Dispersion of red sunflower seed weevil damage in commercial sunflower fields

The red sunflower seed weevil (RSSW), Smicronyx fulvus LeConte, is a key pest of cultivated sunflower in the Northern Great Plains. Larval feeding on the achenes can cause a significant reduction in final oil yield and test weight. A previous study has shown that RSSW feeding damage significantly decreases with increasing distance from the field edge; however, that study was conducted in situations where RSSW population densities were relatively low. Recently RSSW populations have been 10 to 200 times the threshold (4 to 12 RSSW head-1) in South Dakota. Thus, mature sunflower heads were taken from 10 central South Dakota fields at 7 distances ranging from 0 to 200 m in 2024 and 2025. Subsamples were taken for X-ray analysis of RSSW feeding damage, calculation of test weight, and determination of oil content. RSSW feeding damage was greatest in the 10 m nearest the edge. RSSW feeding explained at least half of the variability in sample oil content and test weight. From the results, the following conclusions may be drawn: first, the edge effect is still present when RSSW populations are very high and scouting must account for this dispersion; second, fields with a high edge to area ratio may have more damage and grouping fields together may have a protective effect; third, limited area treatments (eg insecticide applications) may be effective under certain conditions.