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66 D. G. Harcourt, J. C. Guppy and C. R. Ellis
Table 10
Current Status of T. julis
Hastings Counties (Harcourt et al. 1977) contained larvae of T. julis. These findings,
which were unexpected in light of the Michigan experience, prompted closer examination
of larval collections made during the course of annual CLB population surveys by Ontario
Ministry of Agriculture and Food personnel at 8 sites within the province during 1975.
The surveys consisted of 250 net sweeps from each of 30 fields of spring grain located
within a 75 km! area. Eight such areas were sampled in late June following the
accumulation of 475°D>9°C.
Table 10 shows that T. julis had dispersed widely across Ontario by 1975. Rates of
parasitism were highest in southwestern Ontario, lowest in eastern Ontario, and
intermediate in other parts of the province. Overall parasitism was 84%. The number
of parasitoids per host ranged from 1 to 44 and averaged 5.9. The four host instars were
parasitized in roughly the same proportion (75, 87, 84,84), suggesting that the larvae
were attacked when small.
Parasitism of the cereal leaf beetle,
(Walker) in Ontario, 1975.
County Township No. host
larvae collected
Oulema melanopus (L.), by Tetrastiehus julis
Kent Merlin 894 95.0
Middlesex Glencoe 212 91.9
Essex Malden 526 88.6
Wellington Fergus 227 83.7
York Whitchurch 143 83.2
Ontario Brock 15 73.3
Bruce Mildmay 156 68.6
Renfrew Bromley 147 15.0
TOTALS 2320 84.4
% parasitized
Sweepnet samples of spring grain on Manitoulin Island in early July 1976 showed that
the parasitism rate was 87% (Ellis et al. 1979).
Since 1974, there have been no reports of economic damage by the cereal leaf beetle in
Ontario other than on Manitoulin Island. In most fields of spring grain it has been
difficult to find larvae. This raised fears that the host population might become too low to
maintain the parasitoid which would set the stage for a resurgence of the pest. For this
reason, extensive sampling has been carried out in representative sites in Ontario since
1977. Rates of parasitism in the past 4 years have ranged from 31-68%, indicating that T.
julis has the capacity to maintain itself as an effective biological control agent at very low
host densities. There has been no indication of resurgence of CLB populations and no
damage from the pest was reported in Ontario during the 1977-80 period.
Because the distribution of T. julis in 1975 was more extensive than the area of the 1974
releases and because its population density at most sites was inexplicably high, it is
concluded that the parasitoid spread into Ontario as a result of the earlier releases in
Michigan and other parts of the United States. It is also clear that the parasitoid has
better powers of dispersal than originally reported.