Evaluating the impact of planting date on Dectes stem borer (Coleoptera: Cerambycidae) infestations in South Dakota sunflowers

Dectes stem borer, Dectes texanus LeConte (Coleoptera: Cerambycidae), is a common pest of sunflowers, Helianthus annuus L., in South Dakota. Its stalk boring activity causes negligible economic losses in sunflowers. However, yield reductions occur when sunflowers lodge due to the preharvest girdling activity. Increased infestations of Dectes stem borer have recently been observed, which have resulted in increased preharvest lodging. In addition, South Dakota sunflower production is greatly challenged by pyrethroid resistant red sunflower seed weevils, Smicronyx fulvus LeConte (Coleoptera: Cerambycidae). To reduce the impact of red sunflower seed weevil populations, the adoption of early planting dates has been recommended. However, this cultural change could result in increased lodging caused by Dectes stem borer girdling. The objective of this research was to determine if sunflower planting date in South Dakota affects Dectes stem borer infestations and the subsequent lodging caused by larval girdling. During 2023 and 2024, a study was conducted at two locations in South Dakota with two planting dates. The results from both years indicate high infestations present in both planting dates. Lodging increased in the earlier-planted sunflower, which was strongly correlated with increased Dectes stem borer infestations. We conclude that earlier planted sunflowers have increased Dectes stem borer infestation, increased preharvest lodging due to girdling, and decreased yield when compared to later planted sunflowers.