Skip to main content

Casting the Mop Fly for Trout

Mop Up with Spring Terrestrials

Casting the Mop Fly for Trout
Caterpillars constitute an important food source for trout in forest streams around the world, yet they’re vastly under-represented in fly fishers’ boxes, which are often filled with other terrestrials like ants, beetles, and grasshoppers.

This story was originally titled “When Caterpillars Fall: Moth larvae may be the most important items in your terrestrial toolkit.” It appeared in the June-July 2017 issue of Fly Fisherman. 

Terrestrial or land-born insects play an important role in the diets of trout throughout the season. These food items include ants, beetles, grasshoppers, cicadas, crickets, and among other things, caterpillars. Wormlike, with spines and covered by hair, or sometimes hairless, caterpillars feed primarily and aggressively on leaves. Spend any time in a forest with a gypsy moth infestation, and you'll notice that you can actually hear the caterpillars feasting, and the droppings falling out of the infected trees.

The first time I realized how important caterpillars can be to fly fishers was in the early 70s. I was fishing on the upper Huntington, a small freestone trout stream in northeastern Pennsylvania. My favorite pool was an old jack dam that backed up a hip-boot-deep pool for about 50 feet and it always held good numbers of wild brown and brook trout. The right bank was deeply undercut and densely tree-lined, creating the perfect environment of shade and security for the resident trout.

It was a Wednesday afternoon, and as usual I had the afternoon off from working at my parents' tackle shop. I could hear the trout even before I approached the bottom of the dam. The quickly dissipating wave from the rise gave up a feeding trout's position. A quick look at the surface showed no clues, so I started by tying on a Parachute Adams.

A cool May wind was blowing as I made my first cast and watched what I thought would be a perfect cast disappear into the trees. A few coaxing tugs and the Adams came free from the tree limb, and then two fish immediately came to the surface and took something with a vicious swirl. I checked my fly and waited to make another cast. I looked to my left, and at eye level was a small green worm hanging from a thin silken thread. It was about an inch long, and with the next wind gust it disappeared into the water.

I watched it float for a short distance, slowly sink to the stream bottom, and then I forgot about it. An hour later and still fishless, I was starting to get a little frustrated. A number of trout continued to feed but not on a regular basis, it was here and there, and my flies went unnoticed. The only observation I could make was that there was more feeding activity after a gust of wind. Whatever these fish were feeding on had to be falling out of the trees. A window of light allowed me to see the backlit glare of another silken string with a green worm attached hanging from a tree branch directly across from me. This could be it! I opened my fly boxes, filled with all kinds of terrestrial patterns, but I had nothing even close to the green worm. There was no question the leaves held a large number of the little green worms, so I collected a few samples and headed home.

That evening at the tying bench I went through my materials trying to match the chartreuse worm, but nothing I had came close. The next day I sent off a sample of the color to E. Hille in Williamsport, Pennsylvania, then a major supplier of tying supplies. Hille responded by sending me a greenish cotton chenille that they thought might work. Thinking that it was not a close enough match, I tried Reed Tackle in New Jersey, and they quickly came back with a nylon chartreuse chenille, which looked almost perfect.

The following Saturday morning found me back on Huntington Creek with a selection of green worms. I checked the trees and the worms were still there, so I quickly geared up and attached one of my new creations. As usual there were a few feeding fish so I added fly floatant to the fly and made a cast. The fly landed and immediately started to sink, which I thought was not good until the leader straightened out and I almost forgot to react. A few minutes later I held a 12-inch wild brown trout with my green worm firmly planted in his jaw. The next two hours of fishing were almost too easy, simply pop the worm in and get ready to strike. Although I saw trout take floating inchworms, I found that the faster my inchworm sank the better it worked, so I eventually added lead wire under the chenille body with a size 12, 2X-long hook. It was the perfect match to the natural insects.

