Sunflower Pest Management (cont.)

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Insects (cont.)

Sunflower Stem Weevil

Species: Cylindrocopturus adspersus (LeConte)

Description: Adults are about 0.19 inch (4 to 5 mm) long and grayish-brown with varying shaped white spots on the elytron (wing covers) and thorax (area between head and abdomen) (Figure 31). The snout, eyes and antennae are black. Larvae of the sunflower stem weevil are 0.25 inch (5 to 6 mm) long at maturity (Figure 32). They are creamy-white with a small, brown head capsule. The larvae will normally be in a curled or C-shaped position when found in sunflower stalk tissue.

Life Cycle: Adults emerge in mid to late June and feed on the epidermal tissue of the sunflower foliage and stem. This feeding does not affect plant vigor. Mating occurs soon after emergence of adults. Just prior to egg laying, females descend to the lower portion of the plant to deposit eggs individually in the epidermal tissue of the stem. Approximately 50 percent of oviposition occurs by mid July. Upon hatching in early July, the first instar (larval growth stage) larvae feed on subepidermal and vascular tissue. Feeding is concentrated in the pith tissue as the larvae develop to third and fourth instar stages. By the last week in August, the larvae have descended while feeding to just above the soil surface. A chamber is constructed in the stem, and the weevils overwinter in this chamber as fifth instar larvae. Pupation of the overwintering larva occurs the following year in early June. There is one generation per year.

Damage: When larval infestations reach 25 to 30 or more per stalk, considerable weakening of stem tissue can result, especially when these larvae begin to create their overwintering cells in the base of the sunflower stalks. Breakage is most likely to occur when plants are under drought stress and/or during periods of high wind. The breakage typically occurs at or slightly above the soil surface in contrast to breakage attributed to stalk disease, which normally occurs farther up on the stalks.

Scouting Method: Survey methods are directed to the adult stage since larvae develop totally within the sunflower plant. The only method for detecting the presence of larvae in the stems is to split the sunflower stalks. Sampling sites for adults should be 75 to 100 feet (23 to 31 m) in from the field margins. The field should be scouted in late June to very early July using the X pattern (Figure 12) and examining five plants per stop for a total of 25 plants at the 8- to 14-leaf stage (developmental stages V-8 to R-1, Figure 4). The average number of adult weevils per plant can then be calculated.

NOTE: When surveying for stem weevils, the scout must move through the field slowly to avoid having the adult stem weevils drop to the soil and "play dead" as they typically will do if care is not taken during survey and counting efforts.

Economic Threshold: The economic thresholdfor the sunflower stem weevil is one adult per three plants.

Management: Insecticidal treatment should be initiated in late June or early July before appreciable egg laying has occurred. Delayed planting of sunflower until late May or early June has been effective in reducing densities of larvae in the stem. Fall tillage practices which either bury or break up sunflower stalks will increase winter mortality of stem weevil larvae.

Natural enemies of the sunflower stem weevil include parasitic wasps that attack both the egg and larval stage.

Black Sunflower Stem Weevil

Species: Apion occidentale (Fall)

Description: Adults are black and only 0.1 inch (2.5 mm) long from the tip of the snout to the tip of the abdomen (Figure 33). The snout is very narrow and protrudes forward from the head, which is small in relation to the rather large, almost globose body. Larvae of A. occidentale are very similar in appearance to C. adspersus except they are only 0.1 to 0.12 inch (2.5 to 3 mm) long at maturity and yellowish in color (Figure 34).

Life Cycle: A. occidentale overwinters as an adult in soil, plant residue, sod and weed clusters and begins to emerge and feed on volunteer sunflower as soon as the plants reach the early seedling stage. Females deposit eggs under the epidermis of the stem. Larvae emerging from these eggs tunnel in the pith area of the stem, pupate and emerge as adults in early August. Little or no adult activity is observed for about two weeks in late July and early August. Black sunflower stem weevil adults emerging in August also feed on the leaves and stems of the plant, but as the plant matures and the leaves begin to die, the adults move under the bracts of the sunflower head where they can be observed feeding until the plants are harvested.

Damage: Adult feeding by both stem weevil species is considered to cause insignificant mechanical injury. However, since the Phoma macdonaldii Boerma organism has been isolated from the adults of both A. occidentale and C. adspersus, these two species are highly suspect in vectoring Phoma black stem disease in sunflower fields. A. occidentale has transmitted P. macdonaldii to sunflower as a result of their feeding activity under greenhouse conditions.

Scouting Method: A scouting method has not been developed for the black sunflower stem weevil because of the negligible damage caused by this weevil to date.

Economic Threshold: None established.

Management: The use of insecticidal control has not been warranted for use against this weevil.

Sunflower Root Weevil

Species: Baris strenua (LeConte)

Description: Adults are rather robust looking weevils with a somewhat oval shaped body (Figure 35). They are 0.25 inch (6 mm) long and have a short, almost blunt, downward projecting snout. Their coloration is dull black in contrast to the shiny, black appearance of A. occidentale. B. strenua larvae are similar in appearance to C. adspersus larvae but much larger (Figure 36).

Life Cycle: Adult root weevils emerge during the latter part of June. They feed on sunflower foliage in early morning and late afternoon. About two weeks after emergence, the adults begin to congregate around the root zone near the soil surface. Continued feeding and copulation occurs during this period. Feeding activity during this period produces callus tissue under which the bright yellow eggs are deposited two or three at a time. Hatching of the larvae normally occurs during the second week in July. B. strenua larvae are not very mobile. Most of the feeding (consisting of circular tunnels) and development to fourth instar takes place in the same area where hatching occurs. At about the time that the fourth larval stage is reached in late August to early September, the plant becomes significantly dehydrated and encapsulation of the larvae within a "soil cocoon" begins. This "larval cocoon" overwinters among the remaining roots in the soil. Overwintering larvae have been recovered from a depth of 15 inches (38 cm) in North Dakota.

