The fiddleneck gall nematode (Anguina amsinckia), also native to California, induces the weed to form floral galls that replace the flowers. Although the mechanism is somewhat of a mystery, feeding by Anguina in the flowers apparently stimulates the floral tissues to form a gall that encloses the nematode. Two complete life cycles take place within the flower galls, and about 40,000 nematodes occur in an average 3/8-inch-diameter gall.

(T)he nematode had been known to attack only common fiddleneck, Amsinckia intermedia (A. menziesii), but we now know it attacks two other important weeds, Amsinckia lycopsoides and A. gloriosa (now considered a variant of A. tessellata). In addition, there may be different races of the nematode that are host-specific to different fiddleneck species or populations.