Aciurina michaeli

Aciurina lutea

Aciurina maculata

Aciurina opaca

[The author mistakenly described this gall as being induced by Aciurina ferruginea, and his description perfectly matches what is now known as Aciurina michaeli] It is monothalamous, smooth, pear-shaped and occurs at the axillary buds on the current year's growth. A few modified leaves usually arise from the gall surface. The color of galls is green, becoming pale green to yellow with age. Some may have pinkish hues. Twenty-five mature galls collected near the Craters of the Moon National Monument, Butte County, in 1975 ranged from 8.5 to 16.0 mm (x = 12.9 mm) in length and 4.5 to 8.0 mm (x = 6.4 mm) in width. The young gall resembles the mature gall in form. The stem and most of the leaves that would normally arise from the axillary bud are incorporated as gall tissue. A few unmodified leaves may remain and grow from the surface. In some instances stem growth occurs from the gall and many leaves will develop from such a stem above the gall. Larval feeding appears to be the stimulus for A. ferruginea [A. michaeli] gall growth. Growth continues up to the pupal stage. If a larva dies early in development, the gall ceases growth. A mature gall shows evidence of desiccation. Following emergence of the gall-inhabitant the gall desiccates rapidly and will abscise from the host plant. No portions of the host plant's foliage appear suitable for gall growth as long as growing stems are present. Galls are usually concentrated on the lower half of branches rather than toward the tips.


The gall of *A. lutea* is a simple but abrupt swelling on a stem (Fig. 2). It may occur on older stems as well as the current year's growth, but it is always woody with thickened walls. Among 3 galls dissected, 2 were monothalamous and measured 9.5-14.5 mm in length, 4.5-5.5 mm in width, and 5.0-8.0 mm in depth. The polythalamous gall contained 2 larvae and measured 14.0 x 5.0 x 8.0 mm. The cavity size for each was 5.0 x 2.0 mm. The only noticeable effect *A. lutea* galls had on the host plant was the stunting or prevention of stem growth from axillary buds on the swelled portions of stems.

The *A. maculata* gall matches an earlier description by Bates (1935). The gall is monothalamous, basically ovoid in shape but usually with an irregular profile due to bracts on the surface (Fig. 3). The entire gall is covered with a thick, white, cottony tomentum. Ten galls ranged from 7.5 to 10.0 mm (Jc = 8.9 mm) at their largest diameter. In cross section there are three distinct tissue layers. The innermost layer is hard surrounded by a thicker, pithy tissue layer which is covered by a thin outer tissue layer. The internal cavity of 10 galls measured 3.0 to 5.0 mm (Jc = 4.35 mm) in length and 1.0 to 3.0 mm (Jc = 1.90 mm) in width. Galls occurring on different subspecies of *C. nauseosus* are not noticeably different in form. Galls first appear in August as shining green, sticky, globular structures at the axillary buds. At this stage they are not distinguishable from young galls of *Aciurina semilucida* (Bates) and *Aciurina trixa* Curran. Within a few weeks they increase in size and a noticeable tomentum forms; although less than half grown, the young galls resemble mature ones. *Aciurina maculata* gall development is unusual in that growth occurs at a time of year when the plant becomes dormant; most commonly, the growth of galls occurs simultaneously with the growth period of the host (Mani, 1964). In such instances it is not always clear whether the formation of the gall is a mechanical reaction associated with the normal plant growth processes or if the gall-former produces growth stimulation substances. In the case of *A. maculata* the occurrence of a growth promoting chemical is a distinct possibility. The apparent stimulus for growth is the feeding of an *A. maculata* larva. Galls grow and develop as long as the gall-former feeds. By fall, when *A. maculata* has reached the third instar and ceases feeding, the gall is fully formed. Galls with dead *A. maculata* larvae were found to be consistently smaller than those containing living larvae. A gall may remain attached up to one or two years following emergence. *Aciurina maculata* galls are found primarily on the lower portions of branches, oftentimes in the adjacent axils of a stem. Also, new galls may form on stems from previous years' growth near the base of the plant. The distribution of galls is often clumped. Galls prevent growth of the affected axillary bud since the tissue of the bud is totally modified in gall formation. 

[pg. 714 photo 3 is a monochrome photo of the gall]

*Aciurina opaca* is so rarely collected that little can be stated about its biology. Each of two specimens collected were reared from galls on *C. nauseosus*. A single female emerged from a gall collected on 18 March 1974 and a single male was reared from a gall collected on 21 April 1974. The flies issued from stem swellings that appeared identical to those described for *Valentibulla spp*. (Wangberg, 1978). It is not known how the apparent gall-forming habit of *A. opaca* differs from *Valentibulla spp*.

Gall-Forming Habits of Aciurina Species (Diptera: Tephritidae) on Rabbitbrush (Compositae: Chrysothamnus spp.) in Idaho