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Table 35
Cirs;llm I'ulg(lre (Savi) Ten. , 149
The establishment and build-up of U. stylata populations were followed in more detail
at Cranbrook, Nanaimo. and particularly Cloverdale and Ladner. British Columbia
(Table 35). The Cranbrook site was along a power right-of-way through forest; Cloverdale
was a permanent pasture on black muck soil in the Fraser delta on which the thistle
grew where the cattle hooves broke the sward. The site was destroyed for development
in 1980, but the fly survives in the area. At Ladner, also in the Fraser delta. the thistle
was confined to the edge of a drainage ditch through an intensely cultivated area used
largely to grow seed sugar beet and carrots. The populations were followed by sampling,
at the end of the summer, all the seed heads on thistle plants growing at approximately 5
m intervals in 4 directions from the release point. The exception was the linear stand at
Ladner. Samples were taken to the edge of the colony or the site. whichever was least.
Size and spread of U. stylata F. colonies at release sites in British Columbia.
Heads/plant No. heads % heads No. larvaeJgall Colony Distance (m) to near-
Location Year ± S.E.M. sampled galled ± S.E.M. radius m est thistle ± SD
Cranbook 1973 21.2 ± 6.0 525 6.7 20 II.R ± 11.1
197-1 12.1 ± 1.8 320 2.8 2.1 ± 0.4 100 2.9 ± 2.9
1976 6.2 ± 1.11 19-1 3.6 2.0 ± 0.4 80 thistle scarce
19111 No thistles prescnt and no U. srylata found on nearby roadside C. vulgart'
Nanaimo 1973 9.4 ± 2.1 113 3.5 30 0.7 ± 0.8
1974 8.7 ± 1.5 180 42.8 3.0 ± 0.3 100 0.2 ±0.3
1975 Area bulldoled, sown to grass and surrounding thistle cut
Cloverdale 1973 53 30.2 3.0 ± 1.1
1974 29.3 ± 3.5 299 41.5 3.2 ± 0.2
1975 32.2 ± 5.0 10M 15.9 140 2.4 ± 3.4
(299 plantslha)
1976 39.7 ± 9.7 1110 65.3 140 + 36.1 ±5.2
1977 39.8 ± 5.9 1\53 84.7 9.1 ± 0.3 200 + 3.5 ± 4.1
1978 38.4 ± 5.7 1283 95.8 13.7 ± 0.6 250+ 2.4 ± 0.4
1979 34.7 ± 7.2 520 92.7
1980 Site destroyed
(site 2) 1978 511.0 ± 19.4 5811 92.2
1979 60.1 ± 14.5 402 83.6
Ladner 1973 22.5 ± 4.7 -149 12.2 3.1 ± 0.3 100 0.3 ± 0.3
1974 Not sampled - all but 2 thistles mowed
1975 54.1 ± 9.2 1165 24.4 125 1.0 ± 1.1
1976 38.6 ± 6.9 1313 5.7 3!!6+ 2.8 ± 6.3
1977 80.6 ± 15.7 2338 6.0 160 + 1.7 ± 1.7
19711 59.7 ± 10.6 1851 31.7 ISO + 11.9 ± 1.3
1979 61.2 ± 10.9 2348 46.3 14.2 ± 5.2
1980 41.8 ± 5.9 1918 39.9
1981 55.3 ± 11.8 775 26.3
1982 39.6 ± 9.0 912 38.8
The results (Table 35) show that at Cloverdale the fly population increased to attack
over 90% of the heads formed. The number of larvae per head increased with the percentage
of heads attacked (Equation 8. Table 36). This is also apparent from Table 35: at
low densities of the fly. a density of 2-3 larvae per gall was found. but at high fly
densities the average increased to around 14 larvae per gall with maximum numbers of
over 30 larvae. Presumably the high larval numbers are the result of multiple ovipositon.
The data for Ladner were excluded from this analysis where it is suspected that the
inability of the fly to attack more than half the heads resulted from their loss over the
surrounding fields which were treated with insecticide during the season. Thus although