Insects and Pests of Field Crops: Corn Insects and Pests
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This section describes insects and pests that affect only corn. The insects and pests listed below affect corn as well as other crops.
Description: Larvae are greyish-black with a paler underside and no distinct markings on their bodies. Mature larvae are about 3.5 cm (1 1/4 in.) long and hide in the soil during the day. They can be found near freshly cut plants, under soil clumps or along a poorly closed seed furrow. Adult moths are grey with a small black dagger marking on each forewing.
Life History: Cutworm moths do not over-winter in Ontario but are carried in from the south on strong southerly weather systems. The heaviest immigration occurs from April to May but may occur as early as March. Cutworms are therefore more frequent in fields with green cover early in the spring. There are several generations per year, however, only the first generation causes economic damage in corn. Warm, clear, calm nights in early spring are ideal for moths to lay eggs. Eggs are laid on dense vegetation, low to the ground, and are usually laid before primary tillage in the spring. In fields with vegetation present before planting, larvae will develop on the weeds until the crop has emerged. In this situation, the larvae that move over to the crop will be larger and more difficult to control. Feeding occurs from early May through to mid-June.
Damage: Fields located along Lake Erie experience frequent cutworm infestations. Plants attacked by young larvae will have small holes or gouges in the leaves. Plants may suddenly wilt, because the stem has been hollowed out or fed on underground. Larger larvae cut off the plant at or just below ground level. Factors that favour cutworm outbreaks include a history of cutworm damage, presence of winter annual weeds such as chickweed and volunteer wheat before planting, no-till and heavy crop residue.
Scouting Technique: Start scouting for cutworm once every 5 days as soon as corn emerges. Scout at least five locations for every 10 ha (25 acres) of field. Pay particular attention to those areas where weeds were heavy just before tillage and planting. Look for leaf-feeding (pinholes) by young climbing larvae as the first sign of damage. Also look for wilting plants, foliage-feeding or for plants being cut off at the ground. Dig around damaged plants and search through the soil, as cutworms like to hide in the soil during the day. Note the size of the cutworms found.
Action Thresholds:If more than 10% of plants show leaf feeding, treating with a foliar insecticide will give nearly 100% control. Once corn reaches the five-leaf stage and begins to produce roots at the base of the plant, the risk has passed. Cutworms that are nearly mature (over 2.5 cm long) are difficult to control with insecticides and will stop feeding in a few days when they reach full size.
Description: Very tiny (1.8 mm), black, shiny beetles with elongated hind legs, which are used for jumping when disturbed.
Life History: This pest over-winters as adults at the base of grasses. In the early spring, beetles emerge, and mated females then lay their eggs in the soil close to the base of corn plants. Within 6 days, eggs hatch into larvae, which then pupate. Within 14 days, the adult beetle emerges from the soil. There are three to four overlapping generations per year. Only those generations occurring from early-May to late-June, during corn emergence, are considered a potential problem.
Damage: Planting seasons following mild winters experience higher beetle activity. Only susceptible varieties and seed corn inbreds show yield loss. Long feeding scratches or window-paning are found on the leaves, usually running parallel with the leaf veins.
Flea beetles are vectors of Stewarts' bacterial wilt, which over-winters in the gut of the insect. Symptoms of Stewarts' wilt include linear lesions on the leaves with wavy edges. Plants may wilt or become stunted from this disease.
Scouting Technique:Scout every 4-5 days. Inspect five sets of 20 seedling plants per field to determine the presence and density of the beetle.
Action Thresholds: For susceptible varieties, 6 beetles per 100 plants prior to the fifth-leaf stage warrant control. For tolerant varieties, an average of five or more beetles per plant prior to the fourth-leaf stage may warrant control.
Models help predict the risk of high adult activity each year based on winter temperatures and adult survival.
Description: Larvae are light purple to pink, and approximately 4 cm (1 1/2 in.) in length when mature. Adults are brown moths that are active during August and September.
Life History: This pest is more severe in the eastern counties, but infestations occur throughout the province. There is one generation per year. This pest over-winters as eggs on grasses. Eggs are laid in two or three parallel rows under the leaf sheath. These hatch in late April, and the small larvae initially bore into the stalks of grass plants. By late May, the larvae outgrow the grass stalks and move to nearby thicker stalk crops such as corn. These larvae continue to feed until late June, pupate and emerge as adults in late July.
