Corn earworm moths begin laying eggs on leaves and silks soon after tassel emergence. They begin laying eggs on emerging silks before pollination.
After hatching, the larvae tunnel into the silk channel to feed. Because the damaged silks have usually completed pollination, a loss of pollination is generally not a problem.
Later instar larvae feed on the kernels at the tip of the ear. Ear damage is usually minor, although an occasional field may have excessive damage.
Corn earworm control is difficult because the eggs are laid throughout the silking period and after pollination (brown silk stage). Because the untreated silks are exposed daily as they lengthen, insecticides must be applied often. Control efforts are usually costly and inconsistent. Currently, control strategies are not suggested in commercial field corn.
Southwestern corn borer
Although the southwestern corn borer has been a major corn pest on the High Plains (Fig. 3), the widespread planting of Bt corn has reduced its populations. It also occurs in Far West and Northeast Texas but is not economically important in these regions.
The larvae cause damage by tunneling in the stalk and later girdling the plant, which then lodges. In the spring, moths emerge from corn stubble and weed hosts to lay first-generation eggs on whorl-stage corn. On the High Plains, first-generation larvae mature and pupate in the stalk in July. Moths begin emerging about mid-July and lay eggs of the second generation, usually after tasseling has occurred. (See Fig. 3 for distribution).
About three-fourths of the second-generation eggs are laid on the upper surfaces of the middle seven leaves—the ear leaf and the two leaves above and four leaves below it. The eggs are laid singly or in masses of two, three, or more. The eggs overlap like fish scales or shingles. Freshly laid eggs are creamy white; 1 day later, three red bands appear across each egg.
The eggs hatch in about 5 days. Small larvae feed behind leaf collars and ears and beneath the shucks of the primary ear. Older larvae bore into the stalk and continue feeding. Mature corn borer larvae reach 1 to 1½ inches long. They are dull white and have a regular pattern of raised black dots over the body (photos on page 11).
As the plant reaches maturity, the larvae prepare to overwinter in the base of the stalk by girdling the plant from 1 to 6 inches above the ground. Wind can easily lodge girdled plants. Lodged plants are difficult to harvest, and yields are reduced.
Southwestern corn borer larvae overwinter in the stalk base or root crown. They are insulated by a frass (excrement) plug in the stalk and by the surrounding soil.
One of the most effective ways to control borers is to destroy this winter habitat to reduce spring moth emergence. A single tandem disc cultivation or shredding will expose the larvae to cold and dry winter conditions while leaving enough residue to prevent soil erosion. Set the shredder to cut the stalks at the soil surface, which removes the protective frass plug.
Shredding is particularly compatible with grazing and minimum tillage operations because it does not bury the plant materials but does expose the corn borer larvae. Also, the stalks can be shredded even when the soil is frozen.
Double disking and deep plowing are effective methods if soil erosion is not a problem. Cultivating or shredding before mid-January will kill many larvae. Timely stubble destruction will reduce local infestations of first-generation larvae. However, to effectively reduce southwestern corn borer populations areawide, every producer must cooperate by destroying stubble. Early-planted corn is less susceptible to lodging caused by corn borers. Plants with large, healthy stalks, combined with proper fertilization and adequate irrigation, will help prevent this lodging.
To reduce losses, rotate crops, use early-maturing varieties, and harvest early with equipment designed to pick up lodged stalks. All of the Bt hybrids with toxins targeted at caterpillars provide excellent control of the southwestern corn borer (Table A9). However, as of early 2016, 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 recommended in case that resistance has spread to Texas.
Insecticide treatments usually are directed toward second-generation larvae (Table A10). Apply the insecticide when the eggs or newly hatched larvae infest 20 to 25 percent of the plants. Check for egg masses to determine the potential infestation and the correct timing of insecticide application.
second-generation European corn borer are usually higher than are those from the first generation. Second-generation moths that emerge in mid-summer are attracted to dense vegetation around cornfields, primarily for mating.
Mated females return to recently tasseled corn to lay eggs on the undersides of the ear leaf and the leaves nearest to it. The eggs are white, with a black dot (the head of the young larva) appearing just before hatching. The eggs hatch in 3 to 5 days.
