 1 to 1½ inches long. They have a regu- lar pattern of raised black dots on a creamy white body.
First-generation eggs and larvae are difficult to detect because they rarely infest more than 5 percent of the plants. However, if infestations are large enough to warrant treatment, apply insecticide (Table A10) before the borers leave the whorl and enter the stalk.
European Corn Borer
European corn borer was first discovered in Texas High Plains corn in 1978. Since then, the widespread planting of Bt corn has reduced
populations drastically. Euro- pean corn borer is effectively controlled with all types of Bt corn that have one or more tox- ins targeting caterpillars (Table A11). Economic infestations affect most corn-growing areas of the Texas Panhandle.
Borers overwinter as full- grown larvae in corncobs, weed stems, or other cornfield debris.
They pupate in May, and the first generation of moths emerges in late spring. The moths are first attracted to the dense vegetation around corn; they remain there for a few days while they mate.
Mated females return to the cornfields to lay eggs. They are attracted to the tallest fields (at least 22- to 35-inch extended leaf height). The eggs, 15 to 30 in a mass, overlap like fish scales and are normally depos- ited near the midribs on the undersides of the leaves.
The eggs hatch in 3 to 7 days. The larvae move to the whorl to feed before entering the stalk to continue feeding.
To determine whether you need to apply insecti- cide to control first-generation European corn borers, examine five random samples of 20 consecutive plants each. An insecticide application is justified if 50 per- cent of the plants are infested with an average of at least one live larva per plant (Table A12).
Lesser Cornstalk Borer
The lesser cornstalk borer occasionally attacks seed- ling corn. The small, slender larva remains in a silken tube in the soil; it injures plants by feeding on the crown area at the soil line.
These insects may occur in damaging numbers on sandy soils, and populations increase in dry conditions. Because rainfall and irrigation will kill many larvae, the tim- ing and amount of irrigation will influence control. Insec- ticides applied at planting for corn rootworms may control other soil pests such as lesser cornstalk borer (Table A13).
Applications of terbufos (Counter CR) for corn rootworms have suppressed lesser cornstalk borer. During the seedling stage, carefully inspect areas where this insect has been a problem. Base the treat- ments on plant damage and the presence of larvae. This insect does not usually affect larger corn plants.
In recent years, most borer damage to corn in the Lower Rio Grande Valley and along the Gulf Coast has been caused by the Mexican rice borer, sugarcane borer, and the neotropical borer (Fig. 3).
These borers typically attack corn before and after tassel. They feed on the leaves for a short time before boring into the stalks.
Sugarcane borer can cause whorl damage, stalk tunnels, shank damage, and grain-feeding injury. Yield losses are usually minor unless the stalk lodges. Sugarcane borer damage to kernels may cause a red coloration that makes the grain unmarketable for some purposes.
Bored stalks fall most often during ear filling or ear maturation, and lodging is often associated with high winds. The stalks may break at any point but not usually near the soil level as with southwestern corn borer infestations.
Transgenic Bt corn hybrids control these bor- ers well. Conventional insecticidal control is most successful if you scout the fields closely and treat before the larvae bore into the stalks.
Corn earworm and fall armyworm
Corn earworm and fall armyworm moths deposit eggs on leaves. Newly hatched larvae begin to feed in the whorl. Although larval feeding will cause the leaves to appear ragged, insecticide treatments of whorl stage corn seldom achieve economical control.
Flea beetles are very tiny, shiny black or greenish black insects that jump when disturbed. They range from a little smaller than a pinhead to several times larger.
Flea beetles feed on the leaves of corn plants. Dam- aged leaves have a whitened, bleached appearance. Plant growth slows as the leaves wilt and hang limp. Flea bee- tles are more likely to dam- age later-maturing corn.
Fields kept clean of weeds the previous season seldom suffer significant flea beetle injury. When enough flea beetles are dam- aging corn, an insecti- cide application may be necessary (Table A14).
Adult chinch bugs are about 1/6 inch long with black bodies and reddish-yellow legs. When fully developed, the white wings develop a triangular black spot near the middle of the back on the outer wing margin. When viewed from above, the insect appears to have a white X or a white hourglass on the back.
Adult and immature chinch bugs suck plant juices and cause the leaves to red- den. Damage by chinch bugs normally occurs from seed- ling emergence until the plants are 18 inches tall.
Large numbers of chinch bugs can move into a corn- field by crawling or flying from wild bunch grasses or small grains. Once in the field, they congregate and feed behind the leaf sheaths of
the corn plant and below the ground line on the plant roots and crowns.
In fields with a history of early-season, economi- cally damaging chinch bug populations (Fig. 4), seed treatments or at-plant, soil-incorporated insecticides can suppress the insect. Granular formulations may protect corn for 2 to 3 weeks if enough rain falls after the application to wash the insecticide off the gran- ules.
Closely monitor young plants for chinch bugs and feeding damage after germination, particularly during dry periods, even when you use seed treatments or at- plant insecticides (Tables A15 and A16).
Make at least five random checks in the field. Apply insecticide when two or more adult chinch bugs infest 20 percent of the seedlings that are less than 6 inches tall. On taller plants, apply insecticides when immature and adult bugs infest 75 percent of the plants.