Sunflower Pest Management (cont.)

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Insects

Sunflower plays host to a number of insect pests. In the major sunflower producing areas of the Dakotas, Minnesota and Manitoba, approximately 15 species of sunflower insects cause plant injury and economic loss depending on the severity of infestation. However, during any one growing season, only a few species will be numerous enough to warrant control measures. The sunflower insects of major importance in the Northern Great Plains have been sunflower midge, Contarinia schulzi Gagne'; sunflower beetle, Zygogramma exclamationis (Fabricius); sunflower stem weevil, Cylindrocopturus adspersus (LeConte); red sunflower seed weevil, Smicronyx fulvus LeConte, and the banded sunflower moth, Cochylis hospes Walsingham.

Infestation of sunflower insects in fields of susceptible hybrids must be monitored regularly (usually weekly) to determine the species present and if populations are at economic threshold levels. Furthermore, proper timing of insecticidal treatment is essential to maximize control.

The following provides information on the identification, life cycle, damage, scouting methods, economic threshold levels and control of some of the most common insect pests of sunflower in the Northern Great Plains. A preliminary quick reference guide to sunflower insects is available.

Cutworms

Species:
Darksided cutworm Euxoa messoria (Harris)
Redbacked cutworm Euxoa ochrogaster (Guenee)
Dingy cutworm Feltia jaculifera (Walker)

Description: Darksided cutworm -- Forewings of the adult darksided cutworm are usually light, powdery and grayish brown with indistinct markings (Figure 14). The larvae are pale brown dorsally and white on the ventral areas (Figure 15). Sides have numerous indistinct stripes. At maturity they are about 1.25 to 1.5 inches (32 to 38 mm) long and 0.19 inch (5 mm) wide.

Redbacked cutworm -- The forewings of the adult redbacked cutworm are reddish-brown with characteristic bean-shaped markings (Figure 16). The larvae are dull gray to brown with soft fleshy bodies and may be 1 to 1.25 inches (25 to 32 mm) long when fully grown (Figure 17). Larvae can be distinguished by two dull reddish stripes along the back.

Dingy cutworm -- Forewings are dark brown with bean-shaped markings as in the redbacked cutworm adults (Figure 18). Hind wings in the male are whitish with a broad, dark border on the outer margin; in the female that are uniform dark gray. The larvae have a dull, dingy, brown body mottled with cream color. The dorsal area is pale with traces of oblique shading (Figure 19).

Life Cycles: The female darksided and redbacked cutworm moths deposit eggs in the soil in late July and early August. The eggs remain dormant until the onset of warm weather the following spring. The larvae of both species emerge from late May to early June. They continue to feed and grow until about the end of June when fully grown larvae pupate in earthen cells near the soil surface. The pupal period lasts about three weeks. There is one generation per year for both species.

The adult dingy cutworms emerge in August and are active until mid October, with peak activity in September. Eggs are deposited in plants in the Compositae family in the fall. The larvae develop to the second or third instar in the fall and overwinter in the soil. Pupation occurs in the spring to early summer. One generation of this species is produced per year.

Damage: Cutworm damage normally consists of crop plants being cut off from 1 inch (25 mm) below the soil surface to as much as 1 to 2 inches (25 to 50 mm) above the soil surface. Young leaves also may be severely chewed as a result of cutworms (notably the darksided species) climbing up to feed on the plant foliage.

Most cutworm feeding occurs at night. During the daytime the cutworms will usually be just under the soil surface near the base of recently damaged plants. Wilted or dead plants frequently indicate the presence of cutworms. Cut off plants may dry and blow away, leaving bare patches in the field as evidence of cutworm infestations.

Scouting Method: Sampling should begin as soon as sunflower plants emerge and fields should be checked at least twice per week until approximately mid June. The Z pattern should be used in scouting fields for cutworms with sampling points one and two near the margin as indicated in Figure 12.

Stand reduction is determined by examining 100 plants per five sampling sites for a total of 500 plants. A trowel or similar tool should be used to dig around damaged plants to determine if cutworms are present, since missing plants in a row do not necessarily indicate cutworm damage (damage may be caused by a defective planter, rodents or birds).

The Z pattern should be used again to determine cutworm infestation level by examining five 1-square-foot (30.48 x 30.48 cm) soil samples per site (in the row) for a total of 25 samples.

Economic Threshold: One larva per square foot (30.48 x 30.48 cm) or 25 percent to 30 percent stand reduction.

