Rhopalomyia baccharis

Rhopalomyia baccharis Felt, 1908

Coyote brush, Baccharis pilularis De Candolle (Asteraceae) is a common shrub of the inner coast ranges of California. Two species of gall midges (Diptera: Cecidomyiidae) have been described from this plant in California, Rhopalomyia californica Felt, 1908, and Rhopalomyia baccharis Felt, 1908. The first is a well known species responsible for a common and conspicuous terminal bud gall (Fig. 1). This gall midge has been used successfully in the biological control of coyote brush in Australia (McFadyen 1973). Several studies on various aspects of the biology of this species have been published (Tilden 1951a, 1951b; Ehler 1982, 1987, 1992; Ehler & Kinsey 1990, 1991, 1993). 

Little, however, is known of the second species, R. baccharis, described originally on the basis of a single male and female taken from an unspecified stem gall. Following the cursory initial description of this species (Felt 1908) and a subsequent longer formal description (Felt 1915), essentially nothing more on the species or its gall was ever published. Pritchard (1953) mentioned that stem galls were common on the prostrate form of the host in the sand dunes of the San Francisco peninsula, but did not describe the gall in any detail. After almost a century since the original description, the gall is finally described and illustrated here (Figs. 2, 3). 

According to Raymond J. Gagne´ (in litt.) adults of R. baccharis and R. californica are essentially similar. This comparison led him to synonymize the two species (Gagne´ & Boldt 1995) under the assumption, no other stem gall having ever been reported, that Felt’s ‘‘stem gall’’ was instead the typical bud gall of R. californica taken from the stems rather than at the host’s apices. Gagne´ (2004) resurrected R. baccharis from synonymy upon learning from the author that the distinctive stem gall on Baccharis described here had been found in California (Gagne´, in litt). Pupae and adults were eventually reared from the integral stem galls by the author and sent to Dr. Gagne´, who reported (in litt.) that the pupae of the two species, unlike the adults, are easily separated.

Dr. Gagne´ has reported (in litt.) that adults from the new galls fit the types of R. baccharis and that the pupae, previously unknown, are distinct from those of R. californica based on the respective lengths of the antennal sheath bases and cephalic sclerites of the pupae. As reflected in the two sketches kindly provided by him and incorporated in this paper (Figs. 4, 5), the antennal sheath bases and cephalic sclerites of Rhopalomyia californica are notably longer than those of R. baccharis. There is no longer any doubt of the distinctiveness of R. baccharis. Dr. Gagne´ suggests that the two species of Rhopalomyia on coyote brush are closely related because of the similarity of adult morphology and that the distinctive pupae reflect different strategies for escaping from the galls.

Rhopalomyia baccharis induces the development of an integral stem gall (Figs. 2, 3) that secondarily bends and twists the stems of both male and female shrubs. Galls are the same color as normal stems and measure 5–10 mm in diameter and 15– 90 mm in length. The galls usually show definite longitudinal ridges. Spring galls turn bark-brown and harden by late summer, with the stem distad of the gall remaining green and growing an additional 180 mm or more beyond the gall. A second round of galls develops in the fall near the tips of the spring-summer shoots. These galls remain green through the winter, but harden and turn brown soon after the emergence of adult gall midges in December and January. Bent and twisted stems are prominent signs of infestation and contrast strongly with normal straight stems. Stem bending apparently develops in response to the characteristic positioning of the larval tunnels along alternating sides of the stems (Fig. 3). Larval chambers form just under the cambium layer to one side of the stem and run longitudinally downward just inside the bark. Small galls (<10 mm long) usually contain a single larval chamber, while longer galls (>15 mm) have two or more chambers depending on length. A gall 90 mm long can have three or four larval chambers on alternate sides of the stem. Each larval chamber contains a single larva. 

Each larval chamber is capped at its apex by a thin layer of tissue. At about the time the larvae pupate, the cap splits longitudinally and curls inwardly resulting in an elliptical opening about 2 mm wide by 7 mm long through which the adult gall midge escapes. These openings often develop in the axils of small branches or leaves that protrude from the galled stem, reflecting the original site of egg deposition or entry by the larvae.

The light orange larvae may be found in various instars in early summer and again in late fall, suggesting that this species is bivoltine. In October and November, some larval chambers were nearly filled with a clear fluid of unknown origin and role, yet the larvae appeared to be active despite being immersed. Other chambers were simply hollow with single larvae within. Upon reaching full size, larvae pupate at the lower end of the chambers. Unlike the adults of many other gall midges, including R. californica, that force their way out of the galls while still encased in the pupal skins, R. baccharis adults emerge from the pupal skins inside the galls and then crawl out of the galls through the elliptical exits already provided. The blackish-brown pupal exuviae are regularly found at the lower end of the larval chambers.

Galls collected during early summer did not yield adults although pupae were present. Whether this circumstance has to do with increased larval vulnerability to parasites or predators during the spring and early summer, laboratory conditions, or other yet to be determined factors is not known. Adults emerged regularly in December and January, however, from galls collected in September through November. Although some adult gall midges were still emerging in February, most galls collected in the fall contained pupal exuviae by late winter.

The opening of the exit hole is apparently timed for adult emergence of R. baccharis because the only other times other free-living insects were found in the chambers were after adult emergence. The two instances of this noted during this study involved an unidentified moth caterpillar and the mealy bugs Pseudococcus maritimus (Ehrhorn 1900) found in the upper portions of empty larval/pupal chambers of galls collected in February.

 Many larvae have internal hymenopterous parasitoids. Infested larval ‘‘mummies’’ and barrel-shaped larval skins of R. baccharis are often found in the galls. No attempt was made to assess parasitoid survival rates or losses

First Description of The Stem Gall of Rhopalomyia baccharis Felt, 1908 (Diptera: Cecidmyiidae), on Baccharis pilularis De Candolle (Asteraceae)