Insects Pests of Wheat

Wheat and triticale can be attacked by a great many insects. Fortunately, only a few insect species are of major importance, causing severe damage over large geographical areas; most species are only occasional pests and/or are not geographically widespread. The "pest status" of many of species is not always well documented.

Aphids (various species)

Symptoms: Aphids are nearly transparent, soft-bodied sucking insects (56). When present in sufficient numbers, aphids can cause yellowing and premature death of leaves. They exude drops of sugary liquid known as "honeydew", which may cause tiny scorch marks on the foliage and tends to encourage the development of sooty molds. The feeding of Schizaphis graminin (57) is especially damaging, resulting in the development of necrotic areas sometimes accompanied by purpling and rolling of the infested leaves. The feeding of Diuraphis noxis produces long white stripes on the leaves (58), leaf rolling, postrate growth habit, and sterile heads.

Life Cycle: The life cycle of aphids involve winged (alates), wingless (apterous), sexual, and asexual forms. When feeding on cereals, the females of most aphid species reproduce asexually (without being fertilized), giving rise to nymphs rather than eggs.

Hosts/Distribution: Species commonly found on cereals throughout the world include:

• Rhopalosiphum padi (bird cherry-oat aphid)
• Schizaphis graminum (greenbug)
• R. maidis (corn leaf aphid)
• Metopoliphium dirhodum (rose grass aphid)
• Sitobion avenae (English grain aphid)
• Diuraphis noxis (Russian wheat aphid)

Importance: Aphids are important and widespread pests on cereal crops. When feeding in sufficient numbers, they can cause significant damage. In addition, the species listed above may act as vectors of barley yellow dwarf virus.

Stink Bugs (various species)

Symptoms: Adult stink bugs feed on stem tissue or developing kernels (59). Saliva from this insect is toxic to the plant, and a single feeding puncture can kill a stem. Feeding on kernels during the milk dough stage will destroy the kernel, while feeding during later development stages will badly shrivel the grain. Feeding on the developing head may cause partial or total sterility. Adult stink bugs have a shield-shaped body (60) and emit a disagreeable odor when crushed.

Life Cycle: Stink bugs over-winter as adults and may diapause. They tend to hibernate under dead leaves and grass. In the spring they migrate to cereal hosts, mate, and lay eggs at various places on the plant. These hatch into nymphs that feed on the plant. Mild winters and low rainfall seem to favor outbreaks of the insects.

Hosts/Distribution: Stink bugs will feed on most cereals and grasses, as well as a large range of weeds (depending on the species). Stink bugs are of major economic importance in Asia Minor.

Importance: Losses due to stink bugs are highly variable and depend on the density of the insects, weather conditions, and duration of the crop growing period. Losses are due primarily to reduced baking quality.

Armyworms, Cutworms, and Stalk Borers (various species)

Symptoms: The primary symptom is defoliation of the plant. Larvae feed on leaves, chewing from the edges to the midrib, or on the heads of cereal plants. Heavy infestations can be very destructive; larvae may climb the plant and sever the neck just below the head. Some species may be found feeding at the soil surface, others underground feeding on roots, and still others feeding inside the stem.

Life Cycle: Adult cutworms (61) and army worms (62) are moths, and the females lay eggs on leaves and leaf sheaths near the ground. These eggs hatch within a few days and initially the larvae (63, cutworm; 64, armyworm) feed close to where they hatch. The larvae are found in cracks in the soil or under rocks during the day, feeding at night or early in the morning. In damp weather, they may feed all day.

Hosts/Distribution: Larvae are generally omnivorous in attacking grasses. Species of these insects are found in most cereal-growing areas of the world.

Importance: Cutworms and armyworms sporadically cause severe damage; when they do, they can devastate large areas.

Cereal Leaf Beetle

Oulema melanopa

Symptoms: Adult beetles are 4-5 mm long, have a black head, light brown thorax, and a shiny blue-green wing cover with parallel lines of small dots (65). Larvae are a dull to bright yellow color, but soon take on the appearance of a slimy, globular, black mass due to the mound of fecal material they produce and accumulate on their backs (66; 67). The most prominent symptom of cereal leaf beetle infestations is the distinct, longitudinal stripes on leaves (68); these stripes are produced by the feeding of adult beetles and of larvae.

Life Cycle: The insect produces one generation per year. Adults begin their feeding activity in the spring. They lay yellow eggs, either singly or in small chains, covering them with a sticky film that the soil and the adults emerge in summer. Adults overwinter underneath plant debris on the soil surface, in leaf sheaths and ears of standing maize, or under the bark of trees.

