Unlike aquatic born insects, terrestrials are born and live on land. Grasshoppers, bees and wasps, crickets, cicadas, spiders, houseflies, butterflies, caterpillars, beetles and ants are all considered terrestrials. A trout’s diet mostly consists of aquatic insects, but terrestrials are a favorite food of trout during the summer months. Terrestrials like the summer weather, and here in Montana they are most active from July until sometime in September when the nights start getting cold.
Most terrestrials become active as the day heats up, and usually around late morning is when trout will start looking for them. Just because you don’t see fish rising doesn’t mean that a terrestrial isn’t working. The takes can often be subtle and few and far between. Windy days can be good when bugs are being blown into the river, and fish will often be looking for them along the deeper banks. High water keeps the fish pushed up close to the banks, but trout will move further out into the river as the water level drops later in summer. A common mistake is to overlook the middle of the river. The fish will still eat terrestrials out in the middle because the bugs can be blown or taken there by currents.
Grass hoppers work great for a while after the rivers clear out from spring run-off, but later in summer the trout often become weary after seeing so many hopper patterns floating by. When this happens you will find ants and beetles can become more effective then hoppers, and a good technique is to fish a smaller beetle or ant pattern trailed behind the hopper. This allows the hopper to act as an indicator so that you can see the takes on the smaller fly trailing behind. Trout can really get crazy for ants and beetles, especially on smaller meadow streams. Long dead-drifts can also be important, as a trout may follow your fly downstream for a while before eating it. Casting too often can result in missed opportunities, and you will get many more strikes with a really long drift. When the surface action slows down, a hopper-dropper rig can be a great way to catch them. You just trail a small nymph behind the hopper, and the hopper then acts as an indicator for the nymph.
Fishing with terrestrials is an exciting way to catch trout on a fly rod. The size of the fly can make a difference, so bring along an assortment of sizes. Try larger patterns when the water is high or a little off color, and smaller sizes for clear water situations where the fish may be picky.