Pest Degree-Days & Predict the Next Stage

A degree-day is one day's worth of heat above the temperature at which an insect can develop. Insects are cold-blooded, so they grow by accumulated warmth, not by the calendar. One day with a mean temperature 10 °C above the base contributes 10 degree-days; warmer days add more, cooler days add less. Add them up and you can predict when a stage will be reached.

The simple average method is degree-days per day = mean temperature − base (lower threshold) temperature, where mean is (daily high + low) ÷ 2. The result is capped so it never goes below zero, and the mean is held at an upper threshold above which development no longer speeds up. So 28 °C mean with a 12 °C base gives 16 degree-days for that day.

The base (lower developmental threshold) is the temperature below which the insect effectively stops developing — accumulating no degree-days. Each pest has its own value, often around 10–13 °C for many crop insects. Using the correct base is essential: too low overcounts heat and predicts events too early, too high does the reverse.

Above a certain temperature, development no longer accelerates and can even slow, so degree-day models cap the daily mean at an upper threshold (often in the low-to-mid 30s °C). Without the cap, hot spells would overstate how fast the pest matures. The horizontal-cutoff used here simply holds the mean at that ceiling before subtracting the base.

Take the degree-days the pest needs to reach the event — egg hatch, a larval instar or the next generation — and divide by the degree-days accumulating each day at your temperatures: days to event = target degree-days ÷ degree-days per day. As temperatures rise the same number of degree-days arrives in fewer calendar days, so warm spells shorten the wait.

Spraying on a fixed date misses the window when warm or cool seasons shift insect development. Degree-days target the vulnerable stage — newly hatched larvae before they bore into fruit, or the start of a new flight — so a contact insecticide lands when it works and you avoid wasted, badly timed applications.

Many: pink bollworm, fall armyworm, codling moth, European corn borer, cabbage maggot, leafminers and more. Each has published base temperatures and degree-day totals to key events. Plug the pest's own numbers into this tool — the method is the same, only the thresholds and targets change.

Degree-day accumulation usually starts from a biological reference point called a biofix — often the first sustained pheromone-trap catch, the date of egg lay, or a fixed calendar start. From that biofix you accumulate degree-days; this tool gives the per-day rate and days-to-target so you can project the date from your biofix.

They're solid planning figures. Real development varies with microclimate, the exact thresholds for your pest strain, day-to-day temperature swings and the method used. Use local thresholds, update with real daily temperatures, confirm with traps and scouting, and treat the days-to-event as a forecast to steer by — not an exact date.