Damage:Larvae are the only damaging stage. Damage is usually most severe at field edges or near grassy, weedy areas. Larvae feed on plants in late May through early June. On young plants, the larvae will burrow into the base of the plant, below the soil line. Plants may be cut at the base, similar to cutworm injury. At the three or more leaf stage, they feed inside the whorl at the base of the corn plant, causing the upper one or two leaves to wilt, while the lower part of the plant remains healthy. Fields at higher risks include grassy or weed-infested fields the previous year; fields following sod and conservation-till fields.
Scouting Technique: The larvae are usually found within the stem or in soil near the base of the plant. Look for potato stem borers along fencerows or in grassy areas.
Action Threshold: No threshold is available at this time.
True armyworm is an important pest of corn. For Information on this insect.
Description: European corn borer (ECB) egg masses are flat, creamy white and layered over each other, making the egg mass appear similar to fish scales.
Mature larvae are creamy white to pale grey with 2 small spots per abdominal segment, approximately 2.5 cm in length and have a black head.
Adults are light-brown moths approximately 2 cm long with dark wavy lines running across each forewing. Male moths are darker and smaller than females.
Life History: There are two distinct strains in Ontario. South of a line from Sarnia to Simcoe, a bivoltine strain can undergo multiple generations (typically two), depending on the length of the season. North of this line, a univoltine strain has only one generation per year. There is a band of overlap for these two strains, about 50-80 km wide along this line.
The insect over-winters as larvae in corn stalks and other residue left on the surface from the previous growing season. As day-length increases and average day temperatures exceed 10°C, the larvae pupate. Pupae are found within larval feeding tunnels and require 2 weeks to develop before adults emerge.
While emergence begins around the third week of May in the southernmost regions of the province, moths do not usually appear until mid-June in Eastern Ontario. Once moths emerge, they fly to nearby "action sites" or vegetative habitats, such as fencerows, ditches and hedgerows along fields.
Once mated, females leave the action sites to lay eggs on the host crop. Eggs are generally laid on the underside of leaves, close to the midrib. Where univoltine ECB are present, larvae develop through the season until autumn, when as fifth instars they prepare for over-wintering. Where bivoltine ECB are present, first-generation larvae will pupate in mid-summer, emerge as adults and complete a second generation before entering diapause in the fall.
Damage: Early-season larvae feed on leaves, creating small pinholes and eventually migrate into the whorl of the plant and attack the enclosed tassel. Later-season larvae feed briefly on the leaves, bore into the midrib of the leaf and then migrate into the stalk of the plant and husk of the ear. Larvae may also feed directly on the developing kernels. Stalk lodging and ear droppage may occur as a result of significant infestations. This pest can carry both stalk rots and ear rots into the plant. High-risk factors include no-till fields with high residue, those with frequent corn crops in the rotation, regions with a high percentage of corn (50% or greater in region) and regions where univoltine and bivoltine strains overlap.
Scouting Technique: Early-season moths are attracted to taller, early-planted corn fields while second-generation corn borer female moths are attracted to later-planted fields that are silking/tasselling. Examine a minimum of five sets of 20 plants per field (100 plants per field).
First generation scouting: Look for leaf-feeding damage. Pull out and unroll the whorl of the damaged plants, looking for small larvae. Split the stalk of the plants from top to bottom to locate older larvae. Record the percentage of damaged plants and number and size of larvae found.
Look for egg masses on the underside of leaves close to the midrib of the plant. Concentrate scouting efforts to the three leaves above and below the ear of the plant. Record the percentage of plants with egg masses. Repeat scouting every 5-7 days until peak moth flights have subsided (approximately 1 month).
Management Strategies for Non-Bt Corn Hybrids:
Management Strategies for ECB Bt Corn Hybrids:
The insect resistance management strategies include:
The Canadian Corn Pest Coalition website, and the publication, A Grower's Handbook: Controlling Corn Insect Pests With Bt Corn Technology, provide specific refuge requirements and ECB biology.
Growers planting stacked Bt corn hybrids containing both ECB and corn rootworm (CRW) Bt must follow the CRW refuge requirements outlined for corn rootworm.
