It is very close to R. hirtipomum and R. calvipomum, which form the same general type of gall. The external anatomy of R. hirtipomum appears identical with R. pomum, but it's gall has a very fuzzy surface and is always monothalamous.

The monothalamous galls were the first to show midge emergence. Under field conditions emergence occurred as early as May 7, 1969 at Site 1 when a few fresh pupal exuviae were found. On May 28th some pupae were present at this site indicating that emergence was still in progress, but it was completed sometime prior to June 13th. Examination of galls at Site 3 at a higher elevation showed that adult emergence had commenced no more than a few days prior to May 19, 1969.

This persistent leaf series gall is either monothalamous or polythalamous. This mature form varies from globose to distinctly irregularly lobed. The surface of the monothalamous galls is uniform while that of the polythalamous galls (Fig. 3) has both deep and shallow fissures giving the gall a lobed appearance. The color ranges from reddish to a common pattern of mottled reddish and greenish to completely green. The surface is covered with short trichomes. Often monothalamous and polythalamous galls are present on the same plant (Table 5) with the latter usually predominatory. A total of 1185 galls (1166 polythalamous and 19 monothalamous) occured on a sampled plant at Site 2.

Under the surface layer the gall consists of a spongy mass of interwoven fibers. Within this fibrous mass near the base of the gall are the sclerenchymatous larval capsules. A single fiber extends from the upper portion of each capsule to a small brown spot on the gall surface.

One hundred fifty galls had a range of 13-45 mm (avg. 26) for their greatest diameter across the gall from it's base. An opposing diameter at the widest point perpendicular to this diameter ranged from 12-40 mm.

Developing gall appear as small irregular swellings on either surface of a persistent leaf (Fig. 5). They have a dense covering of trichomes which gives them a gray color except for small brown spots on the surface which are free of trichomes.

On Artemsia tridentata

Globose, variable, monothalamous or polythalamous leaf, bud or rosette galls or a bladdery gall arising from the leaf, pubescence variable, dimensions 3-15 mm. On Artemisia tridentata.

Rhopalomyia pomum
Sponge Gall Midge

This midge induces large, irregularly shaped, hairy, spongy galls on Great Basin sagebrush.

Gall on leaves...
Gall globular to lobed, soft, spongy...
Gall surface with short trichomes, red to green... Rhopalomyia pomum Gagné

Galls are irregularly lobed and polythalamous to globular and monothalamous, and are 8–45 mm in diameter. This is probably the most common gall on big sagebrush...Host: A. tridentata