Hosts/Distribution: Cereal leaf beetles can be a problem on fall-sown cereals. Wheats with hairy leaves are affected less.

Importance: Significant yield losses can occur in winter wheat and fall-sown spring wheat. Yield losses of from 14% to more than 25% have occurred with natural infestations.

Thrips (various species)

Symptoms: Thrips are small (1 mm long), brown or black insects with a tapering, segmented abdomen. They have piercing and sucking mouthparts and usually have two pairs of narrow wings. They are usually found behind the sheath of the flag leaf, feeding on the stem (69). However, leaves, stems, and heads may be attacked. Adults and nymphs both can cause damage and, if present in large numbers, may cause the tissue on which they are feeding to take on a silver coloration.

Life Cycle: Eggs are inserted into or attached to host tissue. The generation time is very short, and there may be 10 or more generations per year. Heavy rains will usually destroy a high proportion of they population.

Host/Distribution: Several thrips species live exclusively on cereals, and on forage or weed grasses.

Importance: Thrips rarely cause serious damage, and it is unusual to find infestations at such a level as to warrant control.

Hessian Fly

Mayetiola destructor

Symptoms: Severe infestations of Hessian flies result in stunting of the plants, thin stands, lodging, and reduced yield. Injury is caused entirely by the larvae, which suck juices from plant tissues (70). If infestation occurs during jointing, infested stems often will break prior to maturity. The Hessian fly is 3-4 mm long, has a black head and thorax, and a pinkish or yellow-brown abdomen.

Life Cycle: Adult flies emerge in the spring from pupae that have overwintered in straw or stubble. The minute, oblong eggs are reddish in color and are laid in rows on the upper sides of leaves. The eggs hatch within one week; the white, legless larvae settle behind the leaf sheaths and suck the sap of the plant. They develop into translucent, pale green, slug-like maggots. The reddish brown pupae, commonly called "flag seed" because of their resemblance to the seed of the flax plant, are oval shaped, flattened, taper to a point, and are 3-5 mm long. They are found behind leaf sheaths, usually at a node.

Hosts/Distribution: The Hessian fly is mainly a pest of wheat, but it may attack barley, rye, and other grasses. This pest has been reported in most wheat-growing areas of the world.

Importance: This is one of the most destructive insect pests on cereals. Widespread outbreaks have occurred and, in some locations (such as North Africa and the USA), the past recurs annually.

Wheat Stem Maggot

Meromyze americana

Symptoms: When young tillers are attacked in the fall or early spring, the tillers usually die; infested plants show the "white head" condition typically produced by stem-boring insects (71). The adult flies are about 6 mm in length, and pale green to yellow with dark stripes.

Life Cycle: Wheat stem maggot larvae overwinter in cereal plants or grasses (72). The females lay small white eggs, one per stem, near the sheath of the flag leaf; the larvae burrow into and consume the interior of the stem, killing the upper part of the stem and the head. There are normally three generations per year; one in the spring, one in the summer, and a third in the early autumn that overwinter as larvae.

Hosts/Distribution: In addition to wheat, host crops include rye, barley, and other grasses. There are a number of other flies in various parts of the world that attack wheat in a similar fashion and produce the same kind of damage.

Importance: In infested fields, 10-15% of plants may be injured. Damage can be severe in some years, but the insect seldom causes widespread damage. However, heavy infestations of individual wheat stands may kill a significant portion of the tillers.

Sawfly

Cephus cinctus

Symptoms: Damage by sawflies includes premature yellowing of the head and shrivelling of the grain. The larvae girdle the stem (73) and, later in the crop cycle, lodging is common.

Life Cycle: Sawflies produce one generation per year. The larvae overwinter in the straw (74); in the spring they pupate. Adult sawflies are small, fly-like wasps and appear from late spring to midsummer. The females deposit small white eggs in the upper nodes of stems just below the heads. Upon hatching, the legless white larvae bore into the stem and tunnel downward, feeding on the pith of the stem. When they have completed their feeding, they descend further and girdle the stem base.

Hosts/Distribution: Nearly all cultivated cereals and native grasses act as hosts, although wheat is preferred. Fall-sown cereals are more commonly attacked. Wheat lines having solid or partially solid stems are much less susceptible to attack. The wheat stem sawfly is a major problem in the Mediterranean Basin.

Importance: Sawfly can cause significant damage in some years, but infestations are usually discontinuous.

