Diastrophus kincaidii
Thimbleberry gall wasp

Host: Thimbleberry (Rubus parviflorus).

Gall location: On the stem.

Description: Large, irregular, multiple-chambered swellings, up to 6 centimeters in length and 2 centimeters in diameter. Sometimes rounded but often club-shaped, spindle-shaped, vaguely gnarled and twisted, or otherwise deformed. Generally smooth and covered with spots of rough, bark-like tissue. Greenish to reddish and fleshy when fresh to brown when old and dry.

Range: This gall occurs in two distinct, isolated populations: one in Western North America from northern British Columbia south to most of California and east to Montana, and another in the Great Lakes region in Ontario, Minnesota, Wisconsin, and Michigan. It has been theorized that the two distinct populations are the result of glaciation.

A second species, currently known as Diastrophus austrior, has often been split off from this species based on specimens collected in California. The status of this second species is uncertain but will hopefully be resolved in the near future. Galls on this host plant are especially easy to distinguish, as thimbleberry has no thorns.

The Gall

Mature galls on thimbleberry are abrupt swellings on the stem or petiole (Fig. 1). They are irregularly swollen or lumpy and glabrous. Their size and shape vary greatly depending on the number and precise location of cynipid larvae present in the gall. Larval cavities may be numerous and usually are situated closely together. The majority are located in the vascular zone but many extend into the pith and cortex as well. The tissue immediately surrounding each larval cavity is hard and woody as are the outer tissues of the gall but the tissue between cavities is loose and pithy. Each cavity is irregularly ovoid and about 2X3 mm in size. Gall development proceeds rapidly following oviposition by the adult female in the spring. First evidence is a slight swelling on a stem seven to ten days following egg deposition. The swelling is due to enlarge- ment of cortical, xylem and phloem cells. At this stage of development larval cavities are not yet formed and the eggs and first instar larvae are wedged between plant cells. In three to four weeks after oviposition larval cavities have developed and a gall is present as an abrupt and noticeable enlargement of the stem. The cavities are formed by a proliferation of parenchyma tissue that surrounds individual larvae. Xylem and phloem tissues of the gall are disrupted and only the uninfected portions of the stem have normal vascular tissue. The tissues of most galls show signs of woodiness by the end of summer and during the fall all become woody.

Life History

Diastrophus kincaidii is a univoltine species. The adults are relatively short lived following emergence in the spring. During the summer D. kincaidii passes through five larval instars and its galls become noticeable on thimbleberry. A long diapause begins as a prepupa in the fall and is broken the following spring when pupation occurs within the gall. The seasonsal occurrence of the life stages of D. kincaidii is summarized in Figure 2.

Diastrophus kincaidii Gillette, 1893

Nomenclature: Diastrophus minimus Bassett, 1900

Feeds on: Induces galls on Rubus nutkanus Moc. ex Ser. and R. parviflorus Nutt.

Distribution: United States: California, Idaho, Michigan, Minnesota, Montana, Oregon,Washington, Wisconsin; Canada: British Columbia, Ontario

Diastrophus kincaidii Gill.

Rubus nutkanus
Stem gall
Fig 200. Smooth, lumpy, pithy. Poly.

Fig 200. On Rubus nutkanus. Galls half-grown May 18 (Los Gatos), full-grown but fleshy and larvae very small on June 29(Friday Harbor, Wash.), contain pupae Feb. 19 (Seattle, Wash.), adults out Mar. 50 (Placerville), June 5-15 (Siskiyou, Oreg.).

Diastrophus kincaidii
Stem-gall wasp

Hosts: Rubus parviflorus

They often have short bristles or spiny hairs that are pliable when fresh but brittle by fall.

Range: CA, OR, WA, AK

Diastrophus kincaidii variety kincaidii (Gillette)

[Kinsey distinguishes this species from "Diastrophus kincaidii variety austrior," which is now Diastrophus austrior, as follows]

These two varieties are about as similar as any two forms which I should call distinct. Both insects and galls share the similarity. The best single distinction between the insects is the nature of the foveae of the scutellum ; other characters are distinct mostly in their averages. The two have distinct ranges. This species is not so very different from Diastrophus nebulosus and D. turgidus of the eastern parts of the United States, and I am not entirely certain that all of these should not be considered varieties of one species.

GALL. [same description for both varieties] — Large, irregular stem swelling, Polythalamous, with a great many cells. Swelling rather abrupt, elongate, up to 60. mm. in length by 20. mm. in diameter; somewhat irregular, smoothed, covered with bark of normal color. Internally filled with mostly loose tissue in which the larval cells are densely crowded, the cells not separate, but almost separable, broadly oval, averaging 2. x 2.5 mm. On stems of Rubus parviflorus (acc. B. G. Thompson in Mus. Comp, Zool.), and R. nutkanus.

RANGE.— Washington: Seattle (in Amer. Mus. Nat. Hist.) ; Olym- pia (Gillette); White Salmon. Oregon: Corvallis (B. G. Thompson in Mus. Comp. Zool.). Idaho: Cedar Mts. (Beutenmuller det.).

Diastrophus kincaidi Gillette

Gall. (Plate XXVII, Figs. 2, 3). On the stalk of blackberry (Rubus nutkanus). Polythalamous. Abrupt enlargements surrounding the stalk, very much resembling the galls of Diastrophus turgidus, externally. Inside they are literally filled with hard larval cells, which are divided by thin hard partitions and a thin layer of soft pithy substance forms the external portion of the gall. Length 25 to 60 mm.; diameter 12 to 25 mm.

Habitat: California (Alameda Co.); Washington (Olympia; Seattle); Idaho (Cedar Mts.).

This species is allied to D. turgidus but is abundantly distinct. It may be easily known by the rugose scutellum and the median groove on the mesothorax. The gall differs from that of D. turgidus by being filled with hard, oval, larval chambers. The types are with Prof. C. P. Gillette. The galls on Plate XXVII, Figs. 2 and 3, I received from Mr. A. L. Melander.

Diastrophus kincaidii n. sp.

Gall.-Either of the two galls in my possession measure almost exactly 1.25 inches in length by three-fifths of an inch in greatest diameter, and are upon twigs that do not exceed 2.5 mm. in diameter. The galls are abrupt enlargements of the twigs, and they are literally filled with larval cells. The portion of the gall-substance lying outside the cells is quite pithy, but the portion forming the septa between the cells is hard and woody. The surface of the gall is smooth, but is irregularly swollen or lumpy, much as in the gall of D. nebulosus.