**6. Sunflower**

*Helianthus annuus* L.: known as common sunflower, about 60% of the world's production is in Europe and the USSR (Union of Soviet Socialist Republics) (Putnam et al., 1990).

#### **6.1** *Pseudoheteronyx basicollis* **(L.) (Coleoptera: Scarabaeidae)**

**Identification**: it is known as black scarab beetles or black sunflower scarab*.* Beetles are 13 mm long, shiny black with very short hairs. Eggs are cream and spherical, about 1.5 mm in diameter*.* The larvae are creamy with a grey rear end, brown head capsule and up to 25 mm long. They are C-shaped with wrinkly bodies and Y-shaped palidia (Franzmann, 2011).

Oilseed Pests 165

sunflowers have reduced infestations by this pest. There is no varietal resistance to stem borers in sunflowers and chemical treatments are not recommended. Although adults are susceptible to many foliar insecticides, their extended activity period means that a single application will not provide control. However, larvae feed within the plant and are protected from contact insecticides, and even repeated applications of systemic materials fail

**Identification:** Sunflower stem weevil adults are less than 4-5 mm long and grayish brown with white markings on the wing covers and thorax. The snout, eyes and antennae are black. Eggs are very small about 0.51 mm long by 0.33 mm wide, oval and yellow, and are difficult to see. Larvae are approximately 6 mm long at maturity. Larvae are legless and creamy white with a small, brown head capsule. They are normally in a curled or C-shaped.

**Life cycle and damage**: adult emerge from overwintered stalks or root crowns and mating occurs soon after emergence. Adults feed on stem and leaf tissues which causes minor damage to them. Females lay eggs individually in the stem tissue around the first node (cotyledon) or inside the epidermis of sunflower stems. First instar larvae feed on sub epidermal and vascular tissue in the stem. Feeding is concentrated in the pith tissue as the larvae develop to third and fourth instar stages. It has about five to seven larval instars. Larvae feed apically and last instars descend to the lower portion of the stalk or root crown and excavate overwintering chambers by chewing cavities into the stem cortex. They have only one generation per year. The stem may break, causing a loss of the entire head prior to harvest. Stalk lodging is most severe during drought stress or when high winds occur as plants are drying prior to harvest. Lodging typically occurs at or slightly above the soil line, in contrast to breakage attributed to stalk diseases. Lodging is a good indicator of larval densities; however, lodging is influenced by other factors, including stalk diameter, cortex and pith thickness of the stem, weight of sunflower heads, wind velocity and direction,

**Monitoring:** monitor the field and count number of weevils. Adults can be found on both surfaces of the leaves, the lower portions of the stem, in leaf axils, within the dried cotyledons or in soil cracks at the base of the plant. Field scouting for adults should begin when plants are in the eight to 10 leaf stages. Five randomly sampling sites are determined 70-100 feet in from the field margin and in each sample site, five plants is selected. It is recommended to use an X pattern (or W pattern) to space sample sites throughout the entire field (Knodel et al., n.d.). According to the Michaud et al., (2011) scouting adults is really difficult and not recommended in many cases. So, in a region with a history of stem weevil problems growers may assume that any dry land sunflowers will be at risk, and that their

**Economic and action thresholds:** The economic threshold is one adult per three plants, or

**Control**: delayed planting and reducing plant population increases stalk diameter which is useful for controlling *C. adspersus* (Knodel et al., n.d.). Systemic pesticides such as carbofuran give simultaneous control of stem weevils and most other stem-infesting insects,

to provide adequate control in most years (Knodel et al., n.d.; Michaud et al., 2011).

**6.3** *Cylindrocopturus adspersus* **(LeConte) (Coleoptera: Curculionidae)** 

Pupae are similar in size to the adult and are creamy white (Knodel et al., n.d.).

position of larvae in overwintering chambers in the stalk (Knodel et al., n.d.).

impact on the crop will increase with adversity of growing conditions.

about 40 larvae per stalk at the end of the season (Knodel et al., n.d.).

**Life cycle and damage**: females lay their eggs under seedlings. Young larvae feed on taproots causing wilting and deaths of seedlings, older larvae attack the pods. Adult beetles can defoliate and kill plants up to 40 cm tall. Larvae overwinter in the soil and pupate in the spring. Adults often feed in a line across the field. Beetles hide in the soil during the day and emerge in late afternoon to feed. Heavily infested crops may suffer over 30% yield loss. They have one generation per year (Bailey, 2007).

**Monitoring:** check in the soil by digging and sieving for the presence of larvae prior to planting, and at all stages for adults. Count the number of beetles per m2 (Brien, 2011).

**Economic and action thresholds:** action threshold is when four beetles is observed per square meter which can cause severe losses to young seedlings (Franzmann, 2011).

**Control**: Removal of the host parthenium weed is advised. Spray appropriate insecticides when beetles are active on the soil surface. Beetles can also be controlled by application of pelleted baits (alfalfa or similar meal) at planting. Damage is most prevalent where sunflowers follow wheat, sorghum or grass pasture (Brien, 2011).

#### **6.2** *Dectes texanus* **(LeConte) (Coleoptera: Cerambycidae)**

**Identification:** Dectes stem borer is bluish-gray, about 1-2 cm, with antennae as long, or longer than the body. They are generally cryptic in behavior and can often be found hiding under leaves within the plant canopy during daylight hours, but also may be seen actively flying and running around on plant surfaces. The eggs are like small bananas with rounded ends. The larvae are pale, legless, cylindrical, and deeply segmented. They are creamy and tapered towards the rear so that final segments are narrower (Michaud et al., 2011).

