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Publication #ENY-472

Insect Management for Sweet Corn1

G. S. Nuessly and S. E. Webb2

Foliar, ear and root feeding insects can routinely cause economic losses to sweet corn if left untreated. The most important pests of sweet corn in Florida are the fall armyworm, corn earworm, lesser cornstalk borer, cutworms, corn silk fly, cucumber beetles, aphids, and wireworms. Less common pests of sweet corn include grasshoppers, corn blotch leafminer, twospotted spider mites, sap beetles, stink bugs, maize weevils and billbugs, white grubs, and white fringed beetles.

Fall Armyworm, Spodoptera frugiperda (J.E. Smith)

Description

Adults (Figure 1) are light brown to ash gray with a 1 1/2 in. wing span. Several dark and light spots and lines are found on the front wings. The hind wings are lighter with a dark band near the margin. Eggs are deposited in masses of up to 200 eggs, often with more than one layer, and covered with scales from the moth's abdomen. Newly emerged larvae have black heads with all white bodies that become darker and patterned as they grow. Mature larvae grow to about 2 in. and are light green to tan or brown to nearly black. Larvae have six dark bumps (tubercles) on the upper surface of each abdominal segment greater than or equal to the size of their spiracles (small, oval-shaped openings to the trachea on the lateral sides of most segments). The eighth abdominal segment has four distinct dark bumps on the top surface. The top of the first thoracic segment appears as a dark shield, often with three light stripes. Sutures on the front of head form an inverted "Y." Larvae (Figure 2) have three pairs of true legs and five pairs of prolegs. The 3/4 in. long pupae are dark reddish-brown.

Figure 1. 

Fall armyworm adult.


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Figure 2. 

Fall armyworm


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Biology

Adults feed on nectar and other moisture sources. The ¼ to ½ diameter egg masses are usually deposited under leaves and covered with a small amount of scales from the female's abdomen.

Larvae emerge from eggs in 3 to 4 days. Prior to tassel push, young larvae feed between leaf veins often on outer portions of leaves before moving into the whorl to feed on young developing leaves. Later in the season young larvae first feed on tassels before moving to the ears or feed directly and complete development on ears. Last (6th) instar larvae leave the plants to pupate in shallow chambers beneath the soil surface. Larvae complete development within 2 to 3 weeks. Adults emerge from the pupal cases in 7 to 10 days to complete a generation in 24 to 35 days.

Damage

Long thin holes between the leaf veins indicate feeding by first instar larvae. Later instar larvae leave larger holes across veins. Leaves damaged by small to medium sized larvae within the whorl often emerge to display rows of holes. Feeding by mid to late instar larvae frequently results in a shredded appearance to the emerging leaves. Late instar larvae can completely consume the growing point of the plant, and can severely damage the tassel before it emerges from the whorl. Presence of these larvae can be detected by looking into the whorl for a plug of wet excrement (frass plug) blocking the opening. Tassels should be examined for early through mid instar larvae and their damage, including small holes, missing pieces, excrement and tassel pieces on the upper surface of leaves. Fourth through sixth instar larvae pushed from the whorl by the emerging tassel can quickly ruin the ears of at least one of the surrounding plants. All age larvae feed on ears. Young larvae feed on silk and often enter the tip of the ear to feed on silk before damaging kernels, and cob. Examine the silk for damage and presence of small frass. Mid to late instar larvae can enter the ear through the husk, bypassing the silk and causing direct damage to kernels and cob. The husks of ears should be examined for direct entry, particularly in the protected area between the ear and plant stem.

Large larvae can feed on the shank supporting the ear leading to interference with moisture and nutrient flow and resulting in smaller, deformed ears. Medium to large larvae also bore into corn stalks above leaf nodes beneath subtending leaves and can result in broken stalks below the ears.

Table 1. 

Fall armyworm

Management Option

Recommendation

Scouting/ Thresholds

Fields should be monitored at least weekly until tassel push for young larvae on leaves and in whorls. Thresholds used for treatment are plant age specific, with seedlings and plants pushing tassel often treated at 10% or lower infestations. Plants between these stages are usually treated when infestations reach between 15 and 20%. Fields should be scouted at least twice weekly during the ear stage to detect and control populations before they cause economic damage. Treatments during this period are usually initiated as soon as larvae are detected. Infestations above a few percent should elicit more frequent sampling. Pheromone traps can be used to monitor relative presence of the adults in the area.

