The concept of ‘normal’ temperatures might be a thing of the past. After record warm temperatures last year, and record cold temperatures this year, tracking degree days is more crucial than ever. A degree day (DD) at its simplest is an average of the daily high and low temperature minus a developmental threshold. As DDs accumulate over the course of a summer, they can be used to predict certain events like bloom, moth flights, etc., regardless of the calendar date. Currently only the thresholds for the cranberry plant are published (lower threshold: 41°F, upper threshold: 85°F). These thresholds are used for predicting the stage of plant growth but only provide a rough estimate for insect development. We are working on determining the specific thresholds for Sparganothis fruitworm and we should have these results ready by the end of the summer (stay tuned!).
So how delayed are we? The DD accumulations calculated from March 1-May 15 for the past 6 years for Tomah, WI (41/85°F thresholds, single sine method, horizontal cutoff) are:
2008 – 369
2009 – 463
2010 – 607
2011 – 354
2012 – 832 (wow!)
2013 – 336
It seems that slowly, but surely, spring is arriving.
If you are interested in calculating your own DD accumulations, the University of California-IPM has an excellent DD calculator available on their website. Scroll down to the bottom of the page, enter your upper and lower thresholds (the default calculation methods are fine for the rough estimate) and click on calculate. You can then either enter your weather data online or upload a file.