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Pest Status
Background
Chapter 27
Acroptilon repens (L.) DC., Russian
Knapweed (Compositae)
A.K. WATSON and P. HARRIS
Acrop,ilon repens (L.) DC. (Centaurea repens L.) is native to Mongolia. Western
Turkestan, Iran, Turkish Armenia and Asia Minor (Moore & Frankton 1974). and was
first introduced into Canada about 1900 as a contaminant of Turkestan alfalfa seed (Groh
1940). Russian knapweed is now widespread in the southern parts of the four western
provinces and is also found in southern Ontario. although most infestations are relatively
small (50% of infestations in Saskatchewan are less than one acre in size) (Watson 1980).
This noxious weed has a well developed, extensive root system which is the major
means of reproduction and spread (Frazier 1944). It forms dense infestations in cultivated
fields, in grain and alfalfa fields. in pastures, along roadsides and irrigation ditches and in
waste places. Russian knapweed is apparently indifferent to crop association and is able
to survive in almost any crop in tillable soil (Rogers 1928). Infested fields commonly
have knapweed densities of 100-300 shootslm 2 which suppress and essentially eliminate
other plant growth (Selleck 1964). In addition to its intense competitive ability, Russian
knapweed is poisonous to horses (Younget al. 1970). Actual crop losses and other losses
due to Russian knapweed have not been determined nor estimated in Canada.
Because of its serious threat to Canadian agriculture, Russian knapweed was designated
as a "prohibited noxious weed" in the Federal Seeds Act (Agriculture Canada
1967). The benefit of this is debatable as it is difficult to collect viable seed in Canada
(Watson 1975) and germination rarely occurs in the field (Selleck 1964). Control of this
persistent perennial weed is difficult as its extensive root system is not adversely
affected by cultivation and the weed is relatively resistant to commonly used herbicides.
Picloram (4-amino-3,5.6-trichloropicolinic acid) and glyphosate [N-(phosphonomethyl)
glycine) are two of the more effective herbicides for Russian knapweed control (Alley &
Humberg 1979). The biology of Russian knapweed has recently been reviewed (Watson
1980).
In North America, Russian knapweed is relatively free of specialized parasites and is not
extensively attacked by polyphagous feeders, but in its native range, Russian knapweed
is the host of a number of specialized organisms (Watson 1980). The potential biological
control agents of Russian knapweed include seven organisms that attack the seed head:
a seed gall mite (Aceria acropliloni V. Shev. & Kov.), three Diptera (Dasyneura sp.,
Urophora maura (Frfld.), Urophora kasochstanica V. Richter) and three Coleoptera
(Larinus bardus Gyll., Larinus jaceae Fabr., Rhynchaenus dislans Faust); a stem gall
former (Aulacida acroptilonica Beliz); a leaf and stem rust (Puccinia acroptili Syd.); and
a leaf and stem gall nematode (Paranguina picridis Kirj. & Ivan.) (Watson 1980).
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