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134 P. Harris and J. H. Myers
Table 31
density of the weed, so no decline should be expected. On the other hand, Berube &
Myers (1982) suggested that the knapweed would be controlled on sites where there
was a high seedling loss either from drought or in moist sites from strong competition
from other vegetation. In a later study, Myers & Berube (in press) found that the
number of knapweed plants surviving to the flowering stage was directly proportional to
the number of seedlings over a wide range of knapweed densities. If the results of this
study are generally applicable, the flies should eventually achieve a large decline in
knapweed density. The difference in the two predictions may be related to the study
methods. ROle (1981) achieved a low knapweed density by removing seedlings from
plots of almost pure knapweed. This had the effect of temporarily reducing the competition
for the remaining knapweed. In the Myers & Berube (in press) study, the
knapweed was always in competition: with itself at high densities and with grass at low
densities.
The effect of the beetle S. jugoslavica on diffuse knapweed was not studied in detail as
until recently the colony was too small for destructive sampling. That the beetle tends to
reduce seed production is evident from Table 31. Its effect may be much greater if it
Effect of S. jugoslavica Obenb. on diffuse knapweed, C. diffusa L., seed production
per flowering plant at White Lake, British Columbia
Attacked Unattacked
Year X ±SD n X ±SD n
1977 18.6 13.4 5 20.4 14.8 28
1978 69.1 45.9 26 73.5 55.7 82
1979 38.9 33.9 10 66.5 80.4 16
1980 31.6 24.4 25 54.3 52.5 27 P 0.05 U Test
tends to prevent rosettes from bolting: approximately 20% of the rosettes in the fenced
area had been attacked in the previous year, so they had passed at least two years
without flowering. No previously attacked rosettes were found in the grazed area but the
significance of this is not known, nor is the percentage of rosettes that normally take two
years to flower. There was an indication that S. jugsolavica had a synergistic effect on
seed reduction in plants stressed by drought or attacked by the seed-head gall flies. Both
gall flies were present on the site, and in the autumn of 1981, there was an average of
1.63 ± 1.43 (S.D.) galls in the five distal heads on 23 plants; but whether a combination of
S. jugoslavica and the flies reduce seed production below the level needed for
maintenance of the knapweed stand remains to be seen.
The effect of M. paucipunctella on spotted knapweed was less than expected from
European studies as a different species of knapweed appears to be involved in British
Columbia. Also, the overwintering larvae at Westwold, British Columbia, have suffered
a periodic large and unexplained winter mortality that has prevented the rapid increase
and spread of the species. However, without releases in other knapweed stands, it
should not be assumed that this mortality occurs throughout the spotted knapweed
region of North America. The ability of M. paucipunctel/a or the seed fly U.
quadrifasciata to survive in the colder parts of the Canadian prairies is doubtful. In
the winter of 1981, spotted knapweed heads containing overwintering larvae of the three
seed-head insects were tied to a stake at normal plant height at Regina. Only U. affinis
larvae were alive in the spring. If large stands of the weed became established on the
prairies, it might be possible to select cold hardy strains of the insects but in the meantime