Sunflower Pest Management (cont.)

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Insects (cont.)

Red Sunflower Seed Weevil

Species: Smicronyx fulvus LeConte

Description: Red sunflower seed weevil adults are 0.1 to 0.12 inch (2.5 to 3.06 mm) long and reddish-brown (Figure 42). The larvae are small, 0.10 inch (2.54mm) long, cream-colored, legless and C-shaped in appearance (Figure 43).

Life Cycle: Red sunflower seed weevil emergence occurs in late June and early July. The newly emerged adults feed on sunflower floral buds. As the sunflower matures, the adults include pollen in their diet with the female feeding on sunflower pollen for several days prior to egg deposition. As the seeds begin to mature, eggs are deposited within the seed. Normally a single egg is placed in each seed, although 8 to 12 percent of the seeds may contain several eggs.

The small white eggs hatch in approximately one week. The larvae consume a portion of the kernel, and this feeding causes economic damage. After completion of larval development, the majority of the larvae drop to the ground. Larval drop occurs from mid-August through September. The larvae overwinter in the soil at a depth of about 6 inches (15 cm). Larvae pupate in late June and the pupal period lasts about one week. A single generation per year is produced in North Dakota.

Damage: While the kernel of some seeds may be totally eaten, most seeds are only partially consumed. The separation of undamaged from weevil-damaged seed is difficult.

Most larvae drop from the head to the soil after completing their development, but a small percentage may remain in the seed and be present at harvest. Growers who encounter a seed weevil infestation should delay harvest to allow most of the weevil larvae to exit the seeds.

Larvae that are still in the seed at harvest and bin filling time can cause heating and moisture problems. Larvae harvested with the seed can not be controlled until they have completed development and have emerged from the infested seeds. Once emerged, they are susceptible to fumigation. However, the most advantageous time to initiate control is in the field when the adult weevils are active just prior to egg deposition.

Scouting Method: Begin monitoring fields as soon as sunflower ray petals begin to form and every two to three days thereafter. The X pattern (Figure 12) should be used to sample for seed weevils. Five plants should be examined per sampling site for a total of 25 plants. Sampling sites should be at least 75 to 100 feet (21 to 31 m) in from field borders, which often have an inordinately high number of weevils. After sampling has been completed, the average number of weevils per plant can be calculated.

NOTE: To more precisely check individual sunflower heads for red sunflower seed weevils, the face of the heads should be sprayed with a commercial formulation of mosquito repellent containing diethyl toluamide (DEET). This will cause the weevils to move out from between the florets where they can be more accurately counted. Consult the most recent NDSU Extension sunflower seed weevil Circular (E-817) for a table to convert the visual counts to the absolute number of weevils (both counted and uncounted).

Economic Thresholds: The economic threshold varies with differences in plant population, the cost of insecticides and application, and the market price of sunflower. The procedure for calculating the economic threshold is given in the NDSU extension sunflower seed weevil Circular (E-817). Currently, a level of about 14 seed weevil adults per head in oil sunflower or one seed weevil per head in confectionery sunflower is the average economic threshold.

The optimal period for insecticide treatment is when at least three out of ten plants in the field are at early bloom (R-5.1 to R-5.4, Figure 4) and the economic threshold has been reached. If spray application is delayed beyond stage R-5.4 many eggs will already be laid in the developing seeds and those eggs and larvae can not be controlled. If fields are sprayed too early, reinfestation may occur in areas with a high weevil population. After spraying, fields should be periodically rechecked to determine if reinfestation is reaching the economic threshold. Continue rechecking until most of the heads in the field have reached the R-5.7 stage. At that stage, most eggs will already have been laid and most seeds will be too mature to be suitable for further red seed weevil oviposition.

Management: Several federally registered insecticides are available for control of sunflower seed weevils in the United States. Early planting of sunflower reduces achene damage caused by the red sunflower seed weevil without causing a measurable reduction in oil content and achene weight.

Gray Sunflower Seed Weevil

Species: Smicronyx sordidus LeConte

Description: Adults of the gray sunflower seed weevil are slightly larger (0.14 inch long) than S. fulvus and gray in coloration (Figure 44). The larvae are small, 0.12 inch long (3.1 mm), cream-colored, legless and C-shaped in appearance (Figure 43).

