Rabdophaga strobiloides

Rhabdophaga strobiloides

[Photos of this gall appear on page 2 of the pdf]

The pine cone willow gall midge is univoltine in Michigan. Adults emerge from the galls on Salix eriocephala Michx. and other willows on warm days in late April or early May. Eggs are laid singly on the leaves and stems of the host. Head capsule measurements indicate three larval instars. The first-instar larva emerges in early May and penetrates the soft tissues at the base of the developing shoot tip. Shortly thereafter, the gall begins to develop and is about half grown by the time the second-instar larva appears in late May. The last-instar larva appears in July. Before overwintering, the larva constructs a cocoon which may or may not be closed at the top. Pupation occurs in early April. Numerous inquilines and parasites inhabit the developing and mature gall. 

The gall (Fig. 1) is monothalamous and forms by extensive hypertrophy of a developing shoot tip of willow. It is usually apical the first season and sub-apical the next year if it remains on the stem. 

When young, the gall resembles a miniature cabbage because it is green, nearly spherical, and enshrouded with leaves. The inner leaves become scale-like and the outer leaves abort before the gall matures. At maturity in August, the stem at the gall base dies, then the developing scales become heavily setaceous, dry, and turn gray. 

The mature gall is top-shaped, being prolate above and flattened below. It has numerous (60 or more) regularly arranged scales-hence the resemblance to an immature pine cone (Fig. 1A). These scales are broadened and thickened leaves tightly arranged on a barely elongated stem. Brodie (1909) postulated that the base of the gall represents about 1 m of normal branch reduced to 6 mm. The larva or pupa (Fig. 1B) is in a central cavity surrounded by a ring of stiff elongated scales. Galls found on S. eriocephala and containing healthy larvae averaged 22X 19 mm (Table I); parasitized galls are generally smaller and more spherical. Brodie (1909) gives mean measurements of 12 X 15 mm for galls on S. humilis. 

Life history and habits of the pine cone willow gall midge, Rhabdophaga strobiloides (Diptera: Cecidomyiidae), in Michigan