Description: There are two species of corn rootworm (CRW) in Ontario. Western corn rootworm (WCR) adults are yellow to green with three black stripes on their wings Plate 75
The females typically have three wavy black stripes on their wing covers while the three stripes on males are fused and are undifferentiated. Male WCR adults are also slightly smaller, and their antennae are longer. Northern corn rootworm (NCR) adults are uniformly green to yellowish-beige with no particular markings that differentiate males and females Plate 76.
Larvae are white with a brown head and a distinct dark tail plate. They are approximately 1 cm (1/2 in.) in length. Plate 74
Do not confuse the western corn rootworm with the striped cucumber beetle. The striped cucumber beetle's abdomen on the underside is black, and its stripes are distinctly parallel, not wavy.
Life History: Both WCR and NCR are uniformly distributed across Ontario. In Southwestern Ontario, WCR predominate with a ratio of greater than 4:1, WCR to NCR. In Eastern Ontario and Quebec, the ratio is opposite, with 8:1 NCR to WCR. Both have only one generation per year. Eggs are deposited in the soil from July until a killing frost in the fall. The eggs over-winter and begin hatching in early June. Adults emerge in late July where they feed on silks and tassels.
Damage: Both adults and larvae feed on corn. Larvae feed on and within the roots from mid-June to mid-July, interfering with nutrient and water uptake, causing stress to the plant. Larger larvae feed on the brace roots, reducing the stability of the plant, causing it to lodge or gooseneck. Adults feed on pollen and clip the silks, interfering with pollination. If tassels and ears have not emerged, they will feed on the leaves, stripping tissue on the underside of leaves between the veins, leaving "window panes." Risk factors include continuous corn fields, fields with high beetle populations in corn from previous seasons or being the latest field planted in the previous season.
Scouting Technique: Monitor 20 plants in five different locations in your field weekly from when adults emerge at the end of July to the end of August.
Action Thresholds: If there is less than one beetle per corn plant on average throughout the month of August, then no insecticide to control CRW larvae is necessary in the following corn crop (Note: 1 WCR = 2 NCR when counting adults). Field corn can withstand heavy adult activity, usually requiring at least 10 adults per ear before control is necessary, but seed corn may require control if adult populations are causing extensive silk clipping, disrupting pollination. Dry conditions may keep the plants from growing more silk to compensate for the feeding injury. Treatment is warranted when the silks are on average being clipped down to within 1.25 cm (1/2 in.) of the ear tip. After pollination is complete, beetle feeding no longer poses a threat to yield.
Rootworm is one of the most adaptable insect pests and has developed resistance to many forms of control used against it. Therefore, it is very important to use control products against this pest only when necessary.
Management Strategies for CRW Bt Corn Hybrids and Stacked Hybrids Containing Both ECB and CRW Bt:
corn, dry edible beans
Description: Western bean cutworm (WBC) larvae are tan to pink in colour and do not have warts or spots (tubercles) on them, unlike European corn borer. The only distinguishing marking WBC larvae have is on their pronotum, the shield-like structure just behind the head of the larvae. The WBC's pronotum has two broad dark brown stripes. Adult moths are easy to identify from other corn pests. Each wing of the moth has a white band running along the edge or margin of the wing and has a spot or "moon" and boomerang-like mark on it.
Eggs are laid in masses of 5-200 eggs. WBC eggs are the size of a pin head, pearly white when first laid and are shaped like tiny cantaloupe Plate 78. As the eggs mature, they turn tan and then purple in colour. Eggs hatch in about 5-7 days.
Life History: Western bean cutworm is native to North America, though it has resided mainly in the Southwestern U.S., until its recent range expansion northeast across the Midwest and now into Ontario. WBC over-winter in other regions (though it is expected to successfully over-winter in Ontario) as larvae in soil chambers. Adult moths emerge and are actively flying by early June through early July. They lay eggs on the upper leaf surface of the upper leaves of the corn plants and prefer hybrids that hold their leaves upright. Adults prefer fields in the whorl-to-pretassel stages of corn. Once the corn crop is in tassel or beyond, they prefer to lay their eggs on the dry bean crop. Eggs hatch within a week. Unfortunately, the larvae are very mobile and can disperse from the original egg site to other plants in the vicinity both up and across corn rows. High-risk fields include no-till fields and fields with sandy soil.
