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150 P. Harris and A. T. S. Wilkinson
Table 36
Evaluation of Control Attempts
ElTects of U. stylata
on the number of
seed heads on
C. vulgare plants
the high number of larvae/gall in 1979 indicated a high density of flies in early summer.
the percentage of heads attacked suggests that those forming late in the season escaped
the fly.
The average distance of the sampled thistle to its neighbour (Table 35) gives an idea of
density although. except in one instance. this cannot be related to the number of thistles
per unit area as the distribution contagion was not measured. No effect of the fly on the
distance between thistles was apparent; however as C. vulgare seed was found to be still
viable after approximately 36 years under forest cover (Harrington 1972). it will take a
number of years of reduced input into the soil seed-bank before the density of the thistle
is affected.
Evaluation of the effect of U. stylata F. on C. vulgare (Savi) Ten.
Regression
1. No. heads on unattacked plants 0.40 x + 2.0
2. No. heads on attacked plants 0.32 x + 6.2
3. No. plump seedlhead from unattacked plants 3.66 r - 18.7
4. No. plump seedlhead from attacked plants -0.12 r + 67.7
5. Mean no. larvaelhead in head size classes 0.16 r - 1.2
6. Maximum number of larvaelhead in head
size cI asses 0.24 r + 2.8
7. Live larval weight in mg -0.08 n + 9.5
8. No. larvaclgall 0.11 z + 0.3
9. No. larvaelhead -0.006 x + 9.6
10. Total seed production from unattacked plants 55.51 x + 235.2
11. Total seed production from attacked plants 17.69 x + 571.4
Where x = dry weight of plant in g
r = radius of receptacle attachment to the involucre in nun
z = the percentage of heads attacked
n = no. larvae/head
Cor.Coef. n P
0.97 25