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96 P. Harris
Table 16
annual production (Ellison 1960). If the threshold of a target weed is assumed to be
50% of annual production. it will not be controlled by an agent that removes 30% of
production. Nevertheless the addition of new defoliators, seed or root feeders, will
normally increase the total amount of the weed consumed. Thus the practice is to add
agent species until the threshold is exceeded. For this reason I consider any agent that
has become numerous enough to remove a substantial proportion of annual production
as a success. I would prefer to rate them by the actual amount removed but do not
have the data for all the species established in Canada.
The successes of the Canadian programme are listed in Table 16. Several of the
Agents removing a substantial proportion of the annual production of the target weed in
one or more regions of Canada.
Target Weed
Carduus nutans L.
Centaurea diffusa Lam.
e. maculosa Lam.
Cirsium vulgare (Savi) Ten.
Euphorbia cyparissias L.
Hypericum per/oratum L.
Senecio jacobaea L.
Agent
Rhinocyllus conicus Froel.
Urophora affinis Frfld.
U. quadri/asciata (Mg.)
Urophora sty lata L.
Hyles euphorbiae (L.)*
Anaitis plagiala (L.)
Chrysolina hyperici (Forst.)
C. quadrigemina (Suffr.)
Tyria jacobaeae L.
Longitarsus jacobaeae (Wat.)
• The inclusion of this insect is possibly an optimistic estimate of its effect.
Country
Screening Agent
Canada
Canada
Canada
Canada
Canada
Canada
Australia
Australia
New Zealand
United States
agents have in fact achieved density reductions of the target weed that have solved
some of the problems from it. For example, the seedhead weevil, Rhillocyllus conicus
FroeI., has reduced the density of Carduus nutans L. sufficiently on Saskatchewan
rangeland that it does not threaten the cattle stocking rate. On the other hand, the
thistle remains numerous on disturbed sites such as gravel pits, and even at low
densities can be a nuisance in parks and around beaches. Similarly Hypericum
perforatum L. presently has little effect on the stocking rate of British Columbia
rangeland: the dense stands at Elko, British Columbia, are not of concern for cattle
production as stocking is kept low to retain the area as a wildlife overwintering refuge.
The weed, however, is still perceived to be a problem by the ranchers, possibly
because they fear its spread.
The initial work on four of the successful agents (Table 16) was done by other
countries. Insects successful in Australia or California, such as Chrysolina quadrigemina
(Suffr.), have tended to be successful in Canada despite differences in climate and the
converse is true of Canadian pioneered species such as R. conicus. Indeed, the most
effective and widely employed technique for selection of an effective agent is to choose a
proven winner. Such agents are also inexpensive as most of the pre-release studies
required by Canada have been done. Stock for the Chrysolina spp. established in
Canada was obtained from California following success there. It would almost certainly
have achieved an impact on the weed in Canada sooner if the releases had been made
with climatically pre-adapted stock; but there is a trade-off between speed of success
and cost. In North America, savings can be achieved on weeds common to both Canada