Figure 1. Red sunflower seed weevil adult on sunflower leaf.
Credit: A. Varenhorst.
Red Sunflower Seed Weevils
One of the most economically damaging insect pests of sunflower in South Dakota is the red sunflower seed weevil. Although the National Agricultural Statistics Service (USDA) reports that only 23% of sunflowers in South Dakota are blooming, it is important to scout fields for the red sunflower seed weevil. Last week, we observed adults of this pest on non-flowering sunflower near Onida and Highmore (Figure 1). This indicates that the weevils are active and will be searching for plants that have yellow ray petals showing (R4-beginning of R5).
Observing red sunflower seed weevil adults in fields that are not yet flowering is not uncommon. The adults begin emerging as early as mid-July and will continue throughout August. The early emerging adults initially feed on the bracts of developing sunflower buds and eventually transition to feeding on pollen. Although present on the head throughout flowering, the female red sunflower seed weevils will not begin laying eggs until seeds begin to fill. The importance of the red sunflower seed weevil decreases once sunflowers reach 70% pollen shed (R5.7).
Red sunflower seed weevil adults are relatively small at approximately 1/10 to 1/8 of an inch in length. Their name is based on their reddish-brown coloration. Like other weevils, their mouthparts extend forward forming a snout. The red sunflower seed weevil’s antennae originate from their snout, which is another characteristic of weevils (Figure 2). The larvae of the red sunflower seed weevil are quite small, and are white or cream colored with a slightly darker head region. In addition, the larvae are legless and resemble a C-shape when disturbed (Figure 3).
Fig. 2. Red sunflower seed weevil adult. Notice the elongate snout and the antennae originating from it. Credit: A. Varenhorst.
Fig. 3. Red sunflower seed weevil larva. Courtesy: F. Peairs, CSU, Bugwood.org
Scouting & Management
Scouting for the red sunflower seed weevil should begin when the yellow ray petals are visible on the face of the developing bud (R4). Scouting should continue until 70% of pollen shed has occurred (R5.7). To scout, walk approximately 75 feet in from the field edge and examine five random plants from five different locations throughout the field for a total of 25 sunflowers per field. Developing sunflower heads can be scouted by simply rubbing the face of the sunflower head to disturb the weevils and cause them to move. They may also be scouted by using an aerosol insect repellent and spraying each head and waiting for the red sunflower seed weevils to move. After the red sunflower seed weevil adults begin moving, count them and repeat this procedure for each of the examined heads. Once the adults are counted, calculate the average number of weevils present per sunflower head for the field.
The economic threshold for the red sunflower seed weevil depends on the value of the crop, the cost of insecticide application, and the planting population of the sunflower. For oilseed sunflower varieties, the economic threshold for red sunflower seed weevils is 4 to 6 weevils per sunflower head. The economic threshold for confection sunflower varieties is 1 weevil per head. Management of the red sunflower seed weevil should be timed to also provide management of other sunflower head insect pests, such as the banded sunflower moth, sunflower moth, and tarnished plant bug.
Research from neighboring states indicates that the best time to apply insecticides is when at least 30% of a sunflower field has reached ten percent pollen shed (R5.1 growth stage), and pest insect populations have reached their economic thresholds. Because the timing of the insecticide sprays for the red sunflower seed weevil correspond to sunflower blooming, it is important to consider the effects that the insecticides will have on pollinators. To minimize the impacts of the spray on insects such as bees, spray in the later afternoon hours or evening. Also, be sure to check the South Dakota Department of Agriculture Sensitive Site Registry to ensure that honey bee hives will not be affected by an insecticide application. For a list of insecticides available for managing the red sunflower seed weevil please refer to the current edition of the South Dakota Pest Management Guide: Sunflower and Oilseeds.