Notes

It is important to eliminate the larvae being pushed from the whorl by the emerging tassel so they do not damage the sensitive emerging ear shoots or flag leaves and do not pupate within the field. Larvae completing development at tassel push can emerge as adults to lay eggs on the crop during the period of active silk growth. Chemical controls are most effective against the younger instars with higher rates and more frequent applications needed to try to control later instars. Granular formulations of several pesticides are available that reduce the exposure to non-target organisms by concentrating the pesticides in the whorls and leaf axils, particularly when applied in tight bands over the rows. Granular formulations are one of the most effective treatments available to kill larvae once they have formed a frass plug within whorls. The other pesticides are used as broadcast or banded sprays. Control of newly emerged larvae on ears is important to prevent economic damage. Ear feeding by fall armyworm and corn earworm can result in increased infestations of other insect pests (e.g., corn silk fly species complex and sap beetles) and fungi. Control of larvae in the early instars is important for reducing problems with these additional pests.

Natural Enemies

Birds and insect natural enemies (predators and parasitoids) aid in fall armyworm control from germination through tassel push, but generally do not exert enough pressure to prevent yield loss in sweet corn. Birds can cause more damage than fall armyworms by feeding on kernels at ear tips within a week of harvest.

Resistant Varieties

Sweet corn genetically modified to produce Bt endotoxins (e.g., Bt-11 event) can provide effective resistance to fall armyworm, corn earworm and lesser cornstalk borer. Seed for yellow, white and bi-color varieties are commercially available. These Bt-enhanced lines do NOT provide protection against non-Lepidoptera pests (e.g., corn silk fly species complex), therefore, such varieties should still be scouted for other pests to avoid unnecessary losses.

Corn Earworm, Heliocoverpa zea (Boddie)

Description

The front wing color is sex dependent. Female fore wings are yellow to pinkish-brown, while those of males are light greenish-brown. Both sexes usually have a dark spot in the middle of the fore wing (Figure 3). Both fore and hind wings have dark margins and the hind wings have a short, narrow band near the middle and two pale spots near the edge of the wing in the dark margin. Both sexes have a wingspan of 1 1/2 in. The ball shaped, white to yellow to green eggs have ridges running from top to bottom. Newly emerged larvae are translucent white to yellow with a light brown head. Older larvae vary considerably from dark yellow to green or tan to dark brown. Mature larvae can reach 2 in. Larvae (Figure 4) have three pairs of true legs and five pairs of prolegs. The skin (cuticle) of larvae are covered with microspines (need good hand lens or microscope to see microspines). The dark colored tubercles (larger bumps) on abdominal segments 1, 2 and 8 do not have microspines more than 1/4 the way to their apex. The central area on the inside surface of the mandibles do not have a separate toothed area (observed with microscope after spreading mandibles with pins or forceps). The 3/4 in. long pupae are reddish-brown in color.

Figure 3. 

Corn earworm adult.


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Figure 4. 

Corn earworm.


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Biology

Corn earworm moths feed on nectar and other moisture sources. Females deposit their eggs singly or in small groups of less than five on leaves and ears. The eggs are not protected by scales. Larvae emerge from eggs in 3 to 4 days. While most larvae complete development on corn ears in 18 to 26 days, larvae can also feed and develop on leaves. Larvae leave plants to pupate within the soil. Adults emerge in 7 to 10 days. Corn earworms complete their development from egg to adult in 28 to 40 days.

Damage

Young larvae emerge from eggs to initially feed on corn silk, husks or leaves depending on egg placement. The majority of larvae quickly make their way to feed on ears entering through the silk channel or directly through the husk. Uncontrolled larvae cause severe damage to silk, kernels and cob before leaving to pupate in the soil. Twenty years ago this was the primary pest of corn throughout Florida. Fall armyworm has largely replaced it as the primary pest in southern Florida, but outbreaks still occur south of Lake Okeechobee resulting in 100% of untreated ears infested with corn earworm larvae. Corn grown in northern Florida counties during the late spring through fall is more likely to have large populations of corn earworm.

Table 2. 

Corn earworm

Management

Options

Recommendation

Scouting/ Thresholds

Fields should be scouted at least twice weekly during the ear stage to detect and control populations before they cause economic damage. Treatments during this period are usually initiated as soon as larvae are detected. Infestations above a few percent should elicit more frequent sampling. Helicoverpa pheromone traps can be used to monitor relative presence of the adults in the area.

Notes

Ear feeding by corn earworm and fall armyworm can result in increased infestations of other insect pests (e.g., corn silk fly species complex and sap beetles) and fungi. Control of larvae in the early instars is important for reducing problems with these additional pests.

Natural

Enemies

Birds and insect natural enemies aid in earworm control, but generally do not exert enough pressure to prevent yield loss, particularly in sweet corn. Birds can cause more damage than corn earworms by feeding on kernels at ear tips within a week of harvest.

