As the spring season approaches, insects and plant diseases begin their own progression, along with the plants they attack. Rather than calendars, insects use the weather as their determination of when spring begins. When temperatures start warming up, insects begin to exit from their dormant stage. This doesn’t happen immediately. For example, a few unseasonably warm days in February won’t cause insects to emerge. It takes quite a few days that average 50° F for insects to begin emerging. The easiest way to calculate degree days is to use a base of 50. When temperatures reach 50 degrees Fahrenheit most plants, insects that feed above ground and diseases begin to grow and develop. The calculation of degree days can begin January 1st, for any given year. To use a base of 50, add the high temperature and the low temperature for that day. Take that number and divide it by two and subtract 50. The final number is the amount of degree days for that particular day. If the final number is less than zero, that indicates no degree days can be added to the total for that year. Any number above zero would be the number of degree days for that day and can be added to the amount calculated for the year. In Northern Illinois, the number of degree days starts increasing more rapidly the closer we get to April and May. A mild winter may increase the number of degree days dramatically, while a cold winter or cool spring may keep the number of degree days low. Plant and insect growth are dependent on time and temperature. Because insects are cold-blooded they are unable to regulate their body temperature and unable to survive in cold temperatures. This is the reason an average of 50° F is needed for their growth.
In 2012 we experienced a mild late winter and spring temperatures started much sooner in the year, which increased our number of degree days dramatically and insect and plant growth occured much earlier than normal. While it’s too soon to know what 2013 will bring, the degree days are continually tracked and will indicate the type of spring we will experience in the Chicago area. We will keep you posted on the season as it progresses!
Leave a comment with what you’re experiencing in the landscape this year!