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Degree Days_March19_FtSnellingStation_2012

About Insect Degree Days

An important aspect of Integrated Pest Management is being able to know where your target insects are in their life cycle during the growing season. One intention behind the classes I am teaching for Permaculture Research Institute is to encourage urban growers and new growers to grow comfortable with the idea that they can predict when target insects will emerge. The grower who has the right materials needed for managing plant-feeding insects – prepared and ready – will have a better chance of using them effectively if they are used during the window of time when the insects are most vulnerable.

How to Use Degree Day Information

During the winter, insect accumulate heat units called degree days. These heat units are related to the insect’s metabolism and other physiological processes. Gradually, as enough heat units are built up as winter fades into spring, the insect slowly becomes active and will emerge from its winter slumber. The number of degree days (DD) for some “economic insects” is known. For example, the degree day units for squash vine borers is known to be between 1000-1200 DD. This means, according to the simple degree day calculation formula (shown below),  that squash vine borers in Minnesota emerge in late June. But, if growers keep track of the temperatures and degree day units, we can narrow down the window of time from “sometime in late June” to, for example, the third week in June.

Formula for Simple Degree Day Calculation

To calculate insect degree days using the simple formula, you add the MAX daily temperature and the MIN daily temperature, divide by two, and subtract the insects Base Temperature. Any number below zero indicates no degree day units have accumulated.

Example: 38 (MAX) + 12 (MIN) / 2 = 25 – 50 BT = -25 (No degree day units accumulated.)

68 (MAX) + 54 (MIN) / 2 = 61 – 50 = 11 degree day units accumulated

Frequency of Updates

To accomplish this, I am compiling temperature information from the Minnesota Climatology Working Group and entering it into a Microsoft Excel file converted to pdf. I plan to update this information about bi-weekly until April 15. After that, updates will be made as close to daily as possible.

Caution statement regarding the use of degree days

Degree day information featured on this page is being compiled from the Minnesota Climatology Working Group website, and features temperature information from the Twin Cities Station located near the airport at 44.52/93.13. This temperature information is meant to provide an estimate regarding the degree days so that growers can more accurately predict when insects are going to emerge in their gardens and fields. Growers should keep in mind that temperatures can vary within short distances, and that microclimates can also create a range of temperatures among growing areas located within a short distance from each other. Therefore, critical pest decisions should not be based solely on the information provided on this page.

Contact information

Neil Cunningham

leavesofnoise@gmail.com