**Life cycle and damage**: Females become reproductively active about one week after emergence, and require another week or so to mature eggs. Eggs are normally laid in leaf petioles deep in the central pith. After six to 10 days the eggs hatch and newly hatched larvae pass the first instar in the leaf petiole and then tunnel down into the main stalk where they feed selectively on the pith in the central core. Larvae develop through six instars. The effect of larval feeding on yield is negligible in healthy plants, but reduces plant resistance to other insects such as stem weevils. As stalks dry down, mature larvae of *Dectes* descend to the base of the stem, begin to girdle the interior surface near the soil line, and then plug the tunnel with chewed fibers that resemble sawdust before retreating to the base of the plant to overwinter. Plants weakened by complete or partial girdling snap off easily when pressure is applied laterally to the stem. Adults mate and feed on plants, leaving longitudinal feeding scars on stems and petioles. Infestations approaching 100 percent of plants may go completely unnoticed when fields are harvested early, but any delay in harvest can result in serious losses due to lodging. They have only one generation per year (Michaud et al., 2011).

**Monitoring:** monitor in late summer by splitting stalks to determine if Dectes stem borers are present, and harvest as soon as possible if infestation is extensive (Michaud et al., 2011).

**Economic and action thresholds:** no action threshold has been set for Dectes stem borer.

**Control**: Crop rotation may reduce damage from Dectes when the acreage of soybeans and sunflowers in an area is low. Stalk desiccation is an important cue triggering the girdling behavior of *D. texanus* larvae. Later planting dates, fall or winter tillage and irrigation of

**Life cycle and damage**: females lay their eggs under seedlings. Young larvae feed on taproots causing wilting and deaths of seedlings, older larvae attack the pods. Adult beetles can defoliate and kill plants up to 40 cm tall. Larvae overwinter in the soil and pupate in the spring. Adults often feed in a line across the field. Beetles hide in the soil during the day and emerge in late afternoon to feed. Heavily infested crops may suffer over 30% yield loss.

**Monitoring:** check in the soil by digging and sieving for the presence of larvae prior to planting, and at all stages for adults. Count the number of beetles per m2 (Brien, 2011).

**Economic and action thresholds:** action threshold is when four beetles is observed per

**Control**: Removal of the host parthenium weed is advised. Spray appropriate insecticides when beetles are active on the soil surface. Beetles can also be controlled by application of pelleted baits (alfalfa or similar meal) at planting. Damage is most prevalent where

**Identification:** Dectes stem borer is bluish-gray, about 1-2 cm, with antennae as long, or longer than the body. They are generally cryptic in behavior and can often be found hiding under leaves within the plant canopy during daylight hours, but also may be seen actively flying and running around on plant surfaces. The eggs are like small bananas with rounded ends. The larvae are pale, legless, cylindrical, and deeply segmented. They are creamy and

**Life cycle and damage**: Females become reproductively active about one week after emergence, and require another week or so to mature eggs. Eggs are normally laid in leaf petioles deep in the central pith. After six to 10 days the eggs hatch and newly hatched larvae pass the first instar in the leaf petiole and then tunnel down into the main stalk where they feed selectively on the pith in the central core. Larvae develop through six instars. The effect of larval feeding on yield is negligible in healthy plants, but reduces plant resistance to other insects such as stem weevils. As stalks dry down, mature larvae of *Dectes* descend to the base of the stem, begin to girdle the interior surface near the soil line, and then plug the tunnel with chewed fibers that resemble sawdust before retreating to the base of the plant to overwinter. Plants weakened by complete or partial girdling snap off easily when pressure is applied laterally to the stem. Adults mate and feed on plants, leaving longitudinal feeding scars on stems and petioles. Infestations approaching 100 percent of plants may go completely unnoticed when fields are harvested early, but any delay in harvest can result in serious losses due to lodging. They have only one generation per year (Michaud et al., 2011). **Monitoring:** monitor in late summer by splitting stalks to determine if Dectes stem borers are present, and harvest as soon as possible if infestation is extensive (Michaud et al., 2011). **Economic and action thresholds:** no action threshold has been set for Dectes stem borer.

**Control**: Crop rotation may reduce damage from Dectes when the acreage of soybeans and sunflowers in an area is low. Stalk desiccation is an important cue triggering the girdling behavior of *D. texanus* larvae. Later planting dates, fall or winter tillage and irrigation of

tapered towards the rear so that final segments are narrower (Michaud et al., 2011).

square meter which can cause severe losses to young seedlings (Franzmann, 2011).

sunflowers follow wheat, sorghum or grass pasture (Brien, 2011).

**6.2** *Dectes texanus* **(LeConte) (Coleoptera: Cerambycidae)** 

They have one generation per year (Bailey, 2007).

sunflowers have reduced infestations by this pest. There is no varietal resistance to stem borers in sunflowers and chemical treatments are not recommended. Although adults are susceptible to many foliar insecticides, their extended activity period means that a single application will not provide control. However, larvae feed within the plant and are protected from contact insecticides, and even repeated applications of systemic materials fail to provide adequate control in most years (Knodel et al., n.d.; Michaud et al., 2011).
