International Survey of Herbicide-Resistant Weeds

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(Carduus pycnocephalus)

Auxin Mimics HRAC Group 4 (Legacy O)

New Zealand
Italian Thistle (Carduus pycnocephalus) is a dicot weed in the Asteraceae family.  In New Zealand this weed first evolved resistance to Group 4 (Legacy O) herbicides in 1997 and infests Pastures.   Group 4 (Legacy O) herbicides are known as Auxin Mimics (Auxin Mimics ).  Research has shown that these particular biotypes are resistant to 2,4-D, MCPA, and MCPB and they may be cross-resistant to other Group 4 (Legacy O) herbicides.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

If we do not have a picture for this weed, or you can provide a better picture (scan, digital camera etc) please please e-mail it to us.

QUIK STATS (last updated Dec 13, 2013 )

Common NameItalian Thistle
SpeciesCarduus pycnocephalus
GroupAuxin Mimics HRAC Group 4 (Legacy O)
Herbicides2,4-D, MCPA, and MCPB
LocationNew Zealand
Contributors - (Alphabetically)Kerry Harrington 
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Confirmation Tests

Field, and Greenhouse trials comparing a known susceptible Italian Thistle biotype with this Italian Thistle biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetic studies on HRAC Group 4 resistant Italian Thistle have not been reported to the site.  There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature
Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database.  If you know anything about the mechanism of resistance for this biotype then please update the database.
Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes.  If you have any information pertaining to the fitness of Group 4 (Legacy O) resistant Italian Thistle from New Zealand please update the database.
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Associate Professor in Weed Science
Massey University
School of Agriculture
Private Bag 11-222
Palmerston North, 4442, Manawatu
New Zealand
Email Kerry Harrington
Web   : Web Site Link


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in New Zealand have been instrumental in providing you this information. Particular thanks is given to Kerry Harrington for providing detailed information.
Herbicide Resistant Italian Thistle Globally
(Carduus pycnocephalus)
Herbicide Resistant Italian Thistle Globally
(Carduus pycnocephalus)
Drag a column header and drop it here to group by that column
Herbicide Resistant Italian Thistle Globally
(Carduus pycnocephalus)
#CountryFirstYearSituationActive IngredientsSite of Action
1 New ZealandNew Zealand 1997 Pastures 2,4-D, MCPA, and MCPB 30 Auxin Mimics ( HRAC Group 4 (Legacy O)
43Carduus pycnocephalusItalian Thistle513
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