Stimulators are definitely one the most popular "big" flies on trout streams. They are versatile, float well, visible, easy to tie, and can imitate a lot of different things. As soon as Stone Flies or Salmon Flies hatch, Stimulators will be effective and will continue to be right up through the fall. As soon as the Stone and Salmon Flies disappear, the hoppers will be hitting the water. Hopper imitations will work well into the Fall Caddis hatch. With the exception of color variations from river to river and from hatch to hatch, the stimulator will imitate any of these bugs, as well as just looking like a tasty treat floating by.
Each river has its own variations of color and size of certain bugs that the Stimulator will imitate. As a general rule, below is a list of the colors that are typical for certain bugs and the times of year they are generally present throughout North America.
May-July: Stone / Salmon Flies These big bugs hatch as soon as the river starts to clear and warm up. They vary in size greatly with good imitations tied on hooks from size 2 (huge) to size 18 (small) for Little Sallies. Stone Flies also vary in color widely depending on the river, so it is wise to check the colors you will need before you go. Most are some variation or combination of gray, brown, orange, tan, and or yellow.
July-September: Hoppers The hot summer months typically produce smaller aquatic insects than earlier in the year. This however is when land insects attain their full size. Grass Hoppers are something that hungry fish are always looking for as they uncontrollably fly and accidentally land on the water where they are helpless. Hoppers vary in size and color, but any good size stimulator pattern in yellow, tan, brown, green, orange, and maybe some red combination will usually bring trout to the surface. Many times though, they are wary, so the fly needs to be fished naturally and with light tippet, as the water is usually low and clear this time of year.
September-November: Fall Caddis Known by many names such as October, Fall, and Autumn Caddis, it is not hard to figure out when these super-sized caddis are in the rivers. As far as shape goes, Fall Caddis look exactly like any other caddis. But for size, they are much bigger with patterns tied on hook sizes 6-12 being the norm. Regular Caddis can be any color, but the only color you will need for October Caddis is a creamy orange color. A stimulator with an orange body is about as good as it gets when chasing trout in the fall, almost no matter which North American river you are trout fishing. The odd thing about this hatch is that you will not see them emerging out of the water and flying all around like other caddis hatches. Turn over some rocks though or look in the bushes along the bank and you will find them. Just when you think there are none around is when one will scare you by crawling up the back of your neck. When they do fly you know what it is as it will struggle to stay in the air, and then hit the water and release its eggs, hopefully before getting eaten. Anyway, if you plan to be on a trout stream in the fall, be sure to have some orange stimulators in your fly box (yellow will probably work too).
Fishing a Stimulator Stimulators are one of my favorite patterns to fish. They are easy to cast, highly visible, and the fish love them. Just squirt a little fly floatant on it and you are ready to fish. I like to fish them in fast water where fish will see them and not have much time to make up their mind as to whether or not it is real. Usually you can get by with heavier tippet with these than with small imitations. I typically use 3X or 4X, unless they are being finicky, then I will go a little lighter to 5X tippet. If I see even one stone fly in the sky throughout the day, I will immediately tie on a Stimulator of some color variation depending on the river. If there are stone flies on the water, those fish will be looking up, and a big juicy insect is too much to pass up.