Kill Billbug: Improving a Predictive Model to Time Management in the Intermountain West.

Billbugs (Sphenophorus spp.) have been identified as the most problematic insect pest to turf in the Intermountain West. Most of what is known about billbug phenology comes from a degree day model developed in the eastern U.S and is based on bluegrass billbug populations. Several aspects of the model, however, have not been verified in our region. We conducted a field study to examine the seasonal activity of billbugs in turfgrass to improve our understanding of billbug biology and management in the Intermountain West. We found a complex of three billbug species affecting turf that included the bluegrass billbug (S. parvulus), hunting billbug (S. venatus vestitus), and Rocky Mountain billbug (S. cicatristriatus). There was variation in species composition and density of adult populations among sites and differences in elevation, annual precipitation, average annual high and low temperatures, and management could be contributing factors. First occurrence of adult billbugs in Utah was calculated to be several weeks earlier (4-15 DD₁₀) than predicted by the current model (155-195 DD₁₀). Populations of the damaging larval stages were highest two weeks after adult peaks. An adjustment of the current model to have an early biofix (January 13 versus March 1) and lower temperature threshold (5 C versus 10 C) appears to improve the model fit for our region. Our proposed model predicts first occurrence of billbugs between 39-111 DD₅, 30% adult emergence between 459-484 DD₅, and peak adult activity between 768-838 DD₅. Data from multiple seasons is being used to further develop the proposed degree-day model and optimize timing of management for billbugs in the Intermountain West. Using this information, we are now testing the efficacy of biological and chemical control tactics with varying irrigation levels on billbug suppression.