Tools for IPM: A degree-day Model for billbugs (Sphenophorus spp.) in intermountain west turf

Billbugs (Sphenophorus spp.) have been identified as the most problematic insect pest to turfgrass in the Intermountain West. Billbugs are typically managed by prophylactic applications of systemic insecticides targeted at young larvae in grass stems, and this requires the manager to have knowledge of billbug seasonal activity. Most of what is known about billbug phenology comes from a degree-day model developed in the eastern U.S. for the bluegrass billbug using a start date of March 1 and a lower development threshold of 10^oC. This model has not been validated in our region, and predictions made using this model may be inaccurate. The objective of this study is to assess the predictive ability of the current degree-day model in the Intermountain West and to make any necessary adjustments to the model. We monitored the seasonal activity of adult billbugs using linear pitfall traps on four golf courses in the Intermountain West and found a complex of three species, including the bluegrass billbug (S. parvulus), hunting billbug (S. venatus vestitus), and Rocky Mountain billbug (S. cicatristriatus). We monitored immature billbugs by taking soil cores with a golf course cup cutter. Using the current model’s parameters to calculate degree-days for our region, first occurrence of adult billbugs in the Intermountain West was found to be much earlier (4-15 DD₁₀) than predicted by the current model (155-195 DD₁₀). An adjustment of the model to an earlier start date (January 13) and cooler lower development threshold (5 ^oC) appears to improve the model’s predictive ability for our region. This proposed model predicts first occurrence of billbugs between 39-111 DD₅, 30% of adult emergence between 459-484 DD₅, and peak adult activity between 768-838 DD₅. Data from multiple seasons is being used to further develop and validate the proposed degree-day model. A degree-day model specific to the complex of billbugs in the Intermountain West region will improve timing of preventive applications of insecticides while promoting the implementation of IPM in turfgrass.