Life Cycle: Gray sunflower seed weevil emergence occurs in late June and early July and reaches 50 percent emergence about 10 days before the red sunflower seed weevil. The newly emerged adults feed on floral buds. Oviposition occurs on flowers in the bud stage and before red sunflower seed weevil oviposition begins. Female gray sunflower seed weevils do not lay as many eggs as do females of the red sunflower seed weevil.

The larvae feed in a single achene, and infested achenes are enlarged and protrude above surrounding uninfested achenes. The majority of the larvae drop to the ground from mid August through September and overwinter in the soil. Larvae pupate in late June and a single generation per year is produced in North Dakota.

Damage: Seeds infested by the gray seed weevil lack a kernel and due to their light weight the seeds may be lost during the harvesting process. Because of their low population levels and low fecundity, the gray sunflower seed weevil usually does not cause economic damage, especially in oil sunflower fields. In confection fields, however, populations of the gray sunflower seed weevil may be sufficiently high to warrant treatment at the late bud stage (R-3 to R-4).

As with the red sunflower seed weevil, larvae normally drop from the head to the soil after completing their development. Larvae that do not emerge will present the grower the same problem as unemerged red sunflower seed weevil larvae.

Scouting Method: Normally, gray sunflower seed weevil populations are too low to cause economic damage. However, if an area has had a history of high populations, fields, especially confection fields, should be sampled beginning at bud stage R-2 (Figure 4). Sampling should be done as for the red sunflower seed weevil and continue until plants are blooming.

Economic Thresholds: None established.

Management: Several insecticides are federally registered for control of sunflower seed weevils in the United States. If fields are to be treated with insecticides, they should be sprayed while the plants are still in early bud stage. By late bud stage most oviposition will have already occurred.

Sunflower Moth

Species: Homoeosoma electellum (Hulst)

Description: The adult is a shiny gray to grayish-tan moth about 0.38 inch (9 mm) long with a wingspan of about 0.75 inch (19 mm) (Figure 45). The hind wings are devoid of markings; however, the forewings have a small, dark discal dot near the center of each wing and two or three small, dark dots near the leading margin of each wing. When at rest, the wings are held tightly to the body giving the moth a somewhat cigar-shaped appearance. The larva has alternate dark and light colored longitudinal stripes on a light brown body (Figure 46). The larva is about 0.75 inch (19 mm) long at maturity.

Life Cycle: Sunflower moth migrations from the south central United States normally appear in North Dakota in early to mid July. The moths are highly attracted to sunflower that is beginning to bloom. Up to 30 eggs per day will be deposited by individual female moths on the surface of open sunflower heads. The eggs hatch within 43 to 72 hours and the newly emerged larvae feed on pollen and florets. The larvae begin tunneling into seeds upon reaching the third instar (larval growth stage). This tunneling continues throughout the remainder of larval development. Larval development from hatching to full maturity takes about 15 to 19 days.

Damage: The young larvae of the sunflower moth feed primarily on florets and pollen. Older larvae tunnel through immature seeds and other parts of the head. A single larva may feed on from three to 12 seeds and forms tunnels in both the seeds and head tissue. Larvae spin silken threads which bind with dying florets and frass to give the head a trashy appearance. Severe larval infestations can cause 30 to 60 percent loss, and in some cases the entire head can be destroyed.

Scouting Method: Sampling sites should be at least 75 to 100 feet (23 to 31 m) from field margins. The X pattern should be used in monitoring a field, counting moths on 20 heads per sampling site for a total of 100 heads.

Economic Threshold: The economic threshold for sunflower moth is one to two adults per five plants at the onset of bloom or within seven days of the adult moth's first appearance. Fields that are in bloom or that bloom two weeks or more after the first adult moth appearance have very low potential for damage despite the presence of moths in threshold numbers.

Management: A number of federally labelled insecticides are registered for control of the sunflower moth.

Banded Sunflower Moth

Species: Cochylis hospes Walsingham

Description: The adult has a dark band across the buff or yellowish-tan forewings (Figure 47). The wingspan is about 0.5 inch (13 mm). Early instar larvae are off-white; late instar larvae are pinkish to red with a brown head capsule (Figure 48). Larvae will be about 0.44 inch (11 mm) at maturity.

