Which term describes the accumulation of heat to predict pest development?

• A. Growing degree days
• B. Pest accumulation index
• C. Thermal unit measure
• D. Heat-time index

Answer: (A)

Growing degree days measure how much heat accumulates over time to drive pest development. Insects need a minimum temperature to progress through their life stages, so we track heat units above that threshold. Each day, take the average of the day's high and low temperatures, subtract the species’ base temperature (the point where development starts), and add that result if it's positive. That sum over many days gives the accumulated heat units, or growing degree days (GDD). When the pest reaches a certain number of GDDs, a developmental event such as egg hatch or larval emergence is expected, helping you time scouting or control measures.

For example, if the base temperature is 10°C and a day has a high of 20°C and a low of 6°C, the average is 13°C and the daily GDD is 3°C. Days with averages below the base contribute zero. Over time, a pest that requires, say, 200 GDD to reach a vulnerable stage would be forecast to do so once the accumulated total reaches 200, guiding management actions.

Other terms like pest accumulation index, thermal unit measure, or heat-time index aren’t standard ways to describe this heat-based forecast approach, which is why growing degree days is the correct concept here.