An article on degree days.....

I received the following article a couple of days ago. I have read it a couple of time and thought about it for a bit, and i think it's an interesting take on the life cycle of our bugs. Over the years i have read many articles and books on midges, mayflies, & caddisflies, but i have never came across this before. Maybe i just missed it, who knows? So here's the skinny on it, this was published somewhere at some point in time. I do not know where and when Just that it was written by  C.E. Cushing. PH.D.  of  Estes Park, Colorado.

"Temperature Triggers"
Water temperature plays a huge and complex role in development and distribution of aquatic insects in streams. However, I've noticed in Fly Fisherman phrases such as "triggers the hatch" and "when the water temperature reaches," all referring to specific temperature ranges that cause insects to hatch. These statements give the impression that specific water temperatures or ranges are the causative factor in emergence. Though not unrelated, these is only part of the story. I thought anglers might be interested in knowing the rest of it.

All aquatic insects require a certain thermal history to mature from egg to adult. This thermal history is measured in degree-days, where a degree-day is defined as the average water temperature during a 24-hour period. Here's an example of how it work: Let's say a mayfly deposits eggs when the water temperature is 15 degrees Celsius. At the end of 24 hours, the eggs have accumulated 15 degree-days. For simplicity, let's say the water temperature averaged 14 degrees for the next 20 days; that would be an additional 280 degree-days for a total of 295 degree-days. As the water cools down as winter approaches, we'll say the water averages 10 degrees for the next 60 days, providing an additional 600 degree-days for a total of 895 degree-days thus far into it's development. This continues until the immature insect has accumulated the required number of degree-days to mature into an adult, then it emerges. This number of degree-days required by each insect varies both with species and with geographic location. Actual values are known for only a few species, and determining these values is tedious work.

The point here is that insects do not necessarily emerge when the water reaches a specific temperature. It only means that they have acquired their necessary number of degree-days to complete their development.

This doesn't invalidate angler observations that a certain hatch often comes off at certain temperature. What it means is that in a normal, average year when the water reaches that magic temperature, the necessary thermal history has also been attained. If water temperatures for a particular year have also been below average, hatching time will be delayed.

Thermal history also explains why small insects such as midges can produce several generations per season--they don't need as many degree-days to mature. Conversely, large nymphs such as salmonflies may require more than a single year to acquire their necessary thermal history, which explains why you find more than one size class of these and other nymphs present at the same time.

As a retired stream ecologist, I've been "jousting at this windmill" with anglers for some time, and find that deep-seated beliefs are hard to change. I hope this explanation convinces a wider audience of anglers that absolute water temperatures in relation to insect emergence are correlative and not causative.

C.E. Cushing. PH.D.
Estes Park, Colorado