Growing Degree Days - What?

I received an email the other day asking about Squash Vine Borer. The individual that contacted me informed me that they had already seen one out and felt that it was early as his squash were just beginning to vine out. Usually squash vine borer is more commonly seen in late June through beginning of July for emergence, but when the weather warms up faster, insect emerge sooner, plants bloom sooner. This is where Growing Degree Days come in.

What exactly are growing degree days – GDD? They are an accumulation of "units" that allow us to know when certain insects will emerge or go through changes in lifecycle or plants will bloom. To measure GDD we begin tracking the units on March 1st with a base temperature of 50 degrees. For those curious as to how we calculate daily GDD here's the formula:

Max Temp – Min Temp
------------------------------ - 50 = GDD Accumulation for the Day

You yourself don't have to calculate the daily GDD accumulation unless you really want to. During the growing season, University of Illinois Extension publishes a weekly online newsletter called the Home, Yard & Garden Pest Newsletter that provides current GDD accumulations based on a few locations throughout Illinois. You can find the weekly newsletter by visiting:

But what does this all mean for you? It gives you an opportunity to have a heads up on when we might expect to see certain insects emerge in the landscape or not be surprised if they show up earlier then in the past. For example, as of June 15, 2015 from the weather station in Perry, IL – there was a total of 1135 GDD accumulated, if you live closer to Monmouth, IL the GDD accumulation was 1012 GDD. To make a note, that is higher than average – the eleven year average is 908 in Perry, IL and 829 for Monmouth, IL. This means that insects are emerging earlier this year.

Japanese beetle adults begin to emerge and feed at 950 accumulated degree days which means that they should already be out feeding as we are well beyond their base minimum for becoming active. To come back around to Squash Vine Borer – adults emerge around 900 GDD and begin laying eggs at 1000 GDD. If you visit the link above and select the article about growing degree days, at the bottom are links to GDD charts for landscape and conifer pests.

By following GDD you can use it as a more effective guide for applying chemical controls for insect pests in the landscape as compared with a general "these insect pests usually arrives in late June/early July." Use GDD as part of an integrated pest management plan – one where you find a balance between chemical, cultural, and mechanical methods of control and plant care.