After hatching, about 75 percent of the small larvae move to the leaf axils; the remaining 25 percent move to the ear sheath and collar tissue. They reduce yield by larval tunneling, ear droppage, and direct kernel feeding.
To determine the need for an insecticide application, examine a minimum of five random samples of 20 consecutive plants each. (Table A12). An insecticide application is justified if you find an average of 10 to 20 hatched and unhatched egg masses per 100 plants. Two applications may be necessary to control European corn borer satisfactorily.
Economic infestations of spider mites occur on corn primarily in the Texas High Plains; they occasionally occur in the Winter Garden region and the Rio Grande Valley.
Large numbers of spider mites may occur on corn after the tassels appear. Mites first appear on the lower leaves, but they may move upward until they kill all the leaves (and in extreme cases, the entire plant).
Yield losses from mite feeding occur during the grain-filling growth stages (tassel to soft dough). Once the corn reaches the fully dent growth stage, the mites do not reduce yield directly. Heavy infestations cause extensive webbing on the leaves and may be associated with stalk rot and lodging.
Mite populations increase rapidly in hot, dry weather. An important factor triggering mite increases is the use of insecticide to control other pests. Insecticides may kill the beneficial arthropods that usually keep spider mite numbers low. Because mite numbers may increase when excessive amounts of fertilizer are used, test the soil and apply only the amount of fertilizer needed.
Proper irrigation timing will help the plants withstand mite-feeding damage. The most important time to prevent water stress is during tassel and early grain filling.
Both the Banks grass mite and twospotted spider mite can occur on corn in Texas. The Banks grass mite is the predominant species in early and mid-season, and it is distributed more widely than is the twospotted spider mite. A few fields, however, will have large numbers of twospotted spider mites or mixed populations of both species.
To distinguish between the species, note the pattern of pigmentation spots on the body. (Fig. 5). The adult twospotted spider mite has a well-defined spot on each side of the front half of the abdomen. The spots on the adult Banks grass mite extend all the way down both sides of the body, sometimes almost touching at the rear of the body. Also, twospotted spider mites produce more webbing than do Banks grass mites.
All of the modern miticides take from a few to several days to begin controlling spider mite populations, and the action thresholds have changed from in the past when we had fast-acting miticides. Therefore, base your control decisions on earlier damage levels from spider mite feeding than in the past.
Scout the fields at least once a week to determine whether predators are keeping mite populations and their damage in check. If so, the mite densities and damage will not increase from week to week.
To estimate when to begin applications for spider mite control, use the rating scale in Table 3. It can help you estimate the damage to individual plants as well as the average damage to several plants in an area. Use the average from several locations to make control decisions.
Field trials of the modern miticides (Tables 3, A17, and A18) have shown that mite control works best on tasseled corn before the mites become aggressively active, when they move up the plant rapidly and damage the ear leaf (damage rating 5).
When the mites have caused damage ratings of 6 and above, they have already reduced yields significantly, and miticides will not control the infestations effectively. Begin applying miticides at damage rating 3, when the mite populations are becoming increasingly active but enough time remains to spray the field.
Adult corn rootworm beetles (Mexican and western corn rootworm)
Although adult rootworm beetles feed on leaves, pollen, and tassels, they prefer silks. When adults are numerous (8 to 10 per plant) during the green silk stage and the silks are chewed back to within ½ inch of the shuck, poor pollination may cause poorly filled ears. When this amount of feeding occurs, or if excessive leaf damage occurs, it is profitable to control the beetles.
Controlling adult beetles usually reduces the number of eggs laid in a field. However, insecticides can cause an outbreak of spider mites by destroy- ing their predators. Because spider mites can greatly damage corn and are difficult to control, do not use a synthetic pyrethroid to control adult corn rootworm beetles. Apply insecticide to control adult beetles only when necessary (Table A19).