Management: There are several different insecticides registered for cutworm control in sunflower.

Sunflower Beetle

Species: Zygogramma exclamationis (Fabricius)

Description: The sunflower beetle is associated exclusively with sunflower. Adults (Figure 20) closely resemble adult Colorado potato beetles and may be confused with potato beetles except that sunflower beetles are smaller and do not feed on potatoes and Colorado potato beetles do not feed on sunflower.

The head of the sunflower beetle is reddish-brown and the thorax (area between head and abdomen) is pale cream with a reddish-brown patch at the base. Each front elytron (wing cover) has three dark stripes that extend the length of the back. A shorter, lateral stripe ends at the middle of the wing in a small dot that resembles an exclamation point. The beetle is 0.25 to 0.5 inch (6 to 13mm)long and 0.16 to 0.19 inch (4 to 5 mm) wide. The larvae are yellowish-green, humped-back and about 0.35 inch (9mm) at maturity.

Life Cycle: The sunflower beetle has one generation per year. The adults overwinter in the soil, emerging in late May or early June. Shortly after emergence the beetles begin to feed, mate and lay eggs singly on stems and undersides of leaves. Egg deposition extends over a period of two or more weeks, and the number of eggs produced by a single female has been estimated to be at least 200. Eggs hatch into larvae in about one week (Figure 21). The larvae are present in fields for about six weeks. When mature, the larvae enter the soil to pupate in earthen cells. The pupal stage lasts from 10 days to two weeks. Adults of the new generation emerge and feed for a short period on the bracts of the sunflower head or on the uppermost leaves of the plant before reentering the soil to overwinter.

Damage: Adult sunflower beetles damage plants soon after they emerge from hibernation. Damage to cotyledons is generally slight, but the first true leaves may be severely damaged or completely consumed. Fields may be severely defoliated if beetles are numerous. Larvae of the sunflower beetle cause damage by chewing holes in the leaves. Normally most feeding occurs at night. Larvae typically rest during the daytime in the terminal growth area. If larvae are numerous, they may completely defoliate a plant during their development.

Scouting Method: Sampling sites should be at least 75 to 100 feet (23 to 31 m) from the field's margins when determining if an entire field should be treated. Adults and/or larvae should be counted on 20 plants at each of five sampling sites along an X pattern for a total of 100 plants. The average number of adults and/or larvae per plant should then be determined.

The average percent defoliation of plants is determined when damage is evident in the field by examining 20 plants per five sampling sites for a total of 100 plants (Figure 22).

Economic Thresholds: Adult -- one to two per seedling. Larvae -- 10 to 15 per plant will cause approximately 25 percent defoliation on the upper eight to 12 leaves. Control is normally advised if average defoliation reaches the 25 percent to 30 percent level (Figure 22) and it appears (based on larval size) that more defoliation will occur. However, if the majority of the larvae have reached maturity at about 25 percent defoliation, then control would not be warranted.

Management: Natural enemies include parasites of the eggs, larvae and adult. General predators such as ladybird beetles, lacewings, nabids and anthocorids destroy both eggs and larvae of the sunflower beetle. Application of insecticide when larvae are actively feeding has shown to be effective in preventing economic damage.

Sunflower Bud Moth

Species: Suleima helianthana (Riley)

Description: Sunflower bud moths have a wingspread of about 0.63 inch (16 to 18 mm). Each gray-brown forewing has two dark transverse bands (Figure 23). One band extends across the middle of the wing, and the second band is located near the wing tip. The larva has a dark head capsule with a smooth, cream-colored body and is 0.31 to 0.43 inch (8 to 11 mm) at maturity (Figure 24).

Life Cycle: Two generations of sunflower bud moth are produced per year in North Dakota. Adults from overwintering pupae emerge during the last week of May to mid June.

A few days after adult emergence, eggs are deposited on the terminals of immature sunflower or on the receptacle of mature sunflower. Eggs also are deposited in leaf axils. The hatched larvae begin tunneling into the sunflower plant. The initial infestation in mid June is characterized by an entrance hole surrounded by black frass (insect excrement).

Mature larvae pupate within the sunflower plant. Pupae move to the opening of the entrance holes formed in the stem or head tissue so that adults can emerge easily.

The second generation adults appear in July and August. Infestation by the second generation larvae is not economically important.

Damage: In early planted sunflower, 65 to 85 percent of the infestations occur in the stalks. In late planted sunflower, most infestations occur in the pith areas of the head.

