Appearance and Life Cycle
Photo credit: Petr Kapitola, State Phytosanitary Administration, Bugwood.org
The willow redgall sawfly has two generations a year, with the first generation adults emerging in late May or early June. The adult sawfly measures approximately 4 mm in length and has a black body with yellow legs. Each adult can produce 2 to 35 eggs. The eggs are deposited singly or in a row on either side of the mid-rib of the developing leaves. At each location an egg is deposited a gall begins to form. By the time the larvae hatch the gall is fully developed. The larvae feed and develop within the galls for approximately five weeks. The full grown larvae are a yellowish-green color with a shiny, black head and measure approximately 7 mm in length. When full grown, the larvae drop to the ground and form cocoons in the topsoil. The larvae pupate and emerge as adults approximately two weeks later in late July or early August. By mid-September the second generation larvae are full grown and drop to the ground where they overwinter in cocoons.
Photo credit: J.K. Lindsey, commanster.eu
The adult sawfly forms the galls on the leaves by injecting a fluid into the leaf along with the egg. The galls measure approximately 8 mm long, 6 mm wide by 6 mm deep. The galls appear red on the upper surface and are yellowish-green on the underside. Although the willow redgall sawfly causes minimal permanent damage, the galls are extremely unsightly especially on ornamental plantings. During a severe outbreak the galls cause a reduction in annual growth.
Chemical control of the willow redgall sawfly is rarely required due to parasites and predators that attack the sawfly and as the insect causes minimal permanent damage to the host. If a severe outbreak is anticipated, control may be achieved by applying carbaryl during the adult flight period. The flight period of the sawfly can be monitored using yellow sticky traps as the adults are attracted to the color yellow.