# Degree-day

> A unit of accumulated heat above a pest's lower developmental threshold; development finishes when a stage's degree-day total is reached, whatever the calendar says.

- Canonical: https://genetixbay.com/knowledge/glossary/degree-day.html
- Also written: degree-days, growing degree-day, GDD, heat units, thermal time
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

A degree-day is one day spent one degree above an organism's [lower developmental threshold](https://genetixbay.com/knowledge/glossary/developmental-threshold.md). 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](https://genetixbay.com/knowledge/glossary/ipm.md) 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](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), 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.

## Related terms

[Developmental threshold](https://genetixbay.com/knowledge/glossary/developmental-threshold.md), [Life table](https://genetixbay.com/knowledge/glossary/life-table.md), [Generation time](https://genetixbay.com/knowledge/glossary/generation-time.md)

## Read more

- [Temperature, humidity and pest population growth in cannabis: every major pest compared](https://genetixbay.com/knowledge/pests/temperature-humidity-and-pest-population-growth.md)
- [Budworms on cannabis: corn earworm and tobacco budworm in numbers](https://genetixbay.com/knowledge/pests/budworms-corn-earworm.md)
- [Caterpillars on cannabis: species, degree-days and control](https://genetixbay.com/knowledge/pests/caterpillars.md)

## Sources

1. UC IPM. Degree-Days: About Degree-Days. University of California ANR. https://ipm.ucanr.edu/WEATHER/ddconcepts.html