How does early planting of sunflower affect parasitism of the red sunflower seed weevil, Smicronyx fulvus?

The red sunflower seed weevil (RSSW), Smicronyx fulvus L. (Coleoptera: Curculionidae), is one of the biggest threats to the sunflower industry in North America. Weevils per head in central South Dakota are often more than 100 times the treatment threshold. Fortunately, moving sunflower plantings from mid-June to early-May is highly effective at reducing damage. One prior study in the 1990s did not find an effect of planting date on RSSW parasitism, though weevil densities were relatively low and range of locations and planting dates were limited. Plantings near Pierre, SD during 2023 (1 May to 15 June) and 2024 (25 April to 19 June) covered the full planting period available for crop insurance as outlined by USDA’s Risk Management Agency (RMA). Plantings near Carrington, ND in 2023 (16 May to 13 June) and 2024 (17 May to 17 June) reflect a shorter period for crop insurance in this area. Heads collected from the plantings and were returned to the laboratory to collect weevil larvae as they emerged. Parasitism was assessed with two methods: (1) collecting parasitoids that emerged following overwintering of weevil larvae in cold storage, and (2) detection of DNA of the parasitoid Triaspis aequoris Martin (Hymenoptera: Braconidae) using primers developed from lab-reared, positively identified specimens. Though emergence of parasitoids from weevils collected in 2024 is ongoing, data suggest parasitism is sensitive to planting date and that DNA-based assessment of parasitism is more effective than rearing.