Many non-native plant species have made their way into Devils Tower National Monument, competing with native plants, and, in many cases, out-competing them. Biologists have identified at least 56 exotic (non-native) plant species in Devils Tower. Three of these - Leafy Spurge, Houndstongue, and non-native species of Thistles - are being actively managed.
Leafy Spurge is primarily controlled by biological means. Spurge Beetles have been introduced from Eurasia. These beetles live on and eat the plant. Beetles lay their eggs on the roots, and when the beetle larvae hatch, they eat the roots. This opens the roots to fungal invasion. It is actually the fungus which kills the Leafy Spurge. Although not a native species, Spurge Beetles eat only Leafy Spurge and do not affect other plants.
Houndstongue is a biennial and requires two years to complete its growth and produce seeds. During the plants' second year of growth, the seed heads are manually removed, preventing regeneration.
Three species of Thistles (Scotch, Musk and Bull) are controlled with herbicides. Biological controls are used on Canada Thistles. The Stem-mining Weevil attacks the stem, eating a hole in the stem and killing the plant. Gall Flies create galls on the thistle, preventing the plant from producing seeds. The Seed-head Weevil eats the seeds.