Resistant

Varieties

Sweet corn genetically modified to produce Bt endotoxins (e.g., Bt-11 event) can provide effective resistance to corn earworm, fall armyworm and lesser cornstalk borer. Seed for yellow, white and bi-color varieties are commercially available. These Bt-enhanced lines do NOT provide protection against non-Lepidoptera pests (e.g., corn silk fly species complex), therefore, such varieties should still be scouted for other pests to avoid unnecessary losses.

Lesser Cornstalk Borer, Elasmopalpus lignosellus (Zeller)

Description

The adults are narrow-winged moths 1/2 to 5/8 in. long (Figure 5). The body and wings of female moths are covered with mostly gray to brown and reddish shiny scales. Males are pale yellow to medium brown with wings bordered with a band of darker scales. Adults fly quickly when disturbed in the field, often landing on the soil several yards away. Males are easily spotted against the darker organic soils. The flat, shingle-like eggs are creamy green and become pink to red at the time of larval emergence. Larvae are patterned with alternating narrow transverse bands of maroon to brown and aqua blue over a cream base. Larvae reach 1/2 to 5/8 in. long at maturity. Larvae have three pairs of true legs and five pairs of prolegs. Larvae are very active when disturbed. Pupae change color from greenish to brown with time and are surrounded by a flimsy silken cocoon.

Figure 5. 

Lesser cornstalk borer.


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Biology

Lesser cornstalk borer moths feed on nectar and other moisture sources. Eggs are deposited singly or in small groups on the soil surface or directly on the stem near the soil surface. Temperature greatly effects development. Larvae emerge in 18 days at 64° F, but in less than 3 days at 91° F. They hide and feed at base of corn stalks from narrow soil-covered silken tubes 1/4 to 1/2 in. beneath the soil surface. Larvae complete development in 17 to 42 days. The larvae pupate within the soil and adults emerge in 10 to 24 days. Complete generation time is 30 to 84 days.

Damage

Larvae bore into the corn stem of young plants just below the soil surface and feed up and down the stem. They also bore into larger brace roots of nearly mature plants. Young plants exhibit severe wilting of the youngest two to three leaves. Plants that do not die from this damage often develop several suckers that are bushy and stunted, but they do not produce marketable ears. While wireworms produce similar damage symptoms, corn plants attacked by these beetle larvae are much less likely to survive to produce suckers as when they are attacked by lesser corn borer larvae. Infestations can cause severe stand loss usually in irregular shaped patterns in the field. Damage is usually more severe during dry, warm periods. Larvae may move down a row to kill several seedlings before completing development.

Table 3. 

Lesser cornstalk borer

Management

Options

Recommendation

Scouting/ Thresholds

Control efforts can be maximized by estimating the potential for damage based on adult monitoring, surrounding crops and previous field history. Delta wing pheromone and black light traps can be used to monitor adults. Adults are easily visible in fields as they are quick to fly as one approaches. To confirm lesser cornstalk borer infestation, carefully examine the soil immediately around the base of the injured plants for soil-covered silken feeding tubes loosely attached to the entry hole at plant base or brace roots.

Notes

They tend to be worse in drier years and on well-drained soils. However, they can cause severe damage to sweet corn on heavy organic soils planted throughout the year. Larvae may already be present in the field when seeds are planted. Wet, cool growing conditions increase their mortality and greatly reduce their developmental rates. In fields with a high potential for infestation, at-plant pesticide application should be considered to reduce damage. Unfortunately, by the time field infestations are observed in corn, damage to the crop has already occurred. Post-emergence applications applied as soon as damage symptoms appear can limit additional damage, but may be too late to prevent economic losses. Post-emergence applications work best when banded over the rows and lightly incorporated into the soil around the plants.

Natural

Enemies

Natural enemies are not thought to have a significant effect on lesser cornstalk borers due to their subterranean habits, silken feeding tubes and sporadic nature.

Resistant Varieties

Host plant resistance is probably the best strategy for control of this borer in the future. Resistance has been noted in lines tested at CIMMYT in Mexico and domestic sweet corn varieties enhanced with a Bt toxin provided overall control several times greater than that of insecticides.

Cultural

Controls

Field cultivation several weeks in advance of planting is advised to allow for decomposition of the plant residues and completion of borer development. Planting corn following corn, sorghum, small grains, sugarcane, beans, peanuts, or too quickly behind recently turned weedy fields can predispose a crop to damage. Lesser cornstalk borer does very well on nutsedge and corn planted behind sugarcane infested with these weeds can suffer severe damage.

