pdf, 57.71Mb - Entomological Society of Canada

Evaluation of Control Attempts
Recommendations
Literature Cited
Acropli/on repms (L.) DC.. 109
Fears were expressed following submission of the laboratory screening tests that if P.
picridis was able to adapt to attack desirable knapweeds or even globe artichoke. it
might be extremely difficult to eradicate. The results from the persistence trials and the
lack of movement of the nematode from the treated to control plots indicate that these
fears are groundless. Removal of the host plant for two growng seasons will result in
eradication of the nematode with little likelihood of reinfestation from surrounding
populations without human assistance. Furthermore, the host range under field conditions
was narrower than in the laboratory. Thus, there were no galls found on Cynara
scolymus in the plots in either Saskatchewan or Quebec, and at Regina stray plants of
Carduus nulans and Cirsium arvense that appeared in the treated plots were also not
infected. Lettuce was not a host under laboratory conditions (Watson 1975) and the two
small swellings were almost certainly from some other cause, as no nematodes were
found in them. From a safety point of view. the nematode should be no problem to any
plant except Russian knapweed and it can be easily eradicated if desired.
The effectiveness of P. picridis is another matter. The lack of movement in or on the
soil suggests that it would have to be spread thoroughly over a knapweed stand in much
the same manner as a granular herbicide. The nematode also requires moist spring
conditions and there is no purpose in using it on the Canadian prairies unless the area to
be treated normally receives a good winter snow cover. The nematode requires a winter
before it will infect Russian knapweed, so the fall seems to be the best time for
application.
The lack of knapweed suppression in the plots treated in Alberta was disappointing.
Almost certainly the effect would have been larger if the knapweed had some grass
competition, as this would have helped suppress the later growing shoots. However, the
main reason for lack of an effect is that the impact may be delayed for a year. The stem
growth in the current year depends on the root reserves of the previous year. The
nematode creates a metabolic sink in the stem that will tend to divert reserves from the
roots so the plant should be less robust for the following year.
1 The nematode, P. picridis, should be released on selected major natural infestations
of Russian knapweed in British Columbia, Saskatchewan, and Manitoba.
2 Since Russian knapweed populations in Canada do not reproduce extensively
from seed, the importation of biological control agents that attack the seed head is not
justified.
3 Since biological weed control programmes usually require more than one biological
control agent to be successful (Harris 1979) and since P. picridis may not provide
sufficient stress to control Russian knapweed in all habitats, studies could be initiated to
evaluate the potential of the stem gall former Aulacida acroplilonica and the potential of
the rust Puccinia acroplili as possible additional biological control agents of Russian
knapweed.
Agriculture Canada (1967) Seeds Act and Regulations. Ottawa: Quecn's Printer. 50 pp.
Alley, H.P.; Humburg. N.E. (1979) Research in weed science. 1978. AgricullUral Experiment Station of the University of Wyoming
Research Journal 137.98 pp.
Frazier. J.C. (1944) Nature and rate of development of the root system of Centaurea picris. Botanical Gazelle (Chicago) 105.345-351.
Groh. H. (1940) Turkestan alfalfa as a medium of weed introduction. Scienct' in Agriculture 21. 36-43.