Using Degree Days to Predict Insect Life Cycles

David Owens, Extension Entomologist,

Because insects generally have limited mechanisms of regulating their temperature, insect development and life history is tied to ambient temperature. This enables certain life history events to be predicted fairly accurately. The easiest way to calculate degree days is DD = (max temp + min temp)/2 – Insect base temperature. A negative number counts as a 0. For example, seedcorn maggot’s base temperature is 39, if a day’s high was 50, low 30 then DD = (50+30)/2 – 39 = 1. This week in Georgetown, we hit peak overwintering seedcorn maggot adult activity (base 39, DD target 360, start Jan 1). Fields with recent manure incorporation may have maggot eggs in them. Other insects of interest are as follows:

Alfalfa weevil DD = 200–300, base 48°F.

San Jose scale DD = 380, base 51°F. Start sampling at DD 300. Maximum crawler activity DD 600.

Cereal leaf beetle egg lay DD = 327, base 46°F.

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