Cutworms

Description

Cutworm adults are large bodied moths with various dark patterns over light to medium brown or gray base color of fore wings. Shapes and coloration of spots on front wings are diagnostic in the identification of adult cutworm species. The wing spans of both the black (Agrotis ipsilon (Hufnagel)) and variegated (Peridroma saucia (Hubner)) cutworms ranges from 1 5/8 to 2 1/8 in, while the wings of the granulate cutworm (Feltia subterranea (F.)) are slightly narrower at 1 1/4 to 1 3/4 in. Male and female black cutworm moths (Figure 6) are colored differently. However, in both sexes the kidney-shaped (reniform) spot is a dark crescent-shaped ring with a dark line emanating outward and hind wings are whitish to gray with dark scales on veins. The claviform spot (proximal to and beneath reniform spot) is an elongate oval, and the orbicular spot (proximal to reniform spot) is small and round to tear-shaped. Wing color in granulate cutworms (Figure 7) varies greatly. Both sexes are recognized by the black bar between the usually pale, basal, round (orbicular) and kidney-shaped (reniform) spots on the fore wings. Hind wings are off-white with veins and margins sometimes darker. There is less distinction between the sexes in wing coloration for variegated cutworms (Figure 8). The kidney-shaped reniform and nearly round orbicular spots (proximal to reniform spot) in the central fore wing are both large and ringed with black. The rear half of the forewings is clay-colored with blackish spots. Hind wings are irridescent to pearly white with brown veins and margins.

Figure 6. 

Black cutworm adult.


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Figure 7. 

Granulate cutworm adult.


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Figure 8. 

Varigated cutworm moth.


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Eggs of all three species are whitish and become variegated brown with age and have ribs radiating downward from the top. Black cutworms eggs are spherical shaped, those of granulate cutworms are rounded with flat bottoms, and variegated cutworms eggs are rounded with rounded bottoms.

All cutworm larvae have sutures on the front of the head that form an inverted "V" and all have three pairs of true legs and five pairs of prolegs. Black cutworm larval color ranges from light gray to black on top and lighter below (Figure 9). Two lighter stripes running down the center of the back are visible on lighter specimens, but all larvae lack an obvious band down their back. Two rows of small, black, raised bumps (tubercles) run down the back; two per segment. Their cuticle is covered with large and small convex, isolated granules (need hand lens or microscope to see granules). Their head is brownish with many dark spots. Granulate cutworm larvae (Figure 10) are gray to reddish brown with dull yellowish subdorsal markings on the abdominal segments. The heads are yellowish to brownish. The undersurface of the body is flecked with white. Granules on their cuticle are isolated, bluntly conical, and project slightly backward. Body color in variegated cutworms (Figure 11) is brownish gray to nearly black on top of back and lighter below. A row of whitish to yellow circular spots run down the center of the back, particularly on the first four abdominal segments. The cuticle is smooth without granulation. An orange to brown line may connect the spiracles, below which there is usually some yellowish to orangish coloration. The head is orange-brown and marked with darker spots. The pupae of all species are reddish brown to brown, 5/8 to 7/8 in. long and are found in the soil.

Figure 9. 

Black cutworm larvae.


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Figure 10. 

Granulate cutworm larva.


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Figure 11. 

Variegated cutworm larva.


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Biology

Cutworm moths feed on nectar and other moisture sources. Moths begin depositing eggs on field debris, stubble or leaves near the soil surface 7 to 10 days after emergence. Black cutworm eggs are deposited singly or in groups of up to 30, granulate cutworms eggs are deposited singly or in small groups and variegated cutworms deposit masses of up to several hundred eggs. Larvae emerge from eggs in 3 to 6 days. Larvae tend to curl up into a ring when disturbed or handled. They may also bite and release a greenish-brown fluid. They feed on leaves and stems of mostly young plants. Older larvae (4th instar and later) can reach 2 in. in length and can cut plants off at their bases and drag them to their burrow in soil. Larvae complete development in 20 to 40 days. Larvae pupate within a chamber in the soil. Adults emerge in 10 to 20 days. Generation time for cutworms is 43 to 76 days, depending on temperature.

Damage

These cutworms are pests of corn and most other vegetables throughout Florida. Damage includes leaf feeding and stand loss due to cutting off entire plants. Black cutworms do most of their feeding at ground level. Larvae feed on young plants, cutting off leaves, or in later instars, entire plants. Populations of this pest tend to be higher in weedy and in wet fields. Granulate cutworm larvae can cut off entire seedling plants, as well as climb and feed on leaves of older plants. This cutworm is not associated with weedy fields as is the black cutworm. First instar larvae stay on plants, while older larvae climb and feed on plants only during night. Variegated cutworm larvae cut off seedlings at ground level or defoliate older plants.

Table 4. 

