A degree-day is one day spent one degree above an organism's lower developmental threshold. A pest needing 150 degree-days (C) per generation with a threshold of 10 C finishes in 15 days at a steady 20 C (10 degrees above threshold) and in 10 days at 25 C. UC IPM describes degree-days as physiological time: heat, not days, drives insect and mite development.
In practice
Outdoors, degree-day models time scouting and treatment, for example predicting moth egg-laying from trap catches. Indoors the temperature is near constant, so you can turn degree-days straight into days: divide the requirement by (room temperature minus threshold). Use the leaf or canopy temperature, not the thermostat. Above the upper threshold the sum stops adding up, because development slows or stops.
Don't confuse with
- Calendar spray schedule: ignores temperature, so it drifts out of step with the pest.
- Growing degree-days for crops: same idea, different thresholds.
