Gordioidea is an order of parasitic horsehair worms. Larvae are parasites of insects, mostly grasshoppers, crickets, and katydids. They feed on and absorb nutrients from the gut of their host. It is thought that they influence the behavior of their host, bringing them near water when the adult is ready to emerge. Adults are free-living. They are found usually in freshwater habitats, sometimes in semi-aquatic habitats, or inside terrestrial hosts usually near water. They do not feed, but may absorb nutrients through their body walls.
Adults are very long, hair-like worms. They are usually 12″ to 16″ long but some can grow up to 47″ in length. The body color is purplish-brown to black in most species, tan in some species. There is a blunt head and a swollen tail, but there are otherwise no distinguishing features that can be seen in the field without magnification.
Visitors to Minnesota’s natural places will occasionally come across a stand of Canada thistle with a few plants that are whitened at the top, appearing bleached. The discoloration is caused by the bacterium Pseudomonas syringae pv. tagetis. It has been called “White‐colour Disease of Canadian Thistle,” “apical chlorosis of Canada thistle,” and “Bacterial Speck”, but it has no widely-accepted common name. It is often referred to in scientific literature as Pst.
Outside of a laboratory, a bacterium is recognized only by the symptoms it produces in its host. Pst produces the substance tagetitoxin, which blocks the production of chloroplasts, preventing photosynthesis. This results in whitened plant growth (chlorosis) on only the upper portion of the plant, stunted growth, fewer shoots, and inhibition of flowering. Pst infects plants in the Aster family, including Canada thistle, common dandelion, common sunflower, common ragweed, giant ragweed, Jerusalem artichoke, and some other plants not found in Minnesota.
Researchers at the University of Minnesota conducted a study in 2002 to assess the viability of using Pst as a biological control agent for Canada thistle.