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Table 44
Evaluation of Control Attempts
Euphorbia esula-virgata complex. 167
Oviposition attempts and success by o. erythrocephala (Schr.) at three sites
No. stems with No. larvae in No. larvae in
Site oviposition scars roots by September stems in Septcmber
Jameson. Saskatchcwan 25 1 3
Caronport. Saskatchewan 93 15 2
Cardston. Alberta 29 17 2
was good in spite of a poor snow cover as live larvae were found in three out of four
plants examined in May. About twice as many larvae were found at the end of the
summer as in the previous year including two plants (untagged the year before) with
larvae developing over a two year period. The colony had a radius of about 25 m in the
fall of 1981. No progeny were found at the Jameson release site.
In 1981, beetles were released at three widely separated sites but as in the previous
year breeding success was poor (Table 43). The table indicates that the date of release had
little influence on breeding success so the problem does not appear to be related to the
synchrony of the plant and larval development.
The larvae developed readily in the stems of spurge plants from the Jameson area
grown in pots but these plants tended to have larger stems than most of those in the field.
The colony at Caronport was still present in 1982 and for the first time one adult was
found (a fertilized female). A search made in mid-September produced one larva which
was still in the stem only a few centimeters from the oviposition point. As most of the
spurge stems were dead by October it had little prospect of survival. The problem is
apparently not cool summer temperatures as the Cardston site had fewer heat units than
the other release sites. The stem diameter at Caronport was, however, considerably less
than at Cardston (the soil is sand and too light to be cultivated) so it appears that the
beetle is only likely to thrive on spurge plants growing on fertile soils.
The biological control of spurge in Canada has so far been a failure. The one insect
established on cypress spurge has had little impact and the possible impact of O.
erythrocephala on leafy spurge will not be apparent until its population has reached its
plateau. Other insects tested have failed to survive on Canadian leafy spurge although
some like C. tenthrediniformis appear to have a major impact on E. esula in Europe. The
initial difficulty arose from equating the E. esula of Europe with the plant called E. esula
in North America. Some difficulties still remain as it is not possible to search Europe
for the Canadian spp. of leafy spurge for possible agents since the precise origin and
identity of this spurge is not clear. Indeed, some of it may have arisen in North America
through hybridization. Thus agents have to be found by testing the acceptability of North
American leafy spurge to organisms found attacking closely related European spurges
(those in the section Esu/a). A number of these are already known and can be subjected to
the usual screening tests to establish their specificity. A few species have already been
screened and effort should be made to establish them as widely as possible.
The difference in the density of spurge in European and North American stands
appears to be related to the cropping pressure by a complex of insects and pathogens in
Europe, although it would be nice to demonstrate this by removing the insects from a
European stand. One possible explanation for the richness of the complex in Europe is