Cutworms

Management

Options

Recommendation

Scouting/ Thresholds

Seedling corn should be scouted as frequently as twice per week to detect cutworms or their damage, particularly in areas known for this pest. Young larvae may be found grouped together on foliage, but older larvae will usually be found singly in soil or beneath leaf trash during the day. Adults can be monitored with black light and pheromone traps.

Notes

Larvae may already be present in the field when seeds are planted. Pesticides are available for at-plant, pre- and post-emergence broadcast and banded applications. Post-emergence applications are the most efficient.

Natural

Enemies

Natural enemies such as parasitic wasps, flies and predacious ground beetles can exert tremendous control pressure that may approach 80%. Larvae are also targets for attack by pathogenic fungi and viruses. However, seedlings emerging in fields without resident natural enemy populations can experience significant stand loss from first generation cutworms.

Cultural Controls

Weedy fields quickly rotated to corn have higher potential for stand loss due to older larvae cutting off the emerging plants.

Corn Silk Fly, Euxesta annoae (F.), E. eluta Loew, E. stigmatias Loew and Chaetopsis massyla (Walker)

Description

Described attacking sweet corn in Florida as early as 1938, Euxesta stigmatias was the first of what we now recognize as four species in two picture-winged fly genera attacking corn in southern Florida. Euxesta annonae is relatively rare in sweet corn fields, but the remaining three are serious pests of sweet corn ears. These fly species can be distinguished by their body and leg colors, as well as by their wing patterns. They resemble predacious long-legged flies in shape and movement on the leaves, but 'corn silk flies' have shorter, stouter legs and lack the bright blue and greenish hues of the long-legged flies. Euxesta spp. flies are 3/16 to ¼ in. long with dark green to black bodies and legs all with a slight metallic sheen. The bodies of Euxesta spp. that attack sweet corn appear brown to black in dead specimens. All three Euxesta spp. have four dark bands across their front wings (Figure 12). The clear portion of the wing between the last two bands (i.e., 3rd and 4th bands) does not rise into the r1 cell in E. annonae and E. stigmatias. The bands become increasingly lighter in color from the front to the rear wing margins of E. stigmatias, while they fade only slightly near the rear wing margin in E. annonae. The clear portion of the wing between the 3rd and 4th bands clearly rises all the way to the front wing margin in E. eluta and frequently forms what appears to be a clear, round to slightly oval spot between the dark 3rd and 4th bands. The first band near the wing base covers about 33% of the basal portion of wing cell c in E. annonae while covering only 15% of the base of cell c in E. eluta and E. stigmatias. Chaetopsis massyla are ¼ to 5/16 in. long with greenish-grey thorax, dark green to black abdomen and yellowish legs. The thorax of C. massyla has a bright metallic sheen, even in dead specimens. Only three wing bands are present on C. massyla wings with the clear portion between the 1st and 2nd bands often twice as wide as the clear portion between the 2nd and 3rd bands.

Figure 12. 

Corn silk fly adult.


Credit: J. F. Butler
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Figure 13. 

Corn silk fly larva.


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Biology

While they are primary pests of sweet corn, these flies are also saprophytic, feeding and reproducing on a wide variety of overripe fruits and vegetables, as well as on subtropical to temperate grasses and broad-leaved plants previously injured by insects, diseases and farm equipment. Females deposit eggs into corn ears, but also into tassels, leaf axils and holes in stalks caused by insect borers. Adults are quite active on the plant surfaces, often performing elaborate wing-flapping behaviors in coordination with moving rapidly at or around other flies. Mated females are often observed on or beneath overhanging corn silks. Larvae emerge from eggs in 2 to 4 days and start feeding on silks just inside the silk channel. Larvae in tassels feed on developing anthers prior to pollen shed. Larvae can complete development on silks without causing any direct injury to kernels or cob. But those that continue into the ear feed extensively on kernels at the ear tips, or may disperse randomly throughout ears with loose-fitting husks. Larvae complete development in 12 to 21 days. Larvae exposed during sampling of tassels or ears quickly seek shelter by crawling or flicking themselves from plant surfaces by grabbing and quickly flexing and releasing the end of their abdomens with their mouth hooks. Most larvae leave the plants to pupate in or near the plant surface, but some pupate in dried silk at or near the opening of the silk channel and in tassels. Adults emerge from their puparia in 7 to 10 days to complete a generation in 21 to 35 days, depending on species and temperature.