Damage: The sunflower root weevil adult, like the other two stem weevils, causes negligible mechanical injury to the foliage of the sunflower plant. The destruction of root tissue by the larvae of the sunflower root weevil causes the plants to wilt and lodge if the infestation is severe. The damage to fields attacked by the weevil tends to be localized.

Scouting Method: A scouting method has not been developed because damage by this pest has been minor.

Economic Threshold: None established.

Management: Insecticide use has not be warranted for the control of the sunflower root weevil.

Thistle Caterpillar (Painted Lady)

Species: Vanessa cardui (Linnaeus)

Description: The body of the adult is about 1 inch (25 mm) long with a wingspread of about 2 inches (50 mm) (Figure 37). The upper wing surfaces are brown with red and orange mottling and white and black spots. The undersides of the wings are marble-gray, buff and white. Each hind wing possesses a row of four distinct and obscure eyespots. Eggs are small, spherical and white. The larvae are brown to black, spiny, with a pale yellow stripe on each side (Figure 38). When mature, the larvae are 1.25 to 1.5 inches (32 to 38 mm) long. The chrysalis, or pupa, is molten gold and about 1 inch (25 mm) long.

Life Cycle: The painted lady butterfly is indigenous to the southern United States and migrates annually to the northern regions and Canada. The painted lady breeds in the North Central States and Canada, migrates south for the winter and returns to the northern areas in early June. Eggs are laid on Canada thistle, wild and cultivated sunflower, and other host plants. Hatching occurs in about one week. Larvae feed on sunflower until they reach maturity in late June or early July. Chrysalids are formed and hang from the leaves of the plant. Butterflies will emerge in about 10 days from the chrysalid and a second generation begins.

Damage: The caterpillars (larvae) feed on the leaves and, when numerous, may defoliate infested plants. The larvae produce a loose silk webbing which covers them during their feeding activity. Black fecal pellets produced by the larvae often are found in close proximity to the webbing.

The effect of defoliation by the larvae on the yield of sunflower is similar to that described for defoliation by sunflower beetle larvae.

Scouting Method: Sampling sites should be at least 75 to 100 feet (23 to 31 m) from the field margins when collecting data to determine whether an entire field should be treated. Infestations frequently will be concentrated in areas of a field where Canada thistle plants are abundant. Plants should be examined carefully for the presence of eggs and/or larvae.

The field should be monitored by using the X pattern, counting 20 plants per sampling site for a total of 100 plants to determine percent defoliation (Figure 22).

Economic Threshold: The threshold is 25 percent defoliation provided that most of the larvae are still under 1.25 inch (32 mm) long. If the majority of the larvae are 1.25 to 1.5 inch (32 to 38 mm) long, most of the feeding damage will have already occurred and treatment is not advised.

Management: Insecticide use has generally not been warranted for control of larvae of the painted lady. However, there may be instances of high localized infestations within certain fields where spot treating may be necessary.

Sunflower Midge

Species: Contarinia schulzi Gagne'

Description: The tan-colored body of the adult sunflower midge is about 0.07 inch (1.69 mm) long with a wingspan of about 0.19 inch (4 mm) (Figure 39). The wings are transparent with no markings except the veins. The larvae attain a length of nearly 0.09 inch (2.42 mm) at maturity and they are a cream to yellowish orange color when fully grown (Figure 40). They are tapered at the front and rear with no legs or head capsule.

Life Cycle: The sunflower midge overwinters in the soil as a cocooned larva and pupates during June in North Dakota and Minnesota. First generation adult emergence occurs in early to mid July. They prefer to lay eggs on sunflower buds with a diameter greater than 1 inch (25mm) and with an exposed disk pad in the center. Larvae initially feed on margins of the head between the bracts surrounding the heads. Larvae migrate to the base of the developing seeds and to the center of the head as the head develops. Presence of the larvae frequently can be determined by necrotic areas at the base of or between the bracts. As midge larvae become mature, they move to the surface of the head and drop to the ground. A second generation may occur.

Damage: Damage to sunflower is a result of larval feeding in developing heads. The plant cell walls are not mechanically disrupted by larval feeding, but salivary secretions from the larvae dissolve the cell walls. Larvae then eat the dissolved material. Damage restricted to the base of the bracts of the head causes localized necrosis but little if any economic loss. When many larvae are present, the growth of the developing sunflower head is distorted. If the abnormal growth is sufficient, the back of the head overgrows the front and little or no seed production occurs (Figure 41).

If midge damage is restricted to field margins or small portions of fields, economic losses are minimal. When populations are very heavy, damage will extend into the field and substantial economic losses will occur.

Scouting Method: The X pattern can be used to check sunflower fields for sunflower midge larvae. Larvae are most easily found when flower buds are 1 to 2 inches (2.5 to 5 mm) in diameter. Older plants can be sampled for the abnormal growth caused by midge infestation.

Economic Threshold: None established.

Management: Because effective chemical controls are not available, sunflower midge management consists of cultural practices done prior to planting. If a midge infestation is anticipated, planting a resistant hybrid should provide adequate control. Delaying planting until late May or early June will reduce damage most years.