Etymology of the word 'roach' to mean the end of a marijuana cigarette?

The origin of roach in the marijuana sense is not clear, but among the different assumptions there is no reference to German words:

Roach is the short for cockroach on the wrong notion that it was a compond:

Roach:

• Meaning "butt of a marijuana cigarette" is first recorded 1938, perhaps from resemblance to the insect, but perhaps a different word entirely.

(Etymonline)

Roach:

• narcotics sense perhaps from earlier roach mane, a horse's mane clipped very short and tied; perhaps from the insect.

(The Dictionary of American Slang)

another possible origin is from Spanish tobacco de cucaracha:

• In Spanish, tabaco de cucaracha ("roach tobacco") refers to powdery, low-quality tobacco.

(Wikipedia)