Damage

These flies have become year round pests of corn in southernmost Florida. Corn in central Florida is attacked in late spring and early summer, while northern Florida corn is prone to attack throughout the summer. The larvae of these flies damage corn in several ways. By feeding on silk (i.e., the stigmas of the corn flowers within the ear) inside the silk channel, the larvae disrupt pollination and reduce kernel density. Presence of larvae in the silk channel and damage to silks (with associated fermentation of decaying silks) can result in reduced grade at harvest. The decaying silks attract secondary infestations of sap (picnic) beetles that also attack the kernels near the ear tips. 'Corn silk fly' larvae also feed on the cob and kernels rendering the ears unmarketable. Larvae slice through the soft pericarp of milk-stage kernels with their retractable mouth hooks and can completely consume the endosperm leaving empty kernels. While adults can be found on almost anything around fields during the spring and summer in infested areas, adults appear to be attracted to fields infested with fall armyworm or corn earworm. Fields infested with fall armyworm larvae during the late whorl and tassel-push stage frequently battle sustained adult movement into fields throughout the ear stage. In southern Florida, and other areas with appropriate local food reservoirs, these flies are quick to reenter treated fields and require frequent applications of insecticide applications to produce a marketable crop.

Table 5. 

Corn silk fly

Management Options

Recommendation

Scouting/ Thresholds

Adults are the target for control, because eggs, larvae and pupae are protected within ears and soil. Adults are frequently found on the tassels and upper leaves early in the morning, late afternoon and early evening. This is the best time to scout for them because they move down into the shaded canopy on stems and leaves at or below ear level by mid morning.

Notes

Organophosphate, carbamate and pyrethroid insecticides provide the best control of adults. Methomyl and thiodicarb provide control on contact but have little if any residual activity once they have dried on the leaves. Pyrethroids generally provide the best residual control, but toxicity is often reduced by over 75% within 60 hours. Sugarcane fields provide reservoirs for fly reintroduction following insecticide treatments of neighboring corn fields.

Natural

Enemies

Spiders feed on adults. Predacious insects (e.g., minute pirate bugs and earwigs) likely feed on eggs and larvae within ears. Ants, predacious ground beetles, earwigs and spiders feed on larvae on the soil surface.

Resistant

Varieties

Cultivars with high maysin content in silks (i.e., > 9% dry weight) impart partial resistance to these insects, but no commercial varieties are currently available with this compound.

Cultural

Controls

Rapid destruction of susceptible primary food crops and saprophytic food sources immediately following final harvest removes important food reservoirs and reduces production of flies that will infest nearby fields. Avoid planting to fields surrounded by fruiting vegetable or cucurbit crops, particularly if it is likely that these crops will be abandoned prior to or during the last three weeks of the corn crop.

Cucumber Beetles

Description

Adult banded cucumber beetles (Diabrotica balteata LeConte) (Figure 14) are small (1/4 to 3/8 in. long) and oval-shaped with a dirty yellow to dark green thorax and abdomen. The yellow transverse bands across the wing covers (elytra) and red to reddish-brown heads serve to separate them from the southern or spotted root worm adults (Diabrotica undecimpunctata howardi Barber) which have 12 black spots arranged in 3 rows across their elytra. The third tarsal segment on all legs in both species is laterally swollen into two lobes with the last (fourth) tarsal segment originating from between the lobes. Both species produce small, oval-shaped, pale yellow eggs. The white, elongate and soft bodied larvae have three pairs of minute legs. Larvae are indistinguishable between the two species. The white to pale green pupae are found within loosely woven cocoons in the soil.

Figure 14. 

Banded cucumber beetle.


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Biology

Adults have a wide host range and feed on numerous vegetable and agronomic crops throughout the state. Pigweeds (Amaranthus spp.) provide an important food source for these beetles in the summer when most vegetables are not available in southern Florida. Adults feed on leaves and flowers. Adults enter the soil to deposit eggs individually or in clusters of up to 15 eggs near roots. Larvae emerge in 6 to 8 days and feed on roots and other soft tissue at the base of plants. After feeding for 15 to 22 days on corn roots, larvae pupate within a loose silken cocoon beneath the soil surface. Adults emerge in 16 to 20 days. Banded cucumber beetles complete development in 35 to 50 days.

Damage

Cucumber beetles are found throughout Florida, with the banded species more common in central and southern Florida and the spotted one more common in northern Florida. Most of the observed damage to sweet corn results from adults feeding on leaves. Adults are good fliers and can invade and damage fields quickly. In southern Florida, large numbers of adults move into sweet corn and other vegetables from weeds following the fallow summer months. Adults feed on corn leaves, frequently in the whorls, producing irregular shaped, often elongate holes in young leave tissue. Delayed growth, plant stunting, and stand loss can result from heavy feeding damage to whorls of seedlings. Feeding damage to older leaves is evidenced by notched leaves, window paining, and irregular patches with only the leaf veins remaining. Adults feed on anthers on the tassel and silks emerging from ears. Silk pruning can disrupt pollination and result in irregular kernel growth and blanks within the ears. Larvae damage roots and can enter the corn stalk just above the roots where they can eat the crown and kill the buds of young plants. Bacterial wilt and other pathogens can enter the plant through such wounds. Severe root feeding to young plants before brace roots are produced can result in lodging. Such root pruning is characterized most frequently by goose-necking as the leaning plants continually try to compensate by redirecting their growth upwards. Estimates of yield effects for larval or adult feeding have not been determined for Florida corn. Goose-necking and lodging of sweet corn are more common during the fall months in southern Florida.

