Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, Amanda Bachmann and Shelby Pritchard.
Originally Submitted: August 5, 2022
Red sunflower seed weevils are active in South Dakota sunflowers. The maturity of the sunflowers in South Dakota currently ranges from R3 to R5 stages. Scouting should begin for fields that are going to be reaching R5 within the next week. We noticed that many of the fields at the R5 stages are already being sprayed, but scouting should continue for those fields after insecticide application. In recent years, the red sunflower seed weevil populations have been higher than normal in many areas of South Dakota. In some areas, the populations have exceeded the economic threshold (4 to 6 adults per head) with as many as 300 to 500 adults per sunflower head. The 2022 populations that have been observed in several fields were well over the economic threshold.
In addition to large populations, since 2017 we have received reports and observed evidence of red sunflower seed weevils that have reduced susceptibility to pyrethroid class insecticides. These populations often remain above threshold, even after the application of insecticides with the active ingredient lambda-cyhalothrin, esfenvalerate or zeta-cypermethrin. For these reasons, scouting in 2022 will be important, both prior to and after insecticide applications to ensure that populations of this pest are effectively reduced.
Our research during 2018 indicated that scouting 24 hours after application may overestimate insecticide failures. For this reason, we are recommending that scouting after a spray occurs 48 hours after application. We will continue to evaluate fields for pyrethroid-resistant populations this year. If you believe you have a pyrethroid resistant red sunflower seed weevil population, please reach out to Dr. Adam Varenhorst at email@example.com.
Red sunflower seed weevil adults are relatively small beetles (1/10 to 1/8 of an inch long) that are a reddish-brown color. Like other weevils, they have mouthparts that extend forward beyond the head and form a snout (Figure 1). The antennae originate on this snout. Red sunflower seed weevil adults are often present in fields prior to flowering and may be observed in the developing bud.
Once flowering begins, they will move to the heads and often will crawl between the florets (Figure 2). Larvae of the red sunflower seed weevil are cream colored and relatively small. However, the larvae are not typically observed, as they feed directly on the developing seed (Figure 3).
Scouting for red sunflower seed weevils should begin when the yellow ray petals are visible on the face of the developing bud (R4 growth stage) and continue until 70% of pollen shed has occurred (R5.7 growth stage). Once most of the sunflowers in the field have reached R5.7, most egg-laying has finished, and the seeds are too mature to be suitable for further red sunflower seed weevil egg oviposition.
To scout for red sunflower seed weevils, walk approximately 75 feet into the field and examine five random plants. Next, walk to another location approximately 75 feet from the first location and scout an additional five plants. Repeat this process until a total of 25 sunflower plants have been examined in the field. Developing heads can be examined by simply rubbing the face of the head to disturb the weevils and make them easier to count. If this method doesn’t work, they can also be scouted for by using an aerosol insect repellant, which is sprayed across the head. After spraying, simply wait for the beetles to begin to emerge and count them. Once the plants have been examined, calculate the average number of red sunflower seed weevil adults per plant, present in the field.
Economic Threshold and Management
For red sunflower seed weevils, the economic threshold is dependent on the value of the crop, cost of insecticide management, and the planting population of the sunflower. Generally speaking, for most oilseed sunflower varieties, the economic threshold for red sunflower seed weevils is 4 to 6 adults per sunflower head. For confection sunflowers, the economic threshold is 1 red sunflower seed weevil per head. Ideally, management of the red sunflower seed weevil populations should also reduce other pests, including the banded sunflower moth, sunflower moth and tarnished plant bug.
For a list of insecticides currently labeled for red sunflower seed weevil management, please refer to the latest South Dakota Pest Management Guide: Alfalfa and Oilseeds.
Since management of the red sunflower seed weevil occurs during flowering, considerations should be made to reduce the potential impact of the spray on pollinators. It is best to treat in the morning or evening when pollinators are least active. To determine if hives are nearby, please refer to Field Watch and register as an applicator.