Written collaboratively by Adam Varenhorst, Patrick Wagner, Amanda Bachmann, Ruth Beck, Philip Rozeboom and Aaron Hargens.
When scouting sunflower, the most observed weevil is the red sunflower seed weevil. However, there are two species of seed weevils that can be present on sunflower heads in South Dakota. The second species is the gray sunflower seed weevil. During the season, it is possible to observe both species on a single sunflower head (Figure 1). When this occurs, determining their identities isn’t difficult as there are obvious size and coloration differences. Generally, the populations of the red sunflower seed weevil are much higher than the populations of the gray sunflower seed weevil. For this reason, thresholds have been calculated for red sunflower seed weevils (4-6 red sunflower seed weevils per head) while the gray sunflower seed weevil is considered a non-economic pest in South Dakota.
Red Sunflower Seed Weevil
Like its name implies, the red sunflower seed weevil has an orange/red appearance due to small hairs that are present on its body (Figure 2). Often, these weevils will appear almost black because these hairs have been rubbed off. The red sunflower seed weevil has a black snout, with small bent antennae that originate from the snout. The adults will also have black legs. The red sunflower seed weevil is smaller than the gray sunflower weevil.
When examining a plant, the red sunflower seed weevil will often be found down between the developing seeds in the head. It can also be observed climbing on the head or on the leaves and stem of the plant. Prior to flowering, pulling back the bracts can often uncover seed weevils. The red sunflower seed weevils are capable of flight and will drop from the plant when disturbed.
Gray Sunflower Seed Weevil
Aptly named, the gray sunflower seed weevil adult is covered in small gray hairs that cover much of its body (Figure 3). The gray sunflower seed weevil is larger in size when compared to the red sunflower seed weevil and has gray legs instead of black. It also has a black snout that has small antennae that originate from it.
The gray seed weevil emerges approximately 10 days before the red sunflower seed weevil.
The larvae of both species are small and feed on the developing seeds. They are cream colored and take on a “C” shape when disturbed.