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384 F. W. Quednau
Evaluation of Control Attempts
humidity (F.W. Quednau unpublished). The biology of