Insect Degree Day Info (2012)

As of April 26, 2012

The attached pdf file (below) contains degree day calculations (using the simple formula) for Minneapolis/St. Paul Minnesota. These calculations are intended to help growers – particularly urban growers in the Twin City Metro Area – plan for upcoming insects of concern.

The unusually warm spring temperatures may throw growers a few “curveballs” this year. The degree day calculations can provide a more accurate estimate than the coarse estimates provided by typical insect factsheets as to when certain insects will be coming into play. Still, growers should be mindful that the simple degree day tool only provides some additional precision; it is meant to provide growers with a “window” of time (about a week or so) that can help narrow down actual emergence times when seasonal temperature fluctuations interfere with the typical insect emergence estimates.

For example, if our weather continues to warm, the initial emergence of cabbage flea beetles may occur as soon as the end of this week, or the week of April 29, 2012.

In Minnesota, Colorado potato beetle adults typically first emerge about the middle of May, so potato growers should loo for adults that might be coming out a week or two early.

Thanks to the Minnesota State Climatology Working Group for the temperature information that was used to make the degree day calculations. Click on the link below for the degree day calculations:

Degree Days_April25_2012

– Neil, April 26, 2012

As of March 11, 2012

For urban farmers and others who are paying attention to insect degree days, this is the moment you have been waiting for. Cue the trumpet fanfare.

According to calculations based on data provided by the Minnesota Climatology Working Group and the National Weather Service (see link below), insects with base temperature of 50° Farenheit started accumulating degree days on or around Sunday, March 11, 2012. In case you missed the fanfare, I repeat: insects with a base temperature of 50° Farenheit have begun to accumulate degree days.

This announcement is pretty much what is needed to know for now. You can stop reading now if you like, or if you want more discussion, read on:


The fact that insects have started accumulating degree days in the middle of March is not unexpected.  The unusual warmth of this early spring is truly something to marvel at. For plant enthusiasts, it is much easier to detect signs of life in the form of a swollen buds, leaves emerging from buds, birch tree catkins, or green shoots poking up from the ground. In contrast, the development of overwintering insects in the ground or under log or stone is more or less hidden from view.

During an atypically warm – or cold – spring, the simple degree day formula offers a more precise way for predicting insect emergence. To calculate insect degree days using the simple method is to take the MAX Temp plus the MIN Temp, divide them by 2 and subtract the insect-of-concern’s base temperature, or the minimum temperature it needs to function or move. The simplified version goes something like this: (TMAX + TMIN) / 2 – 50.

If, after you subtract 50, you have any positive number above zero, that’s the number of “degree day heat units” that your insect has accumulated.

Again, why is insect degree day information useful? It’s useful because there’s oodles of life cycle information out there about your target plant-feeding insects, but much of the life cycle information is “coarse”, or put politely, less than fully useful. What’s really critical to know – for growers especially – is the timing. For example, for a potato grower, it’s very useful to know when Colorado potato beetle adults (CPB) begin to emerge in your field (early May). It is equally useful, if not even slightly more useful, to know when mated adult females start laying eggs – which could be anywhere from mid- to late May, depending on the weather. With degree days, you can pinpoint a specific timeframe (say, a week or so) when mated CPB adult females lay eggs (which happens to be on or around 120 degree days after adults are first seen in the field). Knowing that adult female CPBs lay eggs on or around 120 degree days gives growers – both large and small scale – a more specific time window to start a search-and-destroy mission for egg clutches.

Taking this idea further: once you determine when to look for yellow egg clutches, you can estimate how many degree days their eggs take to hatch (about 65 degree days after the eggs are laid). Knowing when their eggs hatch, you can more confidently target the insects’s most vulnerable life stage (i.e., young larvae) with a low-risk, short residual botanical pesticide such as neem oil or a low-risk, medium-length residual microbial insecticide like spinosad.

Or, if you have a small plot, the degree days should tell you when the time is right for hand crushing young larvae before they become more destructive mature larvae.

So, while daily temperature continues to be variable, the number of degree day heat units an insect needs to complete its development remains fairly constant. As a tool of pest management, degree day heat units and a simple degree day model can be used to add some level of precision to the actions one takes to protect plants from plant-feeding insects.


Fyi, I will continue to provide updates to insect degree day information as the speed of the 2012 spring growing season gains momentum.



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