White Grubs (various species)

Symptoms: White grubs can partially or completely sever the roots of the host plants. This causes patches of wilting and dying wheat plants (especially at the seedling stage), symptoms that could be attributed to root rots. However, when stunted patches are observed, the surrounding soil should be examined for the larvae (75). When fully grown, the largest of these larvae may be several centimeters long and nearly one centimeter thick. The larvae have three pairs of legs on their thorax (76).

Life Cycle: White grubs are the larvae of May or June beetles. Eggs are deposited in the soil and the hatched larvae feed on roots. The duration of the larval stage varies from species to species.

Hosts/Distribution: Many species of white grubs found throughout the world can attack wheat and many other plant species. Cereal crops may suffer significant damage if seeded into heavily infested grasslands.

Importance: When the roots are not completely destroyed, the plants may survive, but are stunted and fail to produce heads. However, the distribution and extent of attack is not uniform.

Wireworms (various species)

Symptoms: Wireworm damage is very similar to that caused by other soil-inhabiting chewing insects; the only sure means of identifying wireworms as the causal agent is to find them in association with the damaged seedlings (77). The name "wireworm" refers to the tough, wire-like appearance of the larvae. They are 20-30 mm long and are often smooth, hard, and highly polished. They have three pairs of legs (78), and their color may vary from a rich cream to shades of brown. Wireworm larvae may attack wheat as soon as the crop is seeded, eating the endosperm of the kernels and leaving only the seed coat. A common sign of woreworm attack is the wilting and/or dying of a number of adjacent plants, either in a row or patch. The stems of affected seedlings will be chewed just above the seed.

Life Cycle: Wireworms are the larvae of click beetles, of which there are many species. The adult beetles (79) lay eggs in the soil, usually in the spring, and the larvae may take several years to develop prior to pupating, depending on species. Generations overlap so that all stages and sizes of larvae may be found in the soil at the same time.

Hosts/Distribution: Many species of wireworms are found throughout the world, all of which can attack wheat. These larvae are capable of attacking many different plant species as well.

Importance: Woreworms are among the most damaging soil-infesting insects. Damage is usually most severe where wheat has been seeded after fallow or after a number of years of grass.

Slugs, Snails, Grasshoppers, and Crickets (various species)

Symptoms: Slugs and snails (80) can feed on the endosperm of germinating seed, bite seedlings off at ground level, and graze older plants, chewing longitudinal stripes on the leaves (81). This gives the adult plant a frayed appearance. Grasshoppers (82) and crickets cause damage that is very similar to that caused by cutworms and armyworms.

Hosts/Distribution: All these insects will attack a large range of plant species, including the cereals. Distribution is worldwide.

Importance: These insects usually are localized in their distribution, but can cause a great deal of damage to individual stands of wheat.

Mites (various species)

Symptoms: Adult mites are usually less than 1 mm long, and most of the plant-inhabiting species have sucking mouth parts. Mites are not insects. Adults typically possess four pairs of legs (83), while larvae have only three pairs. However, the wheat curl mite, Eriophye tulipae (syn. Aceria tulipae), has only two pairs of legs. When present in large numbers, mites cause a silvery flecking on leaves (84). Some species may produce webs and/or may cause infested plants to be severely stunted, to head poorly, and to turn white. Individual mites are so small they they can scarcely be seen with the unaided eye. Even so, if an infested leaf is held over a piece of white paper (folded to form a trough) and tapped sharply several time, hundreds of mites will fall onto the paper and can be seen moving about.

Hosts/Distribution: Mites generally are not an important problem, with the exception of the wheat curl mite, which is a vector of wheat streak mosaic virus (WSMV).

Reference:

Wheat Diseases and Pests: a guide for field identification

Original text by: J. M. Prescott, P. A. Burnett, E. E. Saari, J. Ranson, J. Bowman, W. de Milliano, R. P. Singh, G. Bekele. INTERNATIONAL MAIZE AND WHEAT IMPROVEMENT CENTER (www.cimmyt.org). CENTRO INTERNACIONAL DE MEJORAMIENTO DE MAIZ Y TRIGO. Lisboa 27, Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico.
HTML version developed with permission from CIMMYT by: J. Wong, USDA-ARS-WRRC, Albany, California; D. E. Matthews, Dep. of Plant Breeding and Biometry, Cornell University, Ithaca, New York and K. D. Kephart, Dep. of Agronomy, University of Missouri, Columbia, Missouri.