Table 6. 

Cucumber beetles

Management

Options

Recommendation

Scouting/ Thresholds

Seedling and silking stage plants are most sensitive to adult feeding. Look for adults, feeding damage and thread-like or oily spots of fecal material at feeding sites while scouting for army- and ear-worms.

Notes

Chemical control of adults is through contact or bait insecticides. This latter type selectively treats the beetles as they eat the baits. Numerous pesticides are labeled for mostly at-plant treatment of cucumber beetle larvae. Cucumber beetles are not controlled by Bt-enhanced sweet or field corn plants. Therefore, plants in sensitive seedling and silking stages still require scouting to find and treat these pests.

Natural

Enemies

Several predacious insects and spiders prey on these insects, but migration of large numbers can quickly overwhelm the natural enemy complex.

Cultural Controls

Control of cucumber beetle infested weeds on ditch banks and surrounding fields should be timed so that beetles are not forced to look for food during sensitive periods in sweet corn development.

Wireworms or Click Beetles

Description

Many wireworm species are found in soils feeding on plant roots. Melanatus communis Gyllenhal is the most common species feeding on corn in southern Florida.

The adult stage (Figure 15) of this insect is a slender, somewhat flattened, medium to dark brown or gray beetle between 1/2 and 7/8 in. long. The exoskeleton is smooth or with very short hairs. They have a large tooth-like projection between the rear legs that fits into a groove on the undersurface of the abdomen. These beetles feign death when disturbed and can then right themselves from their backs by quick flexion at the juncture of the thorax and abdomen. This behavior produces a clicking sound in some species of click beetles. The larvae are called wireworms (Figure 16) and have narrow, hardened, creamy yellow to orangish-brown somewhat flattened to tubular shaped bodies, depending on species. Characteristic hardened projections on the next to last abdominal segment of larvae can be used for species identification.

They have three pairs of short true legs and no prolegs and can reach 1 1/4 in. long. Larvae form a hollow, ovoid-shaped chamber in the soil in which to pupate.

Pupae are not formed within a cocoon and their legs, antennae and wing buds are completely visible.

Figure 15. 

Adult stage - Click beetle.


[Click thumbnail to enlarge.]

Figure 16. 

Corn wireworm larva.


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Biology

Adults feed on living and decaying plant material and are frequently observed in corn leaf axils and whorls during the late spring. Eggs are deposited in soil near plant roots, particularly around grasses, during summer months. Wireworms take two to several years to complete larval development. The pupal stage is passed in the soil from which adults emerge within 2 weeks.

Damage

Larvae attack seeds, roots, and crowns of plants below the soil surface. They chew into the base of plants and then hollow out the stem, eliminating the growing points of young plants. Young plants first exhibit severe wilting and desiccation of the youngest leaves. Such damage to young plants results in stand loss. Wireworm damage symptoms are nearly identical to those of lesser cornstalk borer, although young plants damaged by the latter are much more likely to send up suckers than those damaged by wireworms. Wireworms also do not form silken feeding tubes as do lesser cornstalk borer larvae. Lodging results following strong winds and rains when roots have been pruned or damaged by larval feeding.

Table 7. 

Wireworms

Management

Options

Recommendation

Scouting/ Thresholds

Larvae are frequently found in or very near plants where the youngest leaf has just started to desiccate from damage to the seed, hypocotyl, or plant base as indicated by a light greenish grey cast to the youngest leaf. Soil within 8 in. diameter around affected plants should be excavated in search of larvae in or near these plants. If infestation density is in question, then fields should be sampled for these pests to determine the best treatment strategy before corn is planted. Baits of oat, corn or potato buried in fields and recovered in 2 to 3 weeks work well to monitor wireworms. Average counts greater than 2 per bait are enough to recommend treatment.

Notes

The great majority of commercial sweet corn seed now comes pre-treated with effective insecticides for wireworm control. Insecticides also are available for pre-plant broadcast and at-plant banded in-furrow applications for seeds not pre-treated for wireworms. Soil moisture plays an important role in releasing insecticides from their carriers, consequently wireworm control in dry soils is more difficult. Pre- or post- plant irrigation of dry soils and adequate mixing of insecticides with the soil around and in the seed furrow will help to improve control. Wireworm populations tend to be greater on the high organic matter soils of the Everglades Agricultural Area in southern Florida than in the surrounding silica type soils.

