Rabdophaga s-discolor-stem-gall

Rhabdophaga sp.

Host: Salix discolor

[A photo of this gall appears on page 2 of the pdf]

The insect could not be identified with certainty from its gall either. The gall, however, does resemble one made by the nodule willow gall midge, [Rhabdophaga nodula](https://www.gallformers.org/gall/3558) (Felt 1940).

Rhabdophaga sp. makes an exit hole and damages the stem in the same manner as several "shot hole" species of Rhabdophaga that attack willows. (Shot hole refers to the insect's exit hole in the stem.) But it cannot be included in the shot hole group of midges because they do not cause swellings on the stem (Barnes 1935), whereas this midge does--at least on the stems of Salix discolor. 

The gall on S. discolor develops from hypertrophy of the stem tissues during early summer. The larva enters the stem through the leaf bases during early shoot development, so that the gall formed later occurs on the proximal half of the completed shoot (Fig. 1). Some galls may be monothalamous, but most of them are polythalamous because generally more than one larva attacks the same shoot at the same time. 

The young gall is soft and green like the developing shoot; the old gall is hard, has a woody interior, and is colored externally like the bark of the mature shoot. A mature gall is prolate or spindle-shaped and may be partly covered with buds (Fig. 1). Fine elongate striations rib the sides of the gall. The size of the mature gall varies considerably (Table I) depending upon the number and distribution of the larvae within. 

Occasionally an adult midge emerges from the host in an atypical location. Two emerged from large buds that had not developed, and four adults emerged from the sides of a large gall of the willow beaked gall midge, Mayetiola rigidae (O.S.). Without the protruding pupal cases after emergence, the abnormal gall harboring these insects would never have neen noticed. 

Vacated galls remain on the tree indefinitely and increase in diameter each year as long as the shoot puts on diameter increment. Galls dissected on living shoots 1 and 2 years after being vacated showed the larval cells nearly or completely filled with callus tissues. 

Heavily populated shoots often break off at the mid-gall where the larvae are congested and the interior mined by larval cells. Early yellowing of foliage and premature leaf drop also occur on shoots that are heavily galled. Such shoots die before winter, and sometimes the gall on a dead shoot shrinks and kills the larvae within. 

Adult eclosion begins in mid-April shortly after the pupa breaks through and protrudes two-thirds of its body from its cell in the gall. About 12 minutes elapse between the time the pupal skin splits and the time the adult is dry and able to fold its wings over its abdomen. The meconium is voided after that. The adult then walks up and down the stem. 

Nearly all emergence occurs between 0700 and 1130 hours. The peak emergence day in 1964 was 23 April when 768 adults, or more than 70% of the total that year emerged from four small shrubs. Newly emerged adults began flying at 1300 hours and left the shrubs by mid-afternoon. No adults were seen on the shrubs at night. 

The gall develops rapidly during July and August, and is nearly full sized and complete, except for final color changes, by late August when the third-instar larva appears. 

Range: East Lansing, MI

Life history and habits of Rhabdophaga sp. (Diptera: Cecidomyiidae), a gall midge attacking willow in Michigan