Diastrophus nebulosus
Blackberry knot gall wasp

Host: Various brambles (see remarks below).

Gall location: On the main stem.

Description: Large, multiple-chambered swellings, up to 8 centimeters in length and up to 3 centimeters in diameter. Often botryoidal, appearing as a globular, amorphous swelling, but extremely variable overall. Usually with distinct longitudinal grooves running parallel to the stem, but plants growing in poorer soil are apparently smoother and lack grooves. Smaller galls with fewer chambers often appear more nodular. Also quite variable in color, greenish to reddish when fresh and reddish to brown when old.

Range: Widespread in the Eastern United States, from New England west to Minnesota and south to Georgia and Texas. Similar to the blackberry seed gall wasp, host records for this species are not well established. Thus far, verifiable host records for this gall include several species belonging to the bramble subgenus Rubus: Allegheny blackberry (Rubus allegheniensis), sand blackberry (R. cuneifolius), and the hybrid species boysenberry (R. ursinus x idaeus). As with the blackberry seed gall, host plants used by this gall can be difficult to identify, but the distinctive nature of this gall often makes it identifiable in the absence of a proper host identification.

Diastrophus nebulosus (Osten Sacken, 1861)

Nomenclature: Cynips (Diastrophus?) nebulosus Osten Sacken, 1861

Feeds on: Induces galls on Rubus corchorifolius L. f. (=villosus)

Distribution: United States: Arkansas, Connecticut, District of Columbia, Delaware,Georgia, Illinois, Indiana, Kentucky, Louisiana, Massachusetts, Maryland, Michigan,Missouri, Mississippi, North Carolina, New Jersey, New York, Ohio, Oklahoma,Pennsylvania, South Carolina, Tennessee, Texas, Virginia, Vermont, West Virginia;Canada: Ontario

Diastrophus nebulosus n. sp.

Rubus villosus (?). Blackberry. Elongated, abrupt, pithy swelling on the twigs, from an inch to three inches in length.

This deformation, chiefly due to a hypertrophy of the pith, in consequence of the sting of the insect, is very common in the environs of Washington. Its color is generally dark red or reddish brown; its shape oblong; its surface generally uneven with irregular tubercles, or with deep longitudinal furrows, dividing the whole gall in four or five parallel ridges. The full-grown specimens are usually 2 or 3 inches long, and from | to an inch in diameter. A transverse section of the gall shows a large number of oblong cells, about 0.13 long, arranged for the most part near the middle of the gall; their intervals are filled with soft pithy matter and harder woody fibres. From galls collected in the fall, the insects usually come out during the winter and in the spring.

Diastrophus nebulosus

Blackberry
Abrupt, pithy, elongated subterminal stem swelling, 1-3 inches long by an inch in dia, longitudinally grooved, green or reddish-brown, many-celled.

[Photo caption]

331. On wild blackberry. Adults out May 5-13 (DC); June 1-11 (MO); guests emerged Apr 12-May 5 (IL)

Diastrophus nebulosus (Osten Sacken)

Cynips (Diastrophus ?) nebulosus OSTEN SACKEN
Diastrophus nebulosus OSTEN SACKEN

Gall. (Plate XXVI, Figs. 1,2.) On stems of blackberry (Rubus villosus). Very variable in shape and size. Round, elongated, or irregular, sometimes with deep longitudinal furrows which divide the gall more or less completely into four or five parts. Dark green, tuirning reddish as the season advances. Hard, corky or pithy inside with many larval cells. Length 25-75 mm. and diameter about 25-40 mm.

Habitat: Canada (Ontario); New England and Middle States; D. C.; Maryland; Delaware; Virginia; North Carolina, south to Florida; Ohio; Illinois; Indiana; Missouri; Iowa; Wisconsin; Michigan; Oklahoma; Colorado. Very common and widely distributed. The galls are usually very common and are very irregular in size and shape. The types are in the Museum of Comparative Zoology, Cambridge, Mass.