Degree days at a glance

What are degree days and why do we use them? Degree days are an important part of many NEWA tools, but what are they and whey do we use them?

Insect and plant development is controlled by temperature. Warmer temperatures speed development while colder temperatures slow development. This relationship can be used to track the life cycle of an insect or plant with something called a degree-day model.

A degree day is a unit of measurement that is species-specific and represents some proportion of overall insect or plant development. Degree days are calculated by using maximum and minimum temperature to calculate a daily average, then subtracting a base temperature. NEWA weather stations provide daily temperature information to track pest development, and the rate of degree-day accumulation is unique for each location. It also helps to think of degree days as accumulated points, not accumulated days.

Different insects have a different degree day requirements. This caterpillar pest has a base temperature of 50 degrees Fahrenheit (or 10 degrees Celsius) and a developmental requirement 1000 ‘base 50F’ degree days from egg to adult. Image Copyright Dan Olmstead NYS IPM Program.

For a long time, pest management information learned from our parents and grandparents was accurate. But now, the weather is less predictable, especially in the last five or ten years. (And new pests have emerged.) Sadly, cross-generational knowledge is becoming less reliable. Degree-day models are now the best basis for agricultural pest management decisions.

A farmer is going to see changes in pest pressure, planting time, or crop development from one year to the next. Degree-day models, combined with modern technology, give growers a better estimate of pest status. There is still variability, but he or she is better informed to make effective management decisions.

Degree day requirements for an insect won’t change. But the time needed to gather those degree days is variable.

Up next, we will take a closer look at degree day tools available on NEWA.