Why is tracking degree day accumulations more useful than calendar dates for monitoring pest activity?

• A. Insects, diseases and weeds develop based on heat units. The accumulation of heat units varies from year to year.
• B. Calendar dates are always more accurate.
• C. Pests ignore temperature.
• D. Degree days are constant year to year.

Answer: (A)

Pest development is driven by temperature, not the calendar. Insects, diseases, and weeds progress through life stages as they accumulate heat, which we measure as degree days above a base temperature. Because weather each year varies, the same date can bring a very different amount of heat, so the pests’ actual readiness for emergence or infection shifts with the weather. Tracking degree-day accumulations lets you time management actions to when pests are biologically primed, yielding more reliable control than sticking to calendar dates. For example, a disease or insect may reach a vulnerable stage after a certain heat threshold, which could occur in early spring one year and late spring another year. Using degree days accounts for that variability.