Insects attack all parts of the maize plant and attack the plant through all stages of plant growth. Numerous insect species attack maize in North America but the economic importance of the various species differs by region. The insects discussed are grouped according to the plant parts that they feed on: 1) Seed, Root and Lower Stem Feeders 2) Stalk Borers 3) Leaf Feeders and 4) Ear Feeders. This section examines stalk borers.
Insects feeding in the stalks of maize may also feed on the leaf, ear or other plant parts some time in their life cycle. The insects covered in this section are primarily known for the damage that they do when boring in the stalk. These are the European corn borer, southwestern corn borer, southern cornstalk borer, stalk borer and the lesser cornstalk borer.
European Corn Borer
The sexes of the European corn borer, Ostrinia nubilalis Hübner adults differ in color and size. The female moths (left) are larger and are a pale yellowish-brown color, with irregular darker bands in wavy lines across the wings. The male moth (right) is smaller, more slender, and darker than the female. The outer third of its wings is usually crossed by two zigzag streaks of pale yellow, and often there are pale yellow areas on the forewings. The male moth is darker, with distinct pale yellow bands on the wings (right). The adults have a wingspan of 20 to 30 mm. At rest, the tip of the male abdomen protrudes from the folded wings. Eggs are laid in masses of 20 to 30 eggs that are covered with a shining waxy substance. Within the egg mass, the eggs overlap each other like fish scales. Each white egg is about half the size of a pinhead. Eggs change to pale yellow and darken just before hatching, as the brown head of the borer inside becomes visible.
The newly hatched larva, about 1.5 mm long, has a black head, five pairs of prolegs, and a pale yellow body bearing several rows of small black or brown spots. It develops through 5 or 6 instars to become a fully grown larva about 25 mm long. When fully grown, the larva has a reddish tinge or is pinkish in color. The larval head capsule is dark brown and, on top of each abdominal segment, there are several small dark brown or black spots. The reddish-brown to dark-brown pupa is 13 to 15 mm long with a smooth capsule-like body.
Biology and Life Cycle
The European corn borer was introduced into the U.S.A. from Europe in 1909, and has spread west to the Rocky Mountains with the western distribution running from New Mexico in the south to Montana in the north. Its range goes north into Canada. It feeds on more than 200 plant species but maize is a preferred host.
The number of generations per year varies from the southern to the northern portion of its range. There are four generations a year in the southern portion of the range and one generation in the north. The first generation develops from the overwintered fifth instar in stalks, cobs, and plant debris. The larvae change into pupae in the spring and emerge as second generation moths approximately ten to fourteen days later during May and June. Moths aggregate or gather in weedy or grassy areas, normally field margins, to mate and drink water, usually in the form of dew. On warm, calm, humid evenings in June, female moths fly from these protected areas into maize to lay masses of 15 to 25 eggs near the midrib on the underside of the leaves (< 1% of the eggs may be laid elsewhere on the plant).
Eggs hatch within five to seven days depending on temperature. Eggs that are about to hatch have distinct black centers and are referred to as being in the "blackhead" stage. This is due to the black head of the larva showing through the translucent eggshell. Blackheaded eggs will hatch within twenty-four to forty-eight hours. Newly hatched larvae disperse and soon establish themselves deep in the whorl, feeding on developing leaves during the first two larval instars. As the leaves grow and unroll from the whorl, the "shot-hole" feeding signs (small round holes scattered in the leaf tissue) can be seen. Following a feeding period in the plant whorl (approximately 2 weeks for each larva), third instar larvae leave the whorl, bore into stalks and excavate tunnels (cavities), in which they complete development. Fifth instar larvae of this first generation change into pupae within the plant cavity, from which the summer moths emerge in July and August. Occasionally, larvae will pupate outside the stalk on the maize leaf.
The moths of the first generation (summer moths) generally emerge in late July and August. Similar to first spring moths, they move to grassy or other dense low vegetation near or inside maize fields to mate. In weed-free maize fields these areas may be fence line bromegrass or adjacent soybean or alfalfa fields. If grassy weeds are present in the maize field, moths may remain in the field and aggregate in weeds or patches of volunteer maize. Summer moths lay over 85 percent of their eggs on the undersides of the ear leaf and the three leaves above and three leaves below. Adult females may lay up to 500 eggs over their lifetime.
