Pest Degree-Days & Predict the Next Stage
Times egg hatch
Enter mean temperature, the pest's base and the degree-days to the next stage to get the degree-days per dayand the days to egg hatch, the next generation or a control action — so you spray when it counts.
Degree-day pest forecast
Next: plan to scout intensively and place pheromone traps around day 17, when Pink Bollworm is forecast to reach its action threshold — that's the window to time any spray to the vulnerable life stage.
Degree-day models are guides, not guarantees. Use a simple-average model only as a first approximation, calibrate the base/upper thresholds to your local pest biotype, and always confirm with field scouting.
Pest degree-days — key facts
- DD per day
- mean temp − base
- Mean temp
- (high + low) ÷ 2
- Days to event
- target DD ÷ DD per day
- Base (lower)
- ≈ 10–13 °C many pests
- Upper cap
- mean capped at threshold
- Drives by
- heat, not calendar
- Start from
- biofix (first trap catch)
- Privacy
- Runs in your browser; nothing uploaded
Insects keep time by heat, not by the calendar
Insects are cold-blooded, so a warm week pushes them through their life stages far faster than a cool one — and a fixed spray date can land days off the mark. Degree-days count the warmth above the pest's base temperature, capped at an upper threshold, and add it up. When the running total reaches the pest's known degree-days to a stage — egg hatch, a larval instar, the start of the next generation — that's your moment to act.
This tool gives the degree-days accumulating each day, the target degree-days to the event and the calendar days until it arrives from your mean temperature and the pest's thresholds. Use it to time sprays at the vulnerable stage, plan scouting, and project from your trap biofix. Pair it with the Growing Degree Days, Economic Threshold and Spray Schedule tools for a full IPM plan.
Time the spray
Hit the vulnerable stage, not a fixed date.
Forecast the next gen
See the days to the next generation's flight.
Plan scouting
Be in the field as egg hatch approaches.
Use the right base
Each pest's threshold sets the heat clock.
Frequently Asked Questions
What is a degree-day for pests?+
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.
How are degree-days per day calculated?+
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.
What is the base or lower threshold temperature?+
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.
What is the upper threshold?+
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.
How do I predict days to the next stage?+
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.
Why time sprays with degree-days instead of the calendar?+
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.
Which pests use degree-day models?+
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.
Where do I start counting (the biofix)?+
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.
Are the figures precise?+
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.