Natural

Enemies

Birds such as cattle egrets that follow farm equipment through the field eat many wireworms exposed during field disking.

Cultural

Controls

Corn planted to weedy fields, or in rotation from pasture or sugarcane, are often exposed to very large wireworm populations. Summer flooding of fields is an effective cultural control, providing it is maintained for at least several weeks. Water temperature should be above 82° F for maximum control. Rotation through a cycle of rice has been shown to eliminate the need for wireworm treatment in the subsequent crop.

Aphids

Description

The two most commonly encountered aphids in sweet corn are the birdcherry oat aphid, Rhopalosiphum padi (L.) (Figure 17), and the corn leaf aphid, R. maidis (Fitch) (Figure 18). Both species are small to medium sized (3/32 in.), elliptical- to pear-shaped aphids. Aphids have a pair of short, tube-like structures (cornicles) that extend backward and upward from near the end of the top surface of the abdomen. Wingless adult birdcherry oat aphids are medium green with reddish areas toward the end of the abdomen and area around bases of cornicles. Wingless adult corn leaf aphids are light to medium green with short, dark green to black legs and cornicles, but lack the reddish areas around the cornicles as in birdcherry oat aphids. Nymphs of both species are similar in color and shape to adults, only smaller. Winged adult forms of both aphids have black heads and thoraxes, and green abdomens.

Figure 17. 

Birdcherry oat aphid.


[Click thumbnail to enlarge.]

Figure 18. 

Corn leaf aphid.


[Click thumbnail to enlarge.]

Biology

Adult and immature aphids feed on phloem sap sucked from leaves, stems, tassels, and husks through their long piercing-sucking mouth parts. Excess water and sugars are voided from the body. These honeydew deposits on leaves serve as a source of food for sooty mold fungi, ants, sap beetles, and flies. Winged or wingless adult females deposit live nymphs, skipping the external egg stage in Florida. Adults produce from 3 to 5 nymphs per day for up to several weeks. This allows for rapid population development. The nymphs pass through several instars before molting into adults in 7 to 10 days. There is no pupal stage in aphids.

Damage

Aphids in sweet corn are more common during the late winter and spring seasons in southern Florida. Corn leaf aphid can develop to large populations in fields with light fall armyworm infestations that have not required much insecticide treatment. Large populations of corn leaf aphid feeding on the emerging tassels of many plants within an area can result in poor pollen shed and fertilization. Both species of aphids can be found on ears, but corn leaf aphid can become more difficult to control on ears when they have been allowed to build up in the whorls and tassels prior to silking. Presence of aphids on the husk surface or between layers of the husk can throw ears out of grade. Honeydew from aphid feeding makes husks sticky and encourages sooty mold growth that can further result in husk discoloration and grade reduction.

Table 8. 

Aphids

Management

Options

Recommendation

Scouting/ Thresholds

Look for aphids (particularly corn leaf aphids) within the corn whorls, on husks beneath silk or in region between ear and stalk near ear base. Insecticide applications for fall armyworm and corn earworm control often kill aphids before they can develop to damaging densities.

Notes

Aphid populations are easier to control while still infesting the whorl. Aphids feeding on husks are somewhat protected from chemical control strategies by the corn canopy and silks. Pesticides are available for post-emergence foliar treatments. Birdcherry oat aphids are less susceptible than corn leaf aphids to pesticides frequently used for armyworm control. Pyrethroids have shown good control of aphids within the whorl in pre-tassel stage sweet corn.

Natural

Enemies

Many types of natural enemies may control these aphids under low pesticide input situations, such as in field or sileage corn. However, these aphids appear in sweet corn at a time when pesticide use increases for armyworm, thereby greatly reducing the potential impact of their predators and parasitoids. Pathogens kill many winged adults before they begin producing nymphs.

Figure 19. 

Figure 20. 

Figure 21. 

Figure 22. 

Figure 23. 

Figure 24. 

Figure 25. 

Figure 26. 

Footnotes

1.

This document is ENY-472, one of a series of the Department of Entomology and Nematology, UF/IFAS Extension. Original publication date November 2001. Revised March 2010, June 2013, and February 2017. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

G. S. Nuessly, associate professor, crop protection, UF/IFAS Everglades Research and Education Center; and S. E. Webb, associate professor, Department of Entomology and Nematology; UF/IFAS Extension, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label. Use pesticides safely. Read and follow directions on the manufacturer's label.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.