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.

- Louis Nastasi, Charles Davis: (2022) Field Guide to the Herb and Bramble Gall Wasps of North America^©