In areas with lots of grass, hoppers are abundant from mid-summer until the first frosts. If those grassy areas are near a river, then there's always a few hoppers that will fall onto the water--a medium they are ill-equipped to deal with. Once there, they have a difficult time getting airborne and will drift until they drown--or make a large and delectable meal for a trout.
Just because it's summer, though, don't assume that trout want hoppers, or even that they're around. Rivers with narrow riparian zones, such as Oregon's canyon-bound Deschutes, offer poor hopper fishing. Also, trout may be interested in food other than hoppers, even if the big bugs are present. In fact, trout eat far more beetles and ants than hoppers.
But if hoppers are available, and if trout want them, then you can have some outstanding dry fly fishing. Often it's not enough to simply let your imitation drift down the river. You'll probably need to place it within inches of the riverbank. Also, it will probably help if your fly lands with a loud "plop." Trout associate the noise with a hopper that fell into the river; they'll probably come on-the-double.
In the Rockies, the hopper/dropper combo is a popular and useful strategy from mid-July through the first frost. Tie a 18-24 inch dropper leader off the hook bend of the hopper imitation, then put a nymph pattern on the other end of the dropper. The hopper acts as an indicator if a trout takes the nymph, and sometimes the trout will take the hopper. It's an effective two-fer.
Articles About Grasshoppers
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The Perfect Hopper Looking for a hopper imitation that is easy to tie and works on nearly every type of water? How about the Perfect Hopper! Jeff Morgan
Quick Tip: Fishing the Edges--Rivers Many anglers are so eager to get in the water that they miss the best fishing. Bankwater can offer some ot the best dry fly fishing of the year.Scott Richmond
Hoppers . . . At Last! Among terrestrial insects, hoppers are consumed by trout the least, and imitated by anglers the most. But that doesn't mean you shouldn't have hoppers in your fly box. Here's some new patterns as well as some tips for hopper fishing. Jeff Morgan
Terrestrials Trout food comes from above as well as from below. In late summer, you can ask you fishing partner, "Do have an extra terrestrial?" Dave Hughes and Rick Hafele