Every fisherman who has pushed his way through willow thickets along a stream has noticed the smooth, round red-and-green swellings on the leaves of this tree. At first glance these might be mistaken for brilliantly colored fruits hanging from the branches, but closer examination reveals the fact that the swellings have been formed directly from the tissue of the leaf. An abnormal warty growth of this type is known as a gall, and may be likened to a cancerous growth in human tissue caused by local irritation. An individual willow leaf usually bears but one or two galls, although occasionally as many as eight are found, and there may be several dozen infested leaves on a single bush. The average diameter of a full-grown gall is one-half inch, rising about equally above the lower and upper surfaces of the leaf. Young galls are green in color, later turning to bright red.
As remarkable as the outer appearance of the gall is the fact that each forms a one-room chamber housing the larva of an insect. Hatching from an egg buried in the tissue of the leaf, a small, white, worm-like larva starts to feed and grow in size. The irritation due to the rasping of the larva, or perhaps to some chemical secretion causes the leaf to grow scar tissue around the spot, eventually resulting in a round, firm nodule containing a hollow in which the larva continues to feed until mature. The larva may be easily seen by slicing off a section of the gall with a knife. In addition to protection from the drying heat of the sun, the insect finds food and oxygen in its narrow cell and continues to thrive undisturbed until ready to pupate.
When mid-summer arrives, each larva tunnels out of its cell into the light of day, leaving a telltale opening on the gall and a small pile of fine, brown pellet-like castings within. After dropping to the soil underneath the willow tree it burrows just beneath the ground debris and fashions a new kind of chamber in the form of a thin, parchment-like brown capsule about the diameter of a match stick and a quarter-inch long. Here the transformation occurs from the larva to the adult. The adult sawfly resembles roughly an ordinary housefly in shape and size, having a robust, shining black body with brownish markings. Tho termed a "fly" it actually belongs to the order of bees, wasps, and ants (Hymenoptera) and is distinguished by four wings instead. of the two of typical flies, as well as by its sharp saw-toothed ovipositor useful in placing eggs in the center of a leaf. Eggs are laid in the spring when the leaves of the willow are growing actively and furnish ample food for the developing larva.
Although willow leaf galls are observed year after year they occur on a shrub of little economic value and are consequently not classed as serious pests. The sawfly responsible for the damage is probably held in check by other insect neighbors of parasitic habits, rather than by a lack of food supply. As curiosities of the amazing insect world, though, willow galls will always hold interest for those who hike along tho river trails about the lower slopes of the Mountain.
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