Description

Adults are inconspicuous white moths about 3/4 in. in length and have no obvious patterns on the wings (Photo 5-20). Females are slightly larger than males. At rest adults fold wings almost parallel to the abdomen. Newly hatched larvae are less than 1/8 in. long and have a black head capsule and reddish tint on the abdomen. At maturity larvae are about 1.25 in. in length, have a reddish head capsule and possess numerous black tubercles on their integument (Photo 5-21). These spots and the consistency of off white color of the integument on both the dorsal and ventral sides of the larvae are the most obvious distinguishing characteristics of southwestern corn borer larvae. In contrast the spots are reduced and the dorsal integument is darker on European corn borer larvae.

Distribution, damage and impact

The center of southwestern corn borer activity in Arkansas is likely near West Memphis. Recent surveys have detected almost 100% of non-Bt field corn plants near Marianna and Marion infested with southwestern corn borers. As distance from West West Memphis increases frequency of southwestern corn borer generally decreases. However, damage is severe into Clay, Monroe and Woodruff Counties. Damage is similar to that caused by the European corn borer as described above. Loss of foliage by first generation southwestern corn borer larvae is generally minimal and has little direct effect on corn plants in Arkansas. However, some larvae may tunnel down through the whorl to the growing point of the plant. Leaf tissue above the point of feeding can turn white, a condition called “white heart”. If feeding is extensive the plant may be stunted or killed. This condition is called “dead heart” (Photo 5-22). When the southwestern corn borer first invaded Arkansas during the 1950's entire fields were lost to “dead heart”. Recent surveys of field corn in eastern Arkansas detected only low numbers of plants with “dead heart”. In these fields non-infested plants adjacent to plants with “dead heart” appeared to compensate for the lost plants and thereby limit yield loss. Stalk tunneling is often severe and as much as 50% of the entire length of stalk can be damaged. Yield losses are difficult to assess but use of the highly effective Bt corn has resulted in yield increases of up to 80 Bu/A. Tunneling by southwestern corn borer in stalks may weaken them to the point of breaking and falling across the rows . This is called “lodging” (Photo 5-23). In Arkansas lodging from southwestern corn borer has been common in the eastern and northeastern portions of the state. In 1998 lodging in some corn fields was about 30% of the total number of stalks. Lodged stalks slow harvest operations and yield can be significantly reduced.
Second and third generation southwestern corn borer larvae also feed on the ear shanks - the stem connecting the ear to the stalk. Near harvest affected ears droop instead of pointing upward like non-infested ears. In a field located at Marianna, 46% of the ear shanks were infested by corn borers in 1999. Although many of the infested ears are still harvested, many fall to the ground before or during harvest which significantly reduces yield.
Yet another way in which the southwestern corn borer affects corn yield is by reducing the size of kernels. Preliminary studies conducted during 1999 sought to correlate the extent of tunneling in stalks below the ear with kernel size. Although the effect was limited, kernel size was negatively correlated with increase in tunneling in stalks below the ear. Tunneling may reduce water and nutrient movement within stalks thereby limiting kernel development.

Life History

The southwestern corn borer overwinters as a mature cream colored larvae just below the soil line in the stem/root mass of field corn. Adults emerge from corn stubble in spring (May). Mating begins within one night from emergence and moths generally initiate egg laying or oviposition on the second night after emerging. Eggs are off white in color when laid and may occur singly or a small group (Photo 5-24). Individual eggs are very small, i.e., <1/16 in. across, elliptical and flattened on the leaf surface. Female moths may lay more than 300 eggs, however, under field conditions it is uncommon to find more than four southwestern corn borer eggs in a group. The eggs laid by overwintering corn borers represent the first generation of the year. Common plant locations to find first generation eggs are upper and lower leaf surfaces of young corn plants. Although eggs from subsequent generations may be detected on much of the plant, most will occur on leaves within the area located two or three nodes above or below the ear. About one day after being laid, each southwestern corn borer egg develops three reddish orange bars (Photo 5-25). Tiny black spots which are the head capsules of the developing larvae become visible after three or four days. Larvae within these eggs will emerge within about one additional day. The duration of the egg stage is dependent on temperature. Rolston (1955) reported that most southwestern corn borers hatched six or seven days after being laid in early July when temperature averaged 71°F. With average temperatures of 80 to 83°F, most eggs hatched in four to five days, respectively.
First generation larvae that complete development pupate within corn stalks and second generation moths emerge in mid summer. These moths behave similarly to those of the first generation by mating and ovipositing. By the time oviposition of the second generation occurs the corn plant is decidedly different in structure. At this time ears are often present. Second generation southwestern corn borer moths exhibit a strong preference for depositing eggs on foliage near the ear. About 80% of the second generation eggs can be found within three nodes above or below the ear. When these second generation larvae hatch, the corn plant offers much more structure for the young larvae to penetrate and hide within. The outer layers of ear shuck are often penetrated by young southwestern corn borer larvae. By searching for brown spots on the shuck and carefully removing the outer layer, young larvae may be detected.
Second generation larvae can develop within the ear, particularly the ear shank, but more commonly develop by feeding within corn stalks. Second generation southwestern corn borer larvae affect corn plants in several ways. Most commonly, larvae form extensive tunnels within stalks. When stalks are infested with multiple larvae, the tunnels can run from the soil line up through about 15 plant nodes. Just prior to pupation larvae can terminate a tunnel just below the stalk surface. They leave only the outermost layer of stalk which soon turns brown. This forms an exit hole. After larvae pupate within the tunnel, the adult moth pushes out through the exit hole and leaves the stalk to again mate and begin egg laying.