Insecticide baits consisting of carbaryl and a feed- ing attractant from cucurbits are labeled for control of adult corn rootworms. Corn rootworm beetles feed on these baits, which do not destroy as many benefi- cial insects and predatory mites. Therefore, these baits present less risk of outbreaks of spider mites after application than do many other conventional insecticides.
The fall armyworm is a sporadic pest of corn but has become a more consistent pest in recent years. It migrates north during the growing season from over- wintering sites in South Texas and northern Mexico. Recent research from Mississippi indicates that signif- icant whorl damage can decrease yield, but as of early 2016, the final scientific paper has not been published. Infestations occurring from the tassel to dough stage can damage corn greatly.
The larvae feed on the ears and ear shanks and behind the leaf collars. Research on the High Plains has shown that when boring through the side of an ear, one fall army- worm larva reduces yield by an average of 0.20 pound through direct kernel injury and the damage by asso- ciated fungi. Fall armyworm damage may also cause mycotoxin levels to increase in grain.
Heavy infestations may reduce yields substantially because the larvae feed directly on the ear. Additional losses can occur when shank feeding causes the ears to drop and when stalks lodge because of feeding damage to the nodes.
The larvae range from a light tan to a dark green or black. Light and dark stripes run lengthwise on the body. Dark spots or bumps occur in a pattern over the body, especially when viewed from the top. The head has a prominent inverted Y in a light color that contrasts with the dark head capsule.
Scouting for fall armyworms can be difficult. Check corn leaves and grasses in the furrow for egg masses. Each mass may have 50 to 100 eggs. Check also for small larvae behind the leaf collars, in ear tips and at the bases of primary and secondary ears.
Unlike small larvae, late-instar larvae are pale tan and have a small black spot on each side toward the head. This will help distinguish them from corn earworm and southwestern corn borer larvae.
Texas does not have an established economic threshold for this pest. If control is necessary, target the small larvae before they enter the primary ear. The newer, multiple-toxin Bt corn hybrids provide good to excellent control of fall armyworm (Table A20). Recent research has shown that insecticides targeted at ear protection should be applied in the period from 2 days before pollen shed to 4 days after (Table A21). Later applications will be less effective in ear protection.
True armyworms occasionally damage corn heavily. The most damage usually occurs in fields that have jun- glerice (watergrass) and Johnsongrass in the furrows or in fields that have hail-damaged leaves.
True armyworms may go unnoticed as populations build up on the weeds in the furrows. Then, when the weeds are consumed and the larvae grow, they begin feeding on the corn leaves. Large larvae can defoliate corn plants rapidly. Excessive defoliation will reduce yield, and premature drying of the stalk may lead to lodging problems. Apply chemical treatments when larval feeding destroys an average of three leaves per plant (Table A22).
Western bean cutworm
Economic damage from western bean cutworm is restricted to the extreme north- west corner of the Texas Panhandle.
Moth activity begins in early July, with egg lay following shortly thereafter. The
moths lay eggs on the upper surfaces of the corn leaves in masses of 5 to 200. They turn from a pearly white at egg lay to bluish black at hatching time.
At hatching, the young cutworms feed on the egg- shell and then move to one of two sites on the corn plant, depending on the stage of corn development. If the corn has not tasseled, they will feed in the whorl on the developing tassel. If it has tasseled, they will move to the developing ear and feed on the silk. As the larvae mature, they begin feeding on the developing grain.
There is strong evidence that western bean cut- worm is becoming resistant to the Bt toxin Cry1F, and the toxin Cry1Ac never was very effective. Transgenic corn with these toxins can still be effective, but only if it is pyramided with additional toxins.
Apply insecticide when 14 percent of the plants are infested with eggs or larvae and the corn is 95 percent tasseled (Tables A23 and A24).
Grasshoppers occasionally damage corn. Control damaging infestations early while the grasshoppers are small and still in the crop border areas. Ten or more grasshoppers per square yard in the crop margins warrant control measures (Table A25).
Corn sap beetles, or picnic beetles, are attracted to decaying vegetable matter and often invade corn ears damaged by insects. They are not attracted to healthy ears.
These beetles are small (1/3 inch) and black or brown. They sometimes have orange or yellow spots on their wing covers.