Insects and Pests of Field Crops: Cereal Insects and Pests
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811: Agronomy Guide> Insects
and Pests of Field Crops > Cereal Insects and Pests >
Table 13-6, Cereal Insect Symptoms in the
Field, shows insects and pests that may be causing symptoms
in the field.
This section describes insects and pests that affect only cereal crops. The insects and pests listed below affect cereals as well as other crops.
The bird cherry-oat aphid is the most common aphid found in Ontario cereals. These aphids are small, 2 mm (1/12 in.) or less. Adults are olive-green with patches of red-purple near the rear of the abdomen, between a pair of tubes called cornicles Plate 96. The cornicles and legs are pale green; the antennae are long and black. Younger aphids are light green.
The English grain aphid is typically the largest of the three species
and is pale green or light orange with long legs that may appear
green to black. They have long black antennae and cornicles.
The corn leaf aphid is also olive green, but the legs, cornicles and antennae are black and the shape of the body is more rectangular, while the bird cherry-oat aphid is bulb-shaped. Similar to all aphids, these species are soft-bodied, winged or wingless, with piercing and sucking mouthparts that suck the juices (nutrients) from young plant tissues. Aphids secrete a sticky substance referred to as "honeydew," which can cause sooty mould.
Cereal aphids have over-wintered in Ontario, particularly in mild winters with prolonged snow cover. Fields planted in late summer or early fall (August/September) are at highest risk of fall infestations.
Seldom a direct problem in Ontario. On young plants, aphids cluster on the upper sides of leaves near the base of the plant. Eventually, aphids will climb to the top and can be found in the leaf whorls. High populations can result in fields appearing to have large bronze patches. Cereal aphids are vectors of barley yellow dwarf virus (BYDV). See Barley Yellow Dwarf Virus.
Fall scouting for cereal aphids is important, as early-season infection with BYDV is most harmful to cereals. In spring, scout the field weekly prior to heading. Examine 20 stems in five areas across the field. Shake the plants over a piece of paper and count the number of aphids present or look for colonies specifically at the leaf collar. Also make note of any predators present and whether the aphids are parasitized or infected with a fungus.
Prior to the heading stage, 12-15 cereal aphids per stem, and up to 50 aphids per head once headed.
Adult Hessian flies resemble small mosquitoes. They are smoky grey and fragile, and have pointed abdomens that are a dull red. Adults are weak flyers and only live about 3 days. Larvae are 2 mm (1/12 in.), legless, white maggots. Pupae are reddish-brown, shaped like flax seed and can be found at the base of the plant in late fall and early spring.
Two generations occur per year. The Hessian fly over-winters as "flax seed" puparia in the base of old plant crowns. Adults emerge in the spring. Rain triggers adult emergence. Females lay their long, reddish eggs in rows like sausage links, on the upper surface of leaves of young winter wheat or volunteer wheat. Larvae develop and feed for approximately 3 weeks before forming a puparium in mid-June. A second generation emerges, and the insect continues its cycle until late September when it forms a puparium for over-wintering.
Damage can occur in both spring and fall, though it is the fall population that is the main concern, particularly for the winter wheat crop. Other grain crops including barley, oat and rye are considered more tolerant, though infestations may still occur.
Fall plantings may stunt and turn dark-green. Larvae on young plants feed between the leaf sheath at the base of the plant. The enzymes they secrete into the plant cause the stems of the plant to thicken, the plant to stunt and the leaves to broaden. Multiple tillers can develop. Infested plants take on a dark bluish-green appearance. Winter survival of the crop can be impacted.
Spring damage by the first population is concentrated at just above the nodes where the larvae feed. Internodes become shortened, impacting nutrient transport to the head. Stems, when pulled, break easily at infested nodes. Heads can turn white and plants can lodge. High risk fields include susceptible varieties of winter wheat that are planted early, when adults are still flying.
In the spring, scout fields when heads begin to fill. Look for plants that have shortened internodes and white heads. Gently tug at the stem of the plant to see if it breaks easily at the node. Look for larvae within the internode where the stem broke off. In the fall, begin scouting 3 weeks after wheat plants have emerged. Examine 20 plants in five locations across the field. Pull away leaves to view the base of the leaf at the stem. Look for the "flax seed" puparia to determine the percentage of infestation.
None available. Control is based on prevention.
The cereal leaf beetle (CLB) adult is a metallic, blue-green beetle, approximately 5 mm (1/5 in.) in length, with a reddish-orange head and legs Plate 98.