The next weekend, noted authors Vincent Marinaro and Charlie Meck showed up to fish my home stream, Fishing Creek. I was excited to show them the new worm and have them give it a try. Marinaro declined because the fly sank, and he was a committed dry-fly fisherman. Meck had no problems with a sinking fly, and he later added a tail to the pattern and called it the Green Weenie. Tail or not, the fly works!

Inchworms are actually not worms at all, but small caterpillars, and are the larval stages of a moths in this case the geometer moth. Inchworms have legs at both ends and when they move, their bodies arch. As the body straightens out again, the worm moves about one inch. Inchworms hatch in the spring of each year, and they molt several times as they eat leaves and grow. They can spin silky threads which they use to move through the tree canopy.

They are not limited to the Northeast, in fact there are 35,000 different species of geometer moths around the world. In South American rivers like the Collen Cura in Argentina, they literally defoliate the bankside willows, and often provide fly fishers with some of the best fishing of the season.

Mopping them Up

Small inchworms are hard to miss because they are so prolific, but sometimes larger caterpillars are even more important to trout, yet undetected by fly fishers. One of the most common of these is the caterpillar of the sphinx moth (family Sphingidae).


Like inchworms, these chartreuse caterpillars spend their time eating leaves and they often fall into streams and sink toward the bottom. Many of these larger species grow to be three or four inches long, calorie-rich morsels that are comparable in food value to some species of cicadas or stoneflies. In other words, they are a big deal.

Fly Fisherman magazine editor Ross Purnell and I spent one unforgettable May afternoon casting sinking size 10 chartreuse caterpillars under overhanging tree limbs and watching as trout raced out from the undercut banks to charge the flies. The biggest trout in the river were looking for them.

For years I tied oversized chartreuse caterpillars simply by going to a larger size 10, 4X-long hook, doubling the wraps of chartreuse chenille, adding a green beadhead, and creating what looked to be an inchworm on steroids. Then Sage Rod Company's Russ Miller showed up from Seattle with a chartreuse worm tied from the tentacles of a dust mop.

Smiling man looking at a beautiful and large rainbow that he's holding over the stream he's in. Evergreen trees in background.
Russ Miller showed author Barry Beck how to clean up using imitations of caterpillars from the Sphingidae family.

The color and size were perfect, and Miller spent two days showing me just how well the Mop Fly worked. In fact, it was so good that I made a mental note to never fish behind this guy because he literally mopped up every trout in his path. The chartreuse body material he used is actually from a washing mitt or mop that can be found at most any grocery, hardware store, or auto supply. You can visit YouTube and find easy-to-follow videos on how to tie the Mop Fly. You may think this is an off-the-wall pattern and even joke about it, but a chartreuse mop with a green beadhead is the perfect imitation for large chartreuse caterpillars.

There is also a dark side to caterpillar fishing, as there are a lot of caterpillars with black bodies, including the common Eastern tent caterpillar and the invasive gypsy moth.

There are six species of tent caterpillars in North America. These caterpillars build a large tent-like web in trees where they can move from one tent to another to feast on host tree leaves. Fully grown tent caterpillars are about two inches long, and have hairy mostly black bodies trimmed in brown with additional areas of blue or orange.

a hairy brown caterpillar
Eastern tent moths, woolly bears, and other heavily bristled caterpillars float for long distances, making foam-body, heavily hackled dry flies a viable option.

Appearing in about the same sizes and colors is the gypsy moth, brought to the U.S. in 1869 to start a silkworm industry that failed miserably. The gypsy moth has become a major pest in the Northeast. These caterpillars have voracious appetites and can quickly defoliate a tree. Both the tent and the gypsy moth caterpillars have long hairs that increase their surface area and make them more buoyant when they land on the water.

While the inchworms and larger Sphingidae caterpillars float only short distances before they sink, gypsy moth and tent caterpillars float on the surface for long distances.

Floating or Sinking?

