Insects

About insect degree-days

Insect degree-days (°D) are a measurement of heat unit accumulation required for insect development. Degree-days are calculated on a daily basis (thus the "days" part of degree-days) and summed each day, accumulating over a given time period. Often people accumulate degree-days starting from an early spring calendar date for insects that overwinter in the local area, but other dates or events such as peak insect pheromone trap catch can serve as the starting date. The date or event from which degree-days are accumulated is referred to as the "biofix", and is specific to the particular insect species for which life stage development is being estimated.

A biofix is a biological marker that initiates the beginning of degree day (i.e. heat unit) calculation used to forecast temperature-dependent insect developmental stages. For insects, it is usually the calendar date of a particular pest event, such as peak moth flight in spring.

Degree-days are used in entomology because insects develop through each stage in their life cycle incrementally depending on temperature. This development is bounded by lower and upper developmental thresholds. Thresholds will vary depending on the insect species of interest, and this information is obtained through laboratory studies and validated by field experiments.

The lower developmental threshold, or base temperature, for an insect species is the temperature below which development ceases (it is too cold for the insect to develop and reach the next life stage). For example, the seedcorn maggot (Delia platura) overwinters in Wisconsin in the pupal stage in the soil. It does not commence development in spring until air temperature meets and exceeds its lower developmental threshold (base temperature) of 39°F. Once this happens, seedcorn maggot degree-days are accumulated until enough heat units have accumulated for adult flies to emerge from the soil. The upper development threshold is the temperature above which a particular insect species' development starts to decrease or stops.

All insects require a certain amount of heat units to develop from one life stage to the next. This is a constant, unique to the physiological development of each species. Development will occur faster at warmer temperatures, and more slowly at cooler temperatures, within the optimal range between lower and upper developmental thresholds (provided lethal cold or heat temperatures are not reached).

How to Calculate Degree Days

Progression through an insect's life cycle can be estimated using air temperature data collected from weather stations near the field or farm where IPM decisions are being made. This information is accessible online, unique to each state. When you know the degree-days required for a particular insect pest event (e.g. peak adult emergence, first instar larvae, etc.) you can estimate these events by accumulating degree-days from the appropriate biofix (peak pheromone trap catch, calendar date). This information will be specific to the insect pest you are managing. The daily formula to use is: (maximum temperature + minimum temperature/2) - base temperature.

The University of Wisconsin-Extension Ag Weather website provides a degree-day calculator and select field and forage crop insect pest phenology models with degree-day accumulation map display for Wisconsin including alfalfa weevil, European corn borer, common stalk borer (corn) and seedcorn maggot (soybean, corn and vegetables).

These pest-specific thermal models and degree-day accumulation maps guide growers, consultants and educators to regularly and conveniently check accumulation of insect degree-days, particularly when a field crop scouting window and/or control action decison is approaching.

More information on insect degree-days can be found at UC IPM Online.

back to top