Southwestern/European Corn Borer management

Both the southwestern and European corn borers are managed with four methods.

1. Early planting

Corn planted early in the season may reach maturity before the second generation corn borer larvae can negatively impact yields. Planting dates vary with location, weather and ground preparation. In eastern Arkansas corn planted before April 15 can often avoid much of the damage caused by the second generation of corn borers.

2. Stalk destruction following harvest

Both the southwestern and European corn borers overwinter as larvae within old stalks. With environmental changes associated with spring, the larvae initiate pupation and emerging adults begin the cycle of egg laying. Research has consistently shown that stalk destruction can reduce overwintering corn borer populations by greater than 80 %. From the insect management view, the most effective method of stalk destruction is to mow and disc stalks shortly after harvest in late summer or early fall. This leaves the overwintering larvae on or near the soil surface where the effects of low temperatures and rainfall have their greatest effect on producing larvae mortality. Natural enemies including beneficial insects, rodents and birds also contribute to larvae mortality in stalks left on soil surfaces. In late winter prior to corn borer pupation the stalk residue should be turned under as much soil as possible. This makes it more difficult for the emerging moths to make their way out of the soil.

3. Soil and foliar insecticides

Much of the corn acreage in Arkansas receives a soil insecticide at planting. This may be directed at chinch bugs, rootworms, cutworms, or nematodes. The impact of currently registered soil insecticides on corn borers is not clearly understood. At present it is doubtful that currently registered soil insecticides directed only at corn borers provide economically acceptable returns. Additional research is currently underway to more clearly define the benefits of these insecticides.
Corn borers are susceptible to several insecticides applied to corn foliage. Difficulties with foliar insecticide effectiveness are associated with corn borer biology. First generation corn borers penetrate corn structures within about 10 days of hatching and second generation within about 7 days. After entering the plant corn borers are very difficult to target. Thus, foliar applications must be timed precisely to be effective. While the most effective method of timing insecticides is by scouting for corn borer eggs, it is a very difficult task. Use of pheromone trap collections may provide a good indication of when to initiate scouting. Another method of monitoring adult flight is to search for infested corn stalks. By visually inspecting the ground for insect frass and the lower portion of corn stalks for damage, corn borer larvae can be easily detected. Once found, the stalk can be cut with use of a large knife and carefully split. Both larvae and pupae can be detected. When pupae color changes from tan to dark brown, the moth will emerge within a day or two. Mating and egg laying begins one or two days after moth emergence and eggs will hatch in about five days. Thus foliar sprays should be applied approximately seven days from first detection of dark corn borer pupae. If scouting is attempted one method is to cut 20 plants from each of five locations in a field. Plants should be removed from the field and foliage searched. Applications should be initiated if greater that 25% of the plants are infested. For first generation corn borers, granular insecticides are often chosen. Granules likely drop into the whorl better than water based sprays. Granular insecticides are not effective against second generation corn borer. For second generation larvae sprays are the preferred choice. Spray volume is very important. In 1999 correctly timed foliar sprays provided acceptable levels of corn borer management when applied in 20 gpa. Another method of foliar insecticide application that should be successful is through center pivot irrigation where available.

4. Use of genetically modified corn hybrids

Although field corn has been bred for many years to resist attack by corn borers, the recent development of field corn with the gene that codes for the Bacillus thuringiensis (Bt) toxin has met with phenomenal success. Research in eastern Arkansas and numerous other states in recent years have demonstrated that Bt corn is highly effective against both southwestern and European corn borers. With current populations of corn borers, planting of Bt field corn will likely reduce corn borer impact to more than acceptable levels. Critical in use of Bt corn is managing its use to delay the development of resistance in corn borers to Bt. Insects have a well documented resistance to foliar applications of Bt. As highly effective as Bt corn is against corn borers, selection pressure is very high. Thus the corn producer is faced with insects that have developed resistance to similar Bt strains and are now placing substantial selection pressure on the same insect populations. Resistance development is only a matter of time. Currently the accepted method of delaying resistance development is to maintain an susceptible corn borer population through the planting of a portion of land with non-Bt corn. In addition to resistance development, the other major problem with use of Bt corn is with the public’s acceptance of genetically modified organisms (GMO’s).

Insecticide recommendations for corn borers on field corn

See Table 5-2 for insecticide recommendations current at time of publishing. Current updated recommendations can be found in the Cooperative Extension Service publication MP-144 and on the world wide web at WWW.cdms.net/manuf/default.asp. Always follow instructions on pesticide labels.