Scientific Research on Spotted Knapweed

The story of how and why spotted knapweed is able to use a toxin so effectively that it has become the dominant plant in many areas in Montana is documented in a series of scientific articles.  The following discussion comments on important research results in these papers and also has links to original articles.

In 1997 Prof. Ragan Callaway and co-workers at the University of Montana studied the effect of insect predators on spotted knapweed and found that insect-damaged knapweed still thrived and was even more effective in killing neighboring grass.  Marler, Zabinski, and Callaway used a fungicide and found that fungi growing on the roots of spotted knapweed had a large effect on the ability of knapweed to kill neighboring Idaho fescue grass.

In 1999 Ridenour and Callaway used activated carbon to demonstrate that the roots of spotted knapweed exuded a toxin that got into the roots of neighboring Idaho fescue grass and killed it.  Activated carbon, when mixed with the soil, trapped the toxin and the grass thrived.

In 2000 Callaway and Aschehoug studied white knapweed using activated carbon and a radioactive isotope of phosphorus, ³²P, with a 14 day half-life to compare the effect of toxin exuded by this weed on grasses native to Montana compared to grasses native to the Caucasus Mountains in the Republic of Georgia.  They found that the knapweed killed Montana grass much more effectively than grass from Eurasia, where the weed came from.

In 2001 Ridenour and Callaway demonstrated that larvae of the knapweed root moth, which feed on the roots of spotted knapweed, and are native to Eurasia, are ineffective in controlling the growth of the weed. They also found that all of their spotted knapweed plants were killed by a stem rot fungus.  Unfortunately, this fungus damages many different crops and is a major pest.

In 2001 a group of researchers at Colorado State University identified the powerful toxin which is exuded by the roots of spotted knapweed. The toxin is (-)catechin, a bio-flavinoid.  This toxin is so powerful that it killed all of the 6 different plants on which it was tested.  A patent was taken out on its use as a herbacide.  A root fungus must be present in order for spotted knapweed to produce significant amounts of the toxin.

In 2002 the Colorado State research group investigated the toxicity of a number of flavinoids and found that the (-)catechin exuded by spotted knapweed was the most powerful poison.  They concluded that kaempferol and dihydroquercetin were precursors in the biosynthesis of (-)catechin.  They also discussed the need for more research into the processes by which the roots of one plant use chemicals to communicate with the roots of other plants and with microbes and insects in the soil.

Also in 2002 the University of Montana group at Missoula studied additional ways by which spotted knapweed enhances its competitiveness with native grasses and forbs.  Fungi growing on the roots of spotted knapweed, in particular arbuscular mycorrhizas, allow the knapweed to consume more phosphorus from the soil and for the knapweed to act as a parasite and rob carbon from neighboring grass.

Callaway and co-workers studied the difference in biota in soil adjacent to spotted knapweed compared to native grasses. By measuring phospholipid fatty acids using gas chromatography/mass spectrometry they could distinguish fungi from bacteria quantitatively and show systematic differences in the abundance patterns between what was living in the soil next to spotted knapweed compared to native grasses.

A Colorado State study in 2003 showed that seedlings of spotted knapweed begin secreting growth-inhibiting levels of the (-)catechin toxin within 2-3 weeks of emergence.  This effectively produces a "ring of death" around the spotted knapweed seedlings, allowing them uninhibited access to available water and nutrients in the soil.

A collaborative study by the Colorado State and University of Montana groups, published in 2003, followed the effect of the (-)catechin toxin exuded by spotted knapweed when it reached roots of other plants.  When the toxin reached roots of common wall cress, root cells released reactive species of oxygen within 10 seconds, followed by a burst of calcium ions.  Cell death occurred within minutes with one cause of death being the loss of regulation of pH.  Changes in gene expression were also documented in this study.  Video clips showing the effects of the toxin can be accessed as "supplementary materials" through the website listed in the original article.

A 2003 paper by the Montana research group discussed how two species of gall fly were introduced to control spotted knapweed, but resulted in unexpected consequences.  The gall fly was successfully established and only attacked knapweed. However, the gall fly failed to control the spread of spotted knapweed and now has become very abundant in knapweed areas.  Gall fly larvae have become a preferred food of deer mice and constitute 85% of the deer mouse winter diet in grasslands invaded by knapweed.  This has resulted in an estimated two- to threefold increase in deer mouse populations.  Deer mice are a prime carrier of Hanta virus.  [Other studies found that approximately 25%-30% of deer mice tested in Montana are positive for Hanta virus.]

A University of Montana study published in 2004 demonstrated that soil microbes, i.e. bacteria and fungi, living in locations in Europe where spotted knapweed is native are different from the microbes in Montana soil where spotted knapweed is an invader.  Spotted knapweed flourishes in Montana soil, but has restrained growth in soil from France and Italy.

Callaway and Ridenour (University of Montana) provide evidence for the "novel weapons" hypothesis for success of an invading plant species.  More evidence for this hypothesis is presented by Hierro, Maron, and Callaway.

A 2005 study documents how spotted knapweed exudes more of the (-)catechin toxin when it is attacked by larvae of two different root boring biocontrol insects and a parasitic fungus.