Damage in Corn: Young larvae feed on the tassels and silks until they are large enough to tunnel into the ear and feed extensively on the kernels. In whorl-stage corn, larvae will feed on the developing pollen. Entry holes can sometimes be seen on the outside of the husk though they can also enter through the silk channels. Unlike corn earworm, western bean cutworms are not cannibals and therefore multiple larvae can feed on the same ear. Additional impact to quality can be expected from ear rots and secondary pests that may come in and feed on the damaged ears.
Damage in Dry Edible Beans: Damage begins as leaf feeding, but once the larvae get bigger, they will move to feed on and into the pods and seeds.
Scouting Technique in Corn: Scout 20 plants in five areas of the field. Focus efforts on the top three-to-four upper leaves of the plant. Look for egg masses and young larvae. Pheromone traps can be used to monitor for moth flight, which will indicate when eggs are being laid in the field and when to initiate scouting efforts. Contact the provincial entomologist for pheromone trap configurations, supply sources and monitoring protocols.
Scouting Technique in Dry Edible Beans: Thresholds are based on monitoring adults through pheromone traps first to indicate if and when scouting is necessary. Contact the provincial entomologist for pheromone trap configurations, supply sources and monitoring protocols. Monitoring with traps consists of placing two WBC pheromone traps per bean field. Place traps on opposite sides of the field no later than the last week of June and monitor them through the growing season. Moth catch totals are accumulated over time until peak flight occurs. When adult populations start to decline from last week's counts, the previous week was the peak flight. This requires checking traps regularly to ensure all moths are counted.
Action Threshold in Corn: Spray is warranted if 5% of the plants have eggs or small larvae. If the eggs have hatched, spray at 95% tassel emergence or if tassels are already emerged, when most of the eggs are expected to hatch.
Action Threshold in Dry Edible Beans:If the accumulated moth catch is less than 700 moths per trap, there is low risk of reaching damaging levels in that field. If the accumulated catch is between 700 and 1,000 moths, damage risk to beans is moderate and beans must be scouted closely. Check fields for larvae and larval feeding 10-20 days after the peak, and spray if pod feeding is found. If the catch exceeds 1,000 moths by the peak, risk is high for damage in dry bean and an insecticide application is needed 10-20 days after peak flight. Spray any edible bean field that is neighbouring a corn field that has reached threshold for WBC.
Management Strategies for Corn and Dry Edible Beans:
Description: Corn earworm larvae vary greatly in colour from light green to yellow. The full-grown larvae are 4 cm (1 1/2 in.) long with prominent stripes running the length of their bodies.
The size and presence of the stripes differentiate earworm from European corn borer, while its tan head colour differentiates it from fall armyworm.
Adult moths are buff or tan coloured. The forewing has a central brown dot visible from the underside of the wing, and the hind wings are pale in colour, with a darker brown border. Egg masses are difficult to see, as they are the same colour and width of a strand of corn silk.
Life History: Corn earworm, also known as cotton bollworm - a pest on cotton, does not over-winter in Ontario but migrates as adult moths from the southern U.S. Usually they arrive in Ontario in August but they may come as early as late June. The moths lay their eggs individually on fresh silks. The eggs hatch, and the larvae feed on the silks and kernels at the ear tip. Larvae will pupate but die soon after frost.
Damage: Larvae may feed on leaves and tassels but mainly are found feeding on silks and developing kernels. Larvae damage tassels, causing poor pollination, and consume silks, affecting ear development. Fields at risk are those planted late that will be freshly silking during peak larvae population time.
Unlike European corn borer, western bean cutworm and fall armyworm, corn earworm does not leave entry holes into the ear husk as it enters directly via the silk channels. Feeding is concentrated at the top third of the ear tip.
Scouting Technique: Locate five sets of 10 plants per field and open the ear to inspect for feeding damage or larval presence, including the presence of ear moulds carried in by the pest. Determine the percentage of ears infested. Corn earworms are cannibalistic and, therefore, there is usually no more than one larva per ear of corn. Eggs are the same size and colour of a strand of corn silk and therefore are not practical to scout for.
Action Thresholds: This pest is usually only an economic pest in sweet corn but can affect late-planted seed corn fields that are silking at the time of egg-laying.
Full-grown fall armyworms are 4 cm (1 1/2 in.) long, varying in colour from light tan or green to near-black. Three white, thin strips run down the back. One thicker, yellow band with red spots runs along the side, just above the legs of the larvae.