Both caterpillars are easy to imitate by using a size 10, 4X-long hook, a body of peacock ice dubbing with a black foam cylinder, and a palmered ginger grizzly hackle. A floating tent caterpillar has solved some problem fish for me, and provided some explosive surface strikes, but the downside is that the naturals move and twist as they float on the surface and this movement is difficult to imitate just by stripping and twitching the line. The dry flies are stiff and lifeless compared to the movement you can get underwater from a pattern like a Mop Fly. I'm convinced that the twisting and turning of the naturals both in the water and on the surface draws the attention of the trout.

Another caterpillar that shows up in the Northeast in the fall is the banded woolly bear (tiger moth), which if you believe in legends can forecast the severity of coming winter weather. These bristled caterpillars have 13 distinct segments of either rusty brown or black hairs. According to legend, if the coming winter is mild, the caterpillars are dominated by the rusty brown sections; if a severe winter is coming, the caterpillars will be mostly black.

As you might expect, woolly bears are widespread in cold trout-bearing regions and are found as far north as the Arctic. We see a lot of them in September and October in Pennsylvania, New York, and New England, and yes they do often find their way into the water. Like other caterpillars covered in hair or bristles, they often float for long distances and I have had some surprising (yet inconsistent) success with a clipped black and brown deer hair body tied on a size 10, 2X-long hook.

a bright green inch worm in a man's open palm
Inchworms are the larvae of geometer moths, and are extremely common in the forest canopies of most Northeast trout streams. There are 1,400 known species in North America and 35,000 species worldwide.

I have always had my best days with sinking inch worms or the larger chartreuse worms. There have been some productive times with a floating caterpillar, but the sinking chartreuse patterns win hands down. Often, if I'm casting into the shadows with a sinking caterpillar, I use a small strike indicator and try to be ready for that magic moment when the fly hits the water and just starts to sink. It's in that moment that you'll often get an instant response from the trout. Honestly, in the right water conditions when I have good visibility and can sight-fish, I have watched trout move five feet to charge the sinking inchworm.

One thing for sure, if you fish in areas that have inchworm populations you definitely want to carry a selection of chartreuse imitations, and always carry a few floating patterns for the tent moths and gypsy moths in your arsenal.

Cathy and Barry Beck are on the advisory staffs of Sage, Redington, RIO, and Tibor Reels. They are also hosts for Frontiers International.

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Recommended Articles

Recent Videos

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

How to Tie the Useful Speed Blood Knot

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

A Celebration of Life for the Incomparable Dave Whitlock

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

Mending the Line Trailer

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

The Mother's Day Caddis Hatch

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

A Fly Fishing Short Film

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

FF Mag Promo June-July 2023

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

How to Tie Smitherman's Draggin' Nymph

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

Costa Films Presents “Hooked On: Bonefish”

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

Pharmaceutical Pollution Threatens Florida's Shallow-Water Fish

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

Tying Rene Harrop's Hairwing Dun Fly

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

Tying Barr's Damsel Fly

Learn to tie the Pheasant Tail Nymph with master tier Charlie Craven.

Tying the Pheasant Tail Nymph Fly

Fly Fisherman Magazine Covers Print and Tablet Versions

GET THE MAGAZINE Subscribe & Save

Digital Now Included!


Give a Gift   |   Subscriber Services


Buy Digital Single Issues

Magazine App Logo

Don't miss an issue.
Buy single digital issue for your phone or tablet.

Buy Single Digital Issue on the Fly Fisherman App

Other Magazines

See All Other Magazines

Special Interest Magazines

See All Special Interest Magazines

Phone Icon

Get Digital Access.

All Fly Fisherman subscribers now have digital access to their magazine content. This means you have the option to read your magazine on most popular phones and tablets.

To get started, click the link below to visit and learn how to access your digital magazine.

Get Digital Access

Not a Subscriber?
Subscribe Now

Enjoying What You're Reading?

Get a Full Year
of Guns & Ammo
& Digital Access.

Offer only for new subscribers.

Subscribe Now