After hatching, second generation larvae feed on the leaves and in leaf axils for a few days (particularly if pollen is available), then move behind leaf sheaths and the leaf collar area, or into ear tips. Some third, but mostly fourth instar larvae, bore into the stalk, ear shank, or ear. These larvae usually overwinter and do not pupate until the following spring. In years of extended growing seasons with greater than average degree-day accumulations in Nebraska, a small proportion of the larvae pupate, and produce moths, giving rise to a third generation of larvae. Third generation larvae are not of economic significance, because the maize plant at this late date is normally well beyond the period of susceptibility.
In maize, feeding first occurs in the whorl. Shotholes are visible when the leaf unrolls from the whorl. Later, the larvae bore down midribs of leaves into the stalk. Frass and silk near entrance holes are evidence of the presence of larvae. Extensive amounts of frass may be seen at the collar. They also bore into, feed, and tunnel within the tassel, ear, ear shank and stalk, forming cavities.
Cavities produced by borers interfere with the translocation of water and nutrients. Cavities also reduce the strength of the stalk and ear shank, thereby predisposing the corn plants to stalk breakage and ear drop, which is aggravated by high winds or other adverse environmental conditions.
Yield losses due to damage by the larvae are primarily reduced ear and kernel size (physiological losses) as well as broken plants and dropped ears (potential harvest loss). Larvae feeding in the ear may cause seed yield loss and/or reduce quality in seedcorn, popcorn, and fresh market sweet corn.
Yield losses are highly correlated with plant stage, water stress, and the hybrid. Cavity formation by the first generation borer usually occurs before tassel emergence resulting in approximately 5 percent yield loss per borer per plant. Yield losses (per borer) from second generation larvae vary widely because cavity formation may occur over several weeks, and rapid physiological changes occur as the ear approaches maximum size and physiological maturity. As the ear advances from the blister stage to physiological maturity, the yield reduction per borer rapidly decreases. Average grain weight reductions are 5.9, 5.0, 3.1, and 2.4% per larva per plant when feeding at the 10-leaf, 16-leaf, blister, and dough stages respectively.
Southwestern Corn Borer
The adults of the southwestern corn borer, Diatraea grandiosella (Dyar), are a light-straw color with a wing expanse of 1 1/4 inches. The labial palps extend in front of the head like a short beak. Eggs are whitish or yellow, oval in shape, and laid in groups in an overlapping, fish scale-like fashion. Initially eggs are greenish-white, but develop three distinct red transverse lines within 24 to 36 hours. There are two color forms of the larvae. During the growing season, the larva has a white abdomen with conspicuous dark brown or black spots (left figure). During the winter, the southwestern corn borer becomes pale yellow with very faint spots (figure right). The full grown larva is about 30 mm in length.
Biology and Life Cycle
The southwestern corn borer is found throughout the southern Corn Belt. It is now widely distributed in Arizona, New Mexico, Texas, Oklahoma, Colorado, Kansas, Nebraska, Arkansas, Missouri, Louisiana, Alabama, Mississippi, Tennessee, and Kentucky. The southwestern corn borer has two or more generations per year, depending on the severity of the previous winter. First generation larvae first appear in June. For the first two weeks, first generation larvae feed within the whorl of the plant. After feeding in the whorl, larvae move down the stalk and tunnel into the stalk. Larvae may move from one plant to another. Pupation occurs in the stalk. Adults emerge from the pupae in about a week and soon lay eggs. Upon hatching, the larvae feed on the leaf sheath before boring into the stalk. The second generation occurs during mid- to late summer. In the fall, the larvae migrate to the base of the plant and tunnel. Overwintering occurs as a larva in stalk base below the soil. In the spring, pupation takes place in the stalk base. Moths emerge from the stubble, mate, and deposit their eggs on the upper and lower surfaces of the leaves. Eggs are laid singly or in groups of 2 to 5.
The southwestern corn borer is primarily a pest of maize but also attacks sorghum. Major damage by this pest is due to the girdling of the stalk by the second generation larvae. The first generation attacks whorl-stage corn and is associated with losses to yield by stunting or killing plants. Numerous holes in the emerging leaves and leaf breakage due to midrib tunneling are characteristic. While leaf feeding may not lead to serious yield loss, destruction of the bud in the whorl can result in a "deadheart", and stunting, and complete loss of yield by a plant.
