A. Boy, you sure know how to ask a question that is almost impossible to answer. The reason is fairly simple - there are hundreds of different mayflies and each one requires a certain thermal history to hatch. Further, the thermal history may vary for the same species in different geographical areas and other environmental factors, such as food supply, may also influence the developmental time of the nymph. So, you see, there is no definitive answer to your question because I don't know which mayfly you are talking about, where it lives, or the thermal history of the body of water.

Now that I've explained why I can't give you a good answer, I do want to tell you something about how your question could be answered. I've talked about this in previous columns, but here it is again... As I said, each species requires a certain thermal history to mature from egg to adult. These are usually measured in "degree-days," and is a summation of what temperatures the nymphs have experienced and for how long. Thus, if a nymph started growing for 10 days at an average water temperature of 15oC, this would be 150 degree days. If this were followed by a 15 day period when the water warmed to 17oC, this would be an additional 255 degree days and a total of 305 degree days for this 25 day period. Now if our hypothetical species required 4500 degree days to hatch, it would need 4195 more degree days. If the water averaged 19oC the rest of the season, it would take about 221 more days, or a total of 246 days in this particular temperature regime.