For organisms, heat drives development. If you can regulate your own heat like we humans do every day is a balmy 98.6 or so, so development every day is the same and can be measured by the calendar. But for our field crops and most of the pests they face, development is based on the heat they get from the environment. And, less heat means slower growth. For the week of July 12--19 alone, Iowa normally accumulates 200 base-50 degree days as a statewide average. Last week that accumulation was 133, or about four and a half "normal" days for the week. So, for the season since May 1, we are now 143 to 205 degree days behind normal.
Pope, Richard O., "Crops Put In a 4-Day Work Week" (2009). Integrated Crop Management News. 595.
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