The second generation larvae first feed in the leaf axils. Then, the larvae bore inside the maize stalks and move down the stalk in a straight line and feed at about three to five inches above the soil surface (figure left) where they girdle the plants (figure right). Larvae girdle the stalk by chewing a complete or partial internal groove around the stalk near the base. This leaves only a thin outer layer of the stalk for support. As a result, plants lodge and yield losses occur.
Southern Cornstalk Borer
The southern cornstalk borer, Diatraea crambidoides (Grote), adult is a straw-colored or dull white moth with a wingspan of 15 to 40 mm (females larger than males). Adults have the distinct, extended labial palps in the form of a beak. The forewings are slightly darker than the hind wings. The flat, elliptical egg, approximately 1.3 mm by 0.8 mm, is creamy white when laid but later develops an orange hue due to the presence of three transverse, orange-red lines. The larva, which reaches a length of 25 mm, is creamy yellow during the winter and white with black spots in the summer (figure). The pupa is about 22 mm long and the same color as the larva when first formed but later changes to a reddish-brown.
Biology and Life Cycle
The southern cornstalk borer causes damage primarily in the states from Maryland and Kansas on the north to, and including, the southern and southwestern states. It also occurs in Mexico and in South America. This borer attacks primarily maize but also feeds on grain sorghum, sugarcane, broomcorn, and Johnson grass.
The biology and life cycle of the southern cornstalk borer is similar to that of the southwestern corn borer. There are usually two generations each year although three generations can occur. Southern cornstalk borers overwinter as larvae below the ground level within cavities in maize taproots. Prior to pupation in March or April, the larvae make a silk-lined exit tunnel to the outside of the stalk through which they later emerge as adults. Approximately 10 days after pupation, moths emerge, mate, and begin laying eggs at night, usually on the underside of lower leaves. The flat eggs are laid either singly or in small clusters of 2 to 25 overlapping one another like shingles. Each female lays up to 400 eggs. When the eggs hatch 7 to 10 days later, larvae move into the whorl of the plant, feeding on the leaves and spinning a silken thread behind them. Third- or fourth-instars move down the stalks and eventually tunnel inside the stalk at a point near the ground level. They may move from one plant to another, as does the southwestern corn borer. In the summer, southern cornstalk borer larvae live from 20 to 35 days and develop through seven instars. Mature larvae seal off the tunnels with frass and form cells in which to pupate. Summer pupation occurs in the above- ground stalk. The first generation becomes adults in midsummer and the second generation reaches maturity in early fall and remain as larvae during the winter.
The southern cornstalk borer is one of the most damaging maize insects in the southern states. Maize infested by the borer is twisted and stunted, and the stalk at the surface of the ground may be enlarged. The leaves are ragged, broken, and dangling. They have many holes caused by larval feeding when the leaf was still in the whorl stage.
Early-season larvae start to feed in the whorl; as the leaves unfold, rows of irregular holes may appear. Larvae also tunnel in the midribs of eaves, and sometimes destroy growing points within leaf whorls. As larvae grow larger they tunnel into stalks. Masses of frass accumulate outside the entrance holes. Tunneling may be extensive in the lower portion of the stalk, primarily just above the soil line and into the taproot. This damage may be very destructive because of reduced nutrient and water uptake. Late-season larvae feed but little on the leaves, but tunnel through the base of the stalk.
The stalk borer, Papaipema nebris Guenée, adult is a dull, grayish-brown moth that commonly has several white or silver spots in two rows across the front wings. There is a faint whitish line across the wing near the outer edge. The hind wings are dull brownish-gray. The wingspan ranges from 25 to 40 mm. The longitudinally ribbed egg may be spherical or slightly flattened and measures 0.4 to 0.6 mm in diameter. White when first deposited, it gradually turns brownish-gray or amber before hatching. Young larvae are purple to black and have prominent longitudinal white stripes at the front and rear ends of the body. The stripes are interrupted at mid-body by a solid dark purple to black area on the third thoracic segment and first three abdominal segments. Fully grown larvae are cream colored with faint purple markings. There is a distinct dark band on the side of the prothoracic segment and the head (figure). Larvae reach a length of 30 to 40 mm prior to pupation. The light brown pupa gradually darkens as it matures and is 16 to 22 mm in length.
