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152 P. Harris and A. T. S. Wilkinson
Calculation of seed
production by
attacked and
unattacked plants
Recommendations
Literature Cited
The effect of head size on seed production in the attacked plants is nullified because
the number of U. sty/ala also increases with head size (Equation 5, Table 36). The mean
number of larvae per head was considerably lower than the maximum number in the
heads of each size (Equation 6, Table 36). Thus for a head with a diameter of 14 mm the
average number from the regression was 6.6 larvae while the maximum was 14.6 larvae.
Thus the larval population is well below the physical limits of the thistle heads. There
was a rather small effect of crowding on larval weight. A regression of live weight of 980
larvae from heads with 1-33 larvae in a gall showed that one larva per gall averaged 9.4
mg compared with 6.7 mg for those in heads with 33 larvae. The slope of the regression is
highly significant, although as indicated by the small correlation coefficient there was a
large variation in larval weights at all densities (Equation 7, Table 36). For the purpose of
the calculations in this paper the size and hence the effect of a larva was assumed to be
the same whether it was single or crowded.
(a) Methods
Seed production for each head on the unattacked plants was determined from regression
Equation 3 (Table 36). These were summed and the total for each plant regressed on
plant weight so that average production could be determined for a plant of any size. The
same was done for the attacked plants except that the heads were assumed to produce 62
seeds regardless of size.
(b) Results
The production of 4398 seeds for a plant of 75 g (Equation 10, Table 36), is close to the
figure of 4000 seeds/plant given by Salisbury (1964) for the thistle in Britain. The attacked
plant of this size produced 1898 seeds (Equation 11, Table 36), a reduction of
57%. For a plant of 150 g, the reduction was 62%.
The calculations were made for a U. Sly/ala population infesting 88% of the thistle
heads. This is slightly below the population plateau at Cloverdale, so the reduction in
plump seed production at this site should be at least 60%. If the difference in the germination
between attacked and unattacked plants is accepted, the reduction is over 65%.
Under field conditions germination and seedling survival seem to depend on breaks in
the sward rather than competition within the C. vulgare population. This means that the
reduction in thistle density should be similar to the reduction of seed as soon as the soil
seed-bank has reached its new equilibrium.
(1) U. stylala should be distributed to sites across Canada with stable populations of
C. vulgare; but there is little point in releasing it against temporary outbreaks.
(2) The introduction of additional biological control agents does not appear to be
warranted as most of the problems with C. vulgare are either temporary following the
clearing of land or the result of poor farming practice.
Anderson. R.N. (1968) Germination and establishment of weeds for experimental purposes. Weed Science Society of America, 636 pp.
Harrington. J.F. (1972) Seed storage and longevity. In: Kozlowski T.T. (Ed.) Seed biology vol. III. Academic Press. pp. 145-245.