How does water temperature specifically affect trout insect hatches?
Aquatic insect immatures require a specific cumulative thermal regimen to attain maturity. This is customarily measured in degree-days. Without getting unnecessarily technical, each day the water temperature averages above the minimum threshold for growth, an aquatic insect living in it is considered to accumulate the number of degree-days by which the average temperature exceeds that threshold. For example, if the threshold were 34 degrees, and the average water temperature for the day were 39 degrees, any insect immature in that water would have accumulated 5 degree-days. So, if a particular species were to require 500 degree-days of cumulative thermal regimen in order to attain maturity, it would require 100 days at that average temperature to do so. Of course it would be nice if there were a cumulative degree-days requirement published for every aquatic insect species of interest to flyfishers. However, it seems this information is mostly known for the terrestrial insect pest species which negatively impact agriculture, particularly fruit trees.
Do certain insects trend toward hatching at a specific water temperature?
Yes, there are numerous useful rules of thumb, particularly for some of the more popular mayfly emergences. And of course, the reason they are so useful is that, we have neither a good understanding of the degree-days required for most aquatic insect species to mature, nor a practical way of measuring cumulative degree-days for a particular piece of water.
At what water temperature range do trout generally begin to become active top feeders?
I seem to recall it being said to be in the low 50s, but I’m not sure how useful that number is, as trout will actively surface-feed on emerging BWOs when water temperatures are still in the mid 30s.
Would the time of day affect water temp?
Most certainly, particularly on those days when a significant difference exists between nighttime and daytime temperatures. Of course, this effect is largely negated on spring creeks, particularly in downstream proximity to their subterranean source(s).