This article can be found in the Summer 2011 issue of Fly Tyer Magazine. It was written by Aaron Jasper who is always experimenting with new fishing techniques. Checkout his DVD, European Nymphing: Techniques and Fly Tying.
The idea of incorporating hot spots and fluorescent colors into flies started on large lakes and reservoirs in the United Kingdom. Fly tiers added fluorescent colors to the tails of streamers, nymphs, and wet flies to attract the attention of trout from greater distances. Once the fish zeroed in on the hot spots, they came to inspect the flies out of couriosity, realized they were potential food, and struck. The same principle holds true in rivers and streams: Even though there are numerous food items floating n the water, hot spots create triggers in nymphs that elicit reactions from the trout. You will not only catch fish that are actively feeding, but also encourage idle trout to take your flies out of curiosity.
A fluorescent color reflects light of a longer wavelength than it receives. Don't confuse this with phosphorescence, which is when a material continues to emit color in the dark after being exposed to light. Fluorescent colors absorb any color of light in the spectrum and still reflect their own color. Blue, green, yellow, orange, and cyan are absorbed by the water, whereas violet and red continue to disperse through the water column. No matter what color wavelength reflects off the fluorescent oange thread or bead, it will reflect fluorescent orange; even if violet light reflects off the fluorescent orange hot spot, it will reflect fluorescent orange. This is the secret to the hot spot: The fluorescent bead or thread continues to reflect light and stay true to its color even though the rest of the fly changes color as it descends through the water column.
When trout are keying in on sulfur nymphs, a Pheasant-tail Nymph does the job because when it is four feet deep, it changes to black or gray, but the trout still key in on the size and profile of the fly. If the pattern has a hot spot, the fish will be more likely to see the fly and react.
Orange is the hot spot color on most nymphs, whether it's fire orange (a deep fluorescent orange) or standard fluorescent orange. Orange holds it color, even at depths up to 25 feet. The book What Fish See, by Dr. Colin Kageyama, includes photographs of swatches of red, pink, fluorescent fire orange, fluorescent orange, yellow, and blue taken at the surface when the color swatches are submerged to a depth of 25 feet and photographed from a distance of 25 feet, the two varieties of orange are clearly visible, whereas red and blue look black. This proves that a fly tied using fluorescent orange and fluorescent fire orange can be seen from great distances and at great depths.
Fluorescent yellow is a good choice when fishing deep water under low light conditions. Fluorescent yellow is also a great color to use in streams where therre are high populations of Grannom caddis fly larvae; the yellow thread collar behind the bead head not only attracts fish, but it also looks like a case caddis peeping out from its case.
Dr. Kageyama experimented with fluorescent colors at various water depths and under different water conditions. He found that fluorescent orange and yellow are the most visible colors in stained water up to five feet deep.
Fluorescent red is also an option for hot spots. A fish's eyes contain a chemical that make them extra sensitive to the color red; the same way words that are underlined with a highlighter seem to jump off a page, a fly tied with red seem to jump into a fish's field a vision. To a human, red turns dark under the water, but a trout still interprets it as red. A fly tied with a fluorescent red hot spot is best used in clear, shallow water because the fish see the spot in addition to the rest of the fly; at deeper depths or in dirty water, red doesn't create enough contrast with the rest of the fly and it doesn't elicit as many takes.
Placing a fluorescent thread collar behind a bead head creates sharp contrast between the nymph body and the bead. When the nymph is placed under an ultraviolet light, the only color on the entire fly that is
emitting any relection is the fluorescent band of thread. This enables the trout to see the fly coming from a far distance, which is an advantage when fishing turbulent pocket water; in faster water, fish do not have ample time to inspect the fly, so they see the bright color and strike quickly. Instead of tying a fluorescent thread collar behind the bead head, try using the bead itself as the hot spot. Fluorescent orange, pink, and chartreuse beads work well in high, stained water, but will also work well in low, clear water.
Incorporating a tiny hot spot at the tail of the fly works well in areas where the trout have been subjected to heavy angling pressure. By tying the hot spot at the end of the fly, the contrast and color absorption of the fluorescent thread are still the key, but this spot is smaller and more subdued than a thread collar behind the bead.
Using pink and orange UV dubbing in the middle of the abdomen of flies such as caddis larvae makes a huge difference in the ability of these patterns to catch fish, especially in deeper water. The fish seek out the larva shape of the real insects, and the hot spots draw the attention of the fish.
Placing a hot spot at both ends of the fly seems to confuse fish: they see a trigger at both the head and tail of the fly, are unsure which end to explore, and so reject the pattern altogether.