Timing is everything. Like most pursuits, the true master is one that knows the exact right time for something. I’m still getting my watch fixed at the moment but I think I’ve at least narrowed a few different scenarios for fishing terrestrials that are quite distinct from one another.
Abundant food source
Depending on what you read and where you read it you’ll find all kinds of different analysis of what percentage of a trout’s diet comes from terrestrial insects but there can never be a blanket answer. You need to know your water and know what exists there. Heatherfly in Scotland for example are a key food source on some lochs but on others you’ll be lucky if one or two fall in all season. I should have said this earlier but by terrestrial, we mean land dwelling insects (or other interesting things to be discussed later) that end up in the water.
If you have a water where there is abundant terrestrials that are regularly blown onto the water then you stand a very high chance of doing well with imitations during these times. If the water is covered with beetles and the trout are gorging on them then don’t chuck a large dark olive in there… match the hatch! The converse is also true, where the trout are keyed in on a particular abundant hatch, lets use our olives example, don’t expect a big terrestrial to start picking them up left, right and centre. You might take some but it’s probably not the best approach.
Where there’s not much food
Now lets move our minds to think of those waterways that perhaps don’t contain the highest varieties or amounts of any types of food. I’m thinking highland lochs in Scotland, perhaps some waterways in NZ (although with a specific caveat that a lot of these have huge subsurface food sources). Imagine those hungry trout all of a sudden coming across a hearty meal, it’s game on time. Terrestrials simply work too well on these lochs and whilst the analysis might not be the trout are really hungry, there is something at play. Just a surprise perhaps!
Where terrestrials are known
I’m specifically thinking NZ and Tasmania here – cicadas, beetles and mice. Yep mice aren’t insect but you better believe they end up in the water. In these two places very large (and small too – gum beetles for example) terrestrials are reasonably common in the summer but to be honest in my experience you’ll either see loads of them or none at all. In the case of non at all, if you’re looking for fun chuck on a big fly and get it out there. It’s going to work. Those trout are looking for the big wins, a giant cicada is a lot more useful than a size 18 nymph. This isn’t a rule for taking all fish and especially not taking all big fish but it is a pretty great rule for having a lot of fun!
I’m no expert where it comes to fishing mice patterns but I can say across the heads and tails of pools in the dark has been exceptional fun for me. I’ve yet to catch anything massive but the intensity of fishing in pitch darkness going by very limited sight and essentially all hearing and feel is really, really enjoyable. Just wait on the explosion as you tweak that mousey across the water…
You’ve got to think of how these insect (or mice) get to the water in the first place. Most of the time they’re blown there which means they’re going to fall in and probably close to the shore. So get plonking those flies down beside the bank if prospecting. If targeting sighted fish then drop them down with a plop near them. With mice, do as above or speak to someone more experienced!
My last tip would be to thicken your tippet as quite often the bigger flies will spin or just have such poor aerodynamics that they won’t turn over on light tippet. Save the hassle and thicken up a size or two.