I believe that the willow gall pictured in my last post (Willow Galls) was induced by the willow redgall sawfly (Pontania viminalis). The identification of sawflies and their willow galls is confusing so I may be wrong about the species, however, I feel Pontania is the genus.
Sawflies are not flies but rather, with two sets of wings, are small, primitive wasps.
Also commonly called a common sawfly, the willow redgall sawfly female adult initiates gall formation when she ovipisits an egg into the rapidly growing willow leaf in the late spring. Along with the egg an unknown secretion is also injected into the wound. The egg and galling material must be laid along a leaf midrib of the host specific plant. The correct amount of cecidogenous (gall forming) fluid must be deposited with the egg to initiate gall formation by the plant. Some additional stimulation to complete the gall may be provided by the larva once it hatches and begins to feed on the soft leaf tissue. Much research remains necessary to elucidate details of the ceidogenous material and mechanism of gall formation.
Usually a single larva feeds within the gall. The pea-shaped gall hangs from the midrib on the underside of the leaf and can vary in color from yellow to pink to red. Midsummer the larva leaves the gall and falls to the ground where it pupates and overwinters. In late spring the adult willow redgall sawflies emerge and begin the cycle once again. An adult sawfly lives for 35 to 45 days.
In the pictures of willow galls collected along Ash Creek (Lassen County CA), frass (feces) can be seen in the gall chamber. The larva presses frass into the wall of the chamber and ultimately the frass is incorporated into the gall chamber structure and helps keep the chamber clean.
Usually sawflies only have one brood per season. However, under optimal environmental conditions more than one generation can occur in a year.