Life Cycle: The life cycle of the banded sunflower moth is similar to that of the sunflower moth except that the adults emerge from local over-wintering sites rather than migrating into North Dakota from the south. Banded sunflower moths begin to emerge from the soil about mid July and are present in the field until mid August. Adults tend to congregate in field margins on weeds or adjacent crops during the day, then move into the crop in the evening. Within a week after emergence, they begin to lay eggs on the outside of the bracts of the sunflower head. Eggs may be found through early August and hatch in five to eight days. Larvae develop through five instars and are present in sunflower heads from mid July to mid September. After feeding to maturity, larvae drop to the ground and spin cocoons in the soil to overwinter. Pupation takes place in late June or early July the following year. The pupal period lasts about 12 days.

Damage: The newly hatched larvae move from the bracts to the florets of the sunflower head, where they enter open florets to feed. In some instances, when the eggs hatch prior to the opening of the florets, young larvae will feed on bract tissue before moving into the head. A sunflower head is susceptible to infestation only during the flowering period. The larvae feed in the florets until the third instar. During later stages of larval development, the insect tunnels through the base of the floret into the seed. The larva may consume part or all of the contents of the developing seed. The larva usually enters near the top of the seed and leaves by way of the same opening after the contents are eaten. Each larva may destroy several (five to seven) seeds. Small areas of silken webbing on mature sunflower heads indicate the presence of banded sunflower moth larvae within the head.

Scouting Method: Sampling sites should be at least 75 to 100 feet (23 to 31 m) from the field margins. The X pattern (Figure 12) should be used in monitoring a field, counting moths on 20 heads per sampling site for a total of 100 heads.

NOTE: Sampling will normally provide the most accurate counts for banded sunflower moth when conducted in early evening or early morning when the moths are active. During the day they remain quiet, resting on the underside of lower leaves of sunflower plants, but flutter from plant to plant if disturbed.

Economic Threshold: Field trials conducted in North Dakota Dakota indicate that one moth per two plants might be a reasonable economic threshold level for this species. Fields should be monitored when plants are in the late bud (R-4) to early bloom stage (R-5.1) of development.

Management: Deep fall plowing of sunflower stubble in Manitoba has reduced moth emergence the following season by about 80 percent. Research in North Dakota has demonstrated that delaying planting of sunflower until late May or early June may reduce infestation levels of the banded sunflower moth. Parasitic wasps attack both the eggs and larvae of the moth and general predators in the sunflower field also consume both larvae and eggs.

Since banded sunflower moths have a tendency to congregate around field margins just prior to flowering, treatment of the margins at this time can accomplish significant reduction of adults while at the same time minimizing insecticide treatment costs and impact on pollinators.

Sunflower Headclipping Weevil

Species: Haplorhynchites aeneus (Boheman)

Description: The sunflower headclipping weevil adult is shiny-black in appearance (Figure 49). The weevil is about 0.31 inch (8 mm) long from the tip of the snout to the rear of the abdomen. The area behind the head and thorax is large and "squared" in relation to the narrow and prolonged head and snout.

Headclipping weevil larvae are cream-colored, somewhat C-shaped and grub-like in appearance and 0.16 to 0.24 inch (4 to 6 mm) long (Figure 50).

Life Cycle: Adults emerge in mid July and are active for a two to three week period. The females feed on pollen and nectar of flowering heads. In preparation for egg laying, the female makes one nearly complete row of feeding punctures around the circumference of the stalk just below the head and then lays an egg in the head. The girdled head subsequently falls to the ground, where larval development and overwintering occur.

Damage: Head clipping by H. aeneus is the most apparent type of damage caused by this weevil and frequently occurs along field margins. The percent of "clipped heads" in a field is normally very low (1 to 3 percent). However, losses up to 25 percent have been reported in individual fields (Figure 51).

Scouting Method: The weevils' presence is determined using the X scouting pattern. If the adults are encountered only periodically throughout the sampling sites, controls should not be necessary.

Economic Threshold: None established.

Management: Insecticide use has not been warranted for control of the sunflower headclipping weevil.