Which unit is used to express physiological time in pest development monitoring?

• A. Degree days
• B. Degree hours
• C. Degree months
• D. Degree weeks

Answer: (A)

Pest development is driven by temperature, so predicting when a pest reaches a new life stage is about tracking heat accumulation over time. Degree days are the unit used because they combine how long you’ve waited with how warm it has been, giving a single measure of heat that directly relates to biological progress.

A degree day is earned when the daily average temperature is above a base threshold for a species. You subtract that base temperature from the daily average and, if positive, add that amount to a running total. Over time, the accumulated degree days match the amount of physiological development the pest has achieved. For example, with a base temperature of 10°C, an average daily temperature of 15°C adds 5 degree days for that day; after a certain number of degree days, the pest reaches a specific stage, signaling when to scout or apply control measures. Because this method ties time to actual biological progress, it’s far more informative than calendar time alone.

Other units like degree hours or longer calendar intervals don’t align as neatly with how pests develop. Degree hours would be unnecessary precision for most field decisions, and degree weeks or degree months don’t reflect the rapid changes pests can undergo with daily temperature fluctuations. Degree days remain the standard for expressing physiological time in pest development monitoring.