The Famous Willow Worm Hatch

In the middle of the summer, when the temperature is warmest and the hatches are starting to slow down, a very special event takes place: the annual willow worm hatch. During this hatch, trout gorge themselves on the protein-rich worms, presenting anglers with the exciting opportunity to sight-fish to trout feeding very actively on the surface. The fishing can be so unbelievably good that many of our guests schedule their trips around the hatch.

The willow worm is actually the larva of a very small wasp called a sawfly (Nematus desantisi). The sawfly inserts its fertilized eggs inside the leaves of the willow trees lining the rivers. When the eggs hatch, the larvae begin to feed on the leaves and grow in size. Eventually, the larvae become full-grown willow worms. The worms then build cocoons, from which they emerge as sawflies. In perfect conditions, this bug can go through several life cycles over the course of a summer. Thousands of worms will cover a single tree and consume all of its leaves.

Inevitably, the wind will blow the worms out of the willows overhanging the river. On trees that are overpopulated with larvae, worms will fall out of the tree simply because there is not enough room. This happens during the peak of the worm cycles.

Of course trout notice the green worms in the water and take advantage of the easy meals. The fish will position themselves under trees in places that normally don’t hold any trout. The fish will also stack up in pools that before the hatch occurs may only hold one or two fish. During the hatch, there are considerably more trout and they are all actively competing with each other to feed on the precious worms before they sink. Apparently this larva is a great source of nutrients, and trout don’t have to expend much energy to catch them.

The fly patterns to imitate Gusanito (a little worm, which is under three quarters of an inch in length) are pretty basic, mainly using light green (apple green) foam or chenille, with slight variations in the green tones. We also match the size of the fly to the most predominant size of worm we see while fishing.

This special phenomenon occurs around late December through January and on occasions through the first week of February. In certain rivers, during the worm hatch, trout focus their feeding almost exclusively on them. This provides many exciting sight-fishing situations, and anglers can fish all day long with worm patterns.