Invasive thistle is a good example. Aside from a few native thistle species, the rest you see were brought to this continent to beautify gardens. They are beautiful and butterflies love them, but cattle don't. Even vegetarians are disturbed when they take over a prairie. With 10,000 seeds per flower head, it doesn't take long. Canada thistle, Scotch thistle and musk thistle are on the Missouri Noxious Plant list.
There are two weevils which attack the thistle. The thistle-head weevil is a native of Europe. After studies to "be sure" that it would not attack economically important plants, this beetle was released in the 1970s. It can be purchased and placed on flower heads. A less expensive way is to harvest them from infested plants. You simply spread a sheet under the plant and whack at the flower heads, which is not only productive but lots of fun.
rosette weevil , Trichosirocalus horridus, feeds on musk thistle during the rosette stage, killing first-year rosettes. Thistles have a two year life cycle, and killing the rosette prevents it from maturing and producing seed the next season. Weevils require time to go through their complete life cycle. Working in fields without grazing cattle they can reduce the thistle population as much as 95%. There is also a thistle defoliating beetle, Cassida rubiginosa, and a thistle-stem gall fly, Urophora cardui which damage the plants.
Gypsy moths have pathogens also and the good news is that they catch up to them quickly. New research reported in Science Daily suggests that where the moth goes, their infecting virus and fungal pathogens show up soon after.
The fungal pathogen (Entomophaga maimaiga)was first reported in 1989 and attacks the caterpillars. Land managers gather fungal spore-containing caterpillar cadavers and spread them to try and control new populations of gypsy moths. The virus (Lymantria dispar nucleopolyhedrovirus), which was accidentally introduced near Boston in 1906, also infects gypsy moth caterpillars and is used in a spray by the U.S. Forest Service to control the moths in environmentally sensitive areas.The new study suggests that land managers efforts to introduce these organisms may be unnecessary. "Once their traps caught more than 74 moths each in one year, there was a more than 50 percent chance of finding the fungus in that area in the following year. When more than 252 moths were trapped in a year, there was more than a 50 percent chance of finding the virus the next year."
If we could live for a few more centuries, we would probably find some semblance of balance from these invaders- and most likely many new crops of invasive species.