Evaluating insecticide application timings against billbugs (Sphenophorus spp.) using a degree-day model and calendar-based approach in the Intermountain West

Recommendations for timing insecticides against billbugs have historically been based on adult activity and a corresponding degree-day (DD) model that is optimized for the eastern United States. A DD model was recently developed for billbugs in Utah and Idaho that refines predictions of adult activity based on the phenology of species that inhabit the Intermountain West (IMW) region. However, timings still follow eastern US recommendations and have not been verified with field applications in the IMW. We evaluated the synthetic insecticides Merit 75 WP and Acelepryn 1.67 SC and the bioinsecticides Grandevo and Venerate for controlling larvae when using the eastern recommended treatment thresholds of 30% and 50% adult activity (adults collected in pitfall traps) based on the Utah–Idaho model for two years. Applications of insecticides at these adult emergence thresholds provided >75% control of billbug larvae preventively and curatively in Utah, confirming these are appropriate action thresholds to use in the Utah–Idaho model to time insecticides. Insecticides applied at various calendar dates around these recommended timings, but typical for a professional applicator in Utah based on site history and weather, were assigned model-calculated DD for testing in the Utah–Idaho model. Instances where a professional applicator treated within model-predicted timings resulted in larval reductions, further validating that the model parameters are good recommendations for the region and should be adopted. Additionally, several calendar-based applications made earlier or later than optimal timings based on model predictions were effective, suggesting that applicators have flexibility for timing applications when targeting larvae.