Up to 4,000 larvae per acre have been reported in North Dakota and 24,000 larvae per acre have been reported in Texas. Despite these high populations, economic loss due to this insect has been minimal. The only time yield loss is noticeable is when larvae burrow into unopen buds, preventing proper head development. The larvae normally do not feed on developing seeds but confine feeding activities to the fleshy part of the head. Yield loss has not been economically significant, although injury by the larva produces malformations in both the head and stalk.

Scouting Method: A field monitoring scheme for this insect has not been established since it is not of economic significance.

Economic Threshold: None established.

Management: Insecticide use has not been warranted for control of sunflower bud moth.

Sunflower Maggots

Species: Gymnocarena diffusa (Snow)
Strauzia longipennis (Wiedemann)
Neotephritis finalis (Loew)

Description: The adult forms of all three sunflower maggots (flies) have wings with a distinct brown or yellowish-brown pattern. The name "picture-wing fly" has been given to flies of this type. While all three fly species are similar in appearance, they do have distinguishing differences.

Gymnocarena diffusa -- This species is the largest of the three with a body about 0.4 inch (10 mm) long and a wing span of approximately 0.75 inch (19 mm) (Figure 25). The eyes of this species are bright green and the wings have a yellowish-brown and somewhat mottled appearance. G. diffusa larvae attain a length of nearly 0.31 inch (8 mm) at maturity. The larvae taper from the front to rear and are yellowish-white in color (Figure 26).

Strauzia longipennis -- Adults of this species have a wing spread of about 0.5 inch (13 mm) and a body 0.25 inch (6 mm) long (Figure 27). The wings bear broad, dark bands that form a fairly distinct F-shaped mark near the tips. The larvae of S. longipennis are creamy white, headless and legless, as are the other two species (Figure 28). They taper slightly at both ends and attain a length of about 0.28 inch (7 mm) at maturity.

Neotephritis finalis -- This sunflower maggot is the smallest of the three species with the adult having a body length of about 0.25 inch (6 mm) and a wing span of approximately 0.28 inch (7 mm) (Figure 29). The wings have a brown lace-like appearance. N. finalis larvae attain a length of 0.19 inch (4.5 mm) at maturity (Figure 30).

Life Cycles: Adults of G. diffusa emerge in late June to early July after sunflower buds reach 2 to 4 inches (5 to 10 cm) in diameter. Eggs are laid on the bracts of the developing sunflower heads. Egg laying occurs from mid July through August. The hatched larvae tunnel into the spongy tissue of the receptacle. Damage to the head is negligible. After 30 days, the mature larvae cut a small emergence hole on the underside of the receptacle and drop into the soil to pupate. Overwintering pupae are found about 7.5 inches (19 cm) deep in the soil by August or early September. Some larvae will pupate in the sunflower head. There is only one generation per year in North Dakota.

S. longipennis has one generation per year. This insect overwinters as a larva in plant debris in the soil. Pupation and adult emergence are completed in early June. Females lay eggs in stem tissue of young sunflower, and larvae feed in the pith tissue for much of the growing season.

Unlike the other two species of sunflower maggots, two complete generations per year of N. finalis occur in North Dakota. Adults of N. finalis emerge during the first week of July. Egg deposition occurs on the corolla of incompletely opened sunflower inflorescences. The total larval period is 14 days. The first generation of N. finalis pupates in the head; the second generation overwinters in the soil as pupae.

Damage: Damage by G. diffusa and N. finalis has been negligible.

The magnitude of damage to sunflower seeds by N. finalis larvae depends largely on the stage of larval and seed development. Seed sterility occurs when newly hatched larvae tunnel into the corolla of young blooms. Observations indicate that a single larva feeding on young flowers will tunnel through 12 ovaries. Mature larvae feeding on older sunflower heads will destroy only one to three seeds.

While infestation levels of S. longipennis have occasionally reached nearly 100 percent, damage from larval feeding is usually light. Part of a commercial sunflower field next to a grassed waterway or other water source sometimes supports a higher than usual infestation. Under these conditions, high larval numbers of eight to 10 per stalk may be found and stalk breakage can occur. Stalk breakage of up to 30 percent of the plants has been recorded.

Scouting Method: A scouting method has not been developed for sunflower maggots because of the negligible injury caused by these insects.

Economic Threshold: None established.

Management: Insecticide use has not been warranted for control of sunflower maggots.