The larvae are 6 mm (1/4 in.) in length when mature and yellowish
in colour, but this colour is obscured by a black deposit of fecal
material making it slug-like in appearance Plate
CLBs over-winter as adults in leaf litter in sheltered areas such as woodlots and heavy crop debris. These adults emerge in early spring. The mated females then lay their eggs in wheat fields on the upper surface of leaves. The eggs hatch, and larvae are present by mid-May. The larvae will pupate, and adults will emerge by mid-June. Adults feed on wheat briefly and then congregate in corn fields, feeding for a short period before going dormant until fall. In the fall, adults become active again and make their way to their over-wintering sites. There is one generation per year.
Cereal leaf beetles feed on wheat, oat, corn, forages and grassy weeds. Spring plantings are most attractive, particularly late plantings, though some winter wheat can also be infested in the spring. Both adults and larvae cause damage by chewing long strips of tissue between the leaf veins, leaving the top layer of the leaf intact. This creates a window-paning or "skeletonizing" effect. Most of the injury is caused by the larvae in June. Heavily damaged fields appear silver.
Begin scouting in late April. Examine 20 plants in five locations across the field. It is important to scout various areas of the field, as CLB tends to be unevenly distributed across the field. Record the number of beetles and larvae found per plant. Scout every 5 days, as damage can increase dramatically within days.
One CLB adult or larvae per stem warrants control, especially in the earlier stages of the crop (before heading).
corn, forages, cereals
True armyworms (also known as common armyworm), when full grown,
are 4 cm (1 1/2 in.) long. The dull-green to brown larvae can easily
be confused with other caterpillars, including variegated cutworm
and fall armyworm. Two distinguishing features of true armyworm
are white-bordered stripes running laterally along the body and
dark diagonal bands at the top of each abdominal proleg Plate
69. The head is yellow-brown with a network of dark-brown lines
creating a mottled pattern. The adult sand-coloured moth has distinctive
white spots on the centre of each forewing.
True armyworms over-winter as partially grown larvae. In early spring, the moths emerge and prefer to lay their eggs in grassy vegetation, including cereals, grassy forages and rye cover crop. Larvae hatch from the eggs and feed at night or on overcast days for approximately a month. There are two generations per year, but it is the first generation that tends to do the most damage to corn in Ontario. Outbreak years tend to coincide with cool wet springs that are detrimental to the parasites that typically control armyworm.
True armyworm larvae feed at night. Most feeding damage is done on cereals during July. True armyworms do not feed on pure stands of alfalfa but will feed on alfalfa/grass mixtures. In conventional-till corn fields, damage usually occurs first in the border rows, whereas infestations may develop throughout no-till corn following small grains or grass. True armyworm also frequently invades corn fields from neighbouring cereal fields. Larvae strip the leaf margins, moving up the plant to feed on the panicles and flowers leaving only the midrib. As long as the growing point of the plant is not damaged, the corn plant will be able to recover from moderate feeding.
The best time to scout for true armyworm is at or shortly after dusk. Examine 20 plants in five areas in the field (100 plants total). During the day, you may find the larvae in the whorl, leaf axil, amongst the crop debris on the soil surface or under soil clods. Also, you may find brown frass, often mistaken for eggs, in the whorl or on the soil near the plant. When scouting, check the backs of armyworms for eggs. These small, oval, yellowish eggs are usually located just behind the head of the larva. These are eggs of a parasitic fly. The eggs will hatch, and the maggots will kill the armyworm larvae. Record the size and number of larvae. Scout along the field boundaries bordering cereal and corn crops as larvae will "march" in from neighbouring fields and may be controlled prior to entering the corn field.
Action Threshold for Corn:
For corn past the 6-leaf stage, and cereals at any stage; if 50% of the plants are showing damage and are infested with larvae smaller than 2.5 cm (1 in.), insecticide treatment may be warranted.
As long as the growing point of the plant is not damaged, the corn plant is usually able to recover from moderate feeding. With early-season feeding, insecticide may be warranted in seedling corn if there are two or more unparasitized larvae per seedling and feeding damage exceeds 10%.
Action Threshold for Cereals:
For cereal or small grains control is warranted when five or six larvae per square foot (30 cm x 30 cm area) are found. Treat is larvae are less than 2.0 cm long and threshold has been exceeded. Avoid treating with insecticides when large numbers of parasitized larvae are present.
Action Threshold for Forages:
Control is warranted when five or more larvae (smaller than 2.5 cm) per square foot are found. Avoid treating with insecticides when large numbers of parasitized larvae are present. In seedling crops, two to three larvae (smaller than 2.5 cm) per square foot may warrant control.
is an important pest of forages.
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