The fall armyworm larvae can be distinguished from the true armyworm by a white, inverted "Y" on the front of the head. The fall armyworm head is dark brown to black. Though the larvae have similar stripes to the true armyworm, fall armyworm larvae also have elevated spots with hair sticking out of each one. Four of these spots form a square on the top of the last abdominal segment of the larvae. Unlike true armyworm, fall armyworm do not have black bands on their prolegs (chubby back legs).
Adults are dark grey moths with a mottled pattern on their wings and a prominent white spot on the very tip.
Life History: Fall armyworm adult moths migrate from the southern U.S. and show up later in the season, when the corn is fully grown. This insect cannot over-winter in areas where the ground freezes.
Damage: Fall armyworm is an occasional pest of grass crops such as corn and wheat. The larvae feed on the whorl leaves and ears predominately from late July to September. Fall armyworm feeding occurs in the daytime, unlike true armyworm feeding, which occurs at night. Initial leaf feeding appears as tiny holes similar to ECB feeding but as the larvae grow, holes become very large, with ragged edges. Moist, reddish-brown frass can be found nearby.
Damage to the ear is similar to that of corn earworm. However, the fall armyworm entry hole can easily be seen coming from the side of the ear, and feeding is throughout the ear. Corn earworm enters the ear through the silk channels, and feeding is typically concentrated close to the tip of the ear.
Examine 20 plants from five locations in your field to determine the level of infestation. Record the size and number of larvae. Scout the field perimeters, as armyworm can also move in from neighbouring corn fields.
When scouting, check the backs of armyworms for parasite eggs. These small, oval, yellowish eggs are usually located just behind the head of the larva. These are eggs of a parasitic fly whose maggots will kill the armyworm larvae.
Action Threshold for Corn: If 50% of the plants are infested with unparasitized larvae smaller than 2.5 cm (1 in.), insecticide treatment may be warranted. However, damage is usually not economical unless infestations are high, and feeding is concentrated on the undeveloped tassels.
Action Threshold for Winter Cereals:
Two-to-three unparasitized larvae, less than 2.5 cm (1 in.) in length, per linear foot of row, particularly when the crop is in the younger seedling stages.
Management Strategies in Corn:
Management Strategies in Winter Cereals: Fall armyworm populations are rare in winter cereals. If the action threshold is reached, a spray may be necessary.
Description: These aphids are small (2 mm or less), bluish-green, soft-bodied insects with black legs and short black cornicles ("tailpipes") near the rear of the abdomen Plate 81. They have piercing and sucking mouthparts and feed on the juices (nutrients) of young plant tissue (tassel and whorl). They secrete a sticky substance referred to as "honeydew," which can become coated with a blotchy, sooty mould.
Life History: This pest does not over-winter in Ontario but arrives each year on air currents from the south where crops are more advanced. Initial spring migrants feed on cereals, until corn becomes attractive. Migrating populations are comprised of winged females only. Once they settle, these females reproduce without mating and give birth to live wingless nymphs. Both winged and wingless generations of adults develop, depending on the nutrient quality of the plant. Winged aphids then fly to nearby corn fields and enter the whorl. There are several generations per year.
Damage:The degree of feeding injury depends on the size of the population. Nymphs and adults feed primarily on the whorls of the plant, removing nutrients and water. Symptoms include yellowing, wilting and curling of the leaves. During dry periods when the plants are stressed, symptoms may be amplified. As densities increase, leaf surfaces and tassels often become black and sooty as mould begins to grow on the honeydew. Tassels may become gummy, causing poor pollination. They are also vectors of maize dwarf mosaic virus and barley yellow dwarf virus.
Scouting Technique: Examine five sets of 20 plants per field.
Action Thresholds: If 50% of all plants during the late-whorl-to-early tassel stage have 400 aphids per plant, and plants are under moisture stress, control is required.
Management Strategies: Chemical control is warranted only if the natural enemies and parasites of the corn leaf aphids are not present and aphid densities are above threshold. There are several natural enemies that exist and are quite effective at controlling corn leaf aphids. These include lady beetle adults and larvae, lacewing adults and larvae, and a few parasitic wasps.
Chemical control will kill natural enemies and may lead to a resurgence of the aphid population.
For more information:
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