Biology and Life Cycle
The stalk borer occurs in all areas east of the Rocky Mountains from southern Canada to the Gulf of Mexico. This borer tunnels in almost any large- stemmed plant. Their host range encompasses at least 44 families and 176 species of plants. Some cultivated crops subject to infestation include maize, cotton, potato, tomato, alfalfa, rye, barley, pepper, spinach, beet, and sugarbeet. Although many weedy plants are infested, giant ragweed is preferred. Highest populations are associated with fields and fencerows with large-stemmed weeds. Stalk borers overwinter as eggs. Female stalk borer moths lay their eggs primarily on dry grasses such as smooth brome and giant ragweed in late summer and early fall. The moths tend to lay eggs singly or in groups under sheaths and in folded or rolled leaves. Eggs overwinter and hatch in the spring. In May, the newly emerged larvae feed as leaf miners on broadleaf plants or as stem borers on grasses. Larvae eventually bore into the stem. If they kill or outgrow their host, they will emerge at night and tunnel into new plants, including maize seedlings. The migrating stalk borer larvae can attack maize that is between the two and eight leaf stages. Larvae develop through 7 to 10 instars, in about 10 weeks. Pupation occurs in individual cells that are constructed in the soil beginning in late July. Moths emerge during August, September and early October and deposit eggs singly or in masses between the leaf sheath and stems of weeds. One generation occurs each year.
Damage is sporadic but most commonly associated with the border rows of conventionally planted maize and with no-till plantings. Stalk borer larvae can injure corn plants in June and early July in two ways: 1) by burrowing into the base of the plant and tunneling up through the center of the stalk, or 2) by entering the plant through the whorl and tunneling down. Plants attacked at earlier growth stages tend to receive more damage. A single stalk borer larva may attack more than one plant if the first plant does not support the larva as it increases in size.
A damaged plant will show irregular rows of holes through the unfolding leaves (see figure). These irregular rows of holes will be much larger and more ragged than those caused by whorl feeding of first generation European corn borer larvae. If the feeding injury to the central part of the plant is severe, the whorl will appear dead while the outer leaves are green and apparently healthy. In severe cases an infested plant will have a very ragged appearance, with destroyed tassels and with abnormal growth habits such as twisting, bending over, and/or stunting. Suckers may be produced. Frass is usually evident around the base of more mature infested plants. Once past the whorl stages, maize is somewhat resistant to the stalk borer and recovers more readily from damage.
Lesser Cornstalk Borer
The lesser cornstalk borer, Elasmopalpus lignosellus ;(Zeller) is a small insect. The male's front wings are brownish yellow and have grayish margins with several dark spots .The moth has a wingspan of about 25 mm. Wings are wrapped around the body when at rest. The egg is greenish-white and less than 1 mm in diameter. The larva is a slender, bluish-green, brown-striped caterpillar up to 19 mm long. The pupa is brownish and about 8.5 mm long.
Biology and Life Cycle
The lesser cornstalk borer is found throughout the United States but most severe feeding damage occurs in the southern states, particularly Alabama, Georgia and South Carolina. This insect is also found in Mexico, Central America and South America. The lesser cornstalk borer prefers maize but it also feeds on beans, cowpeas, crabgrass, Johnson grass, peas, peanuts, sorghum, soybeans, and wheat.
In the southern states, these borers usually hibernate as larvae. The larvae transform into pupae and the moths emerge early in the spring. The moths lay their eggs on the leaves or stems of the plant upon which they feed. The eggs hatch in about a week. The larvae feed first on the leaves or roots. Later they construct underground silken tubes or burrows from which they bore into plants near the ground line. They become fully grown in 2 to 3 weeks, leave their burrows, and spin silken cocoons under trash on the surface of the ground. In these cocoons, they change to pupae from which moths emerge in 2 to 3 weeks. Two generations occur in most southern states.
The larva of this small moth has been sporadically injurious to the seedlings of many plant species. This larva may be found feeding on leaves or roots of maize but eventually tunnels into the plant's stem usually at or near the soil line. Injury is caused when the larva bores into the stalk of the host plant, thereby disrupting the growing point. The presence of the larvae in maize is evident by the masses of borings that are pushed out through holes in the stalk. A silken, soil-covered tube is often connected to the plant stem at the entrance hole. Damage can be slight, or it can kill the plant. In older plants the larvae may girdle the stem near the ground level resulting in stalk breakage. Damage is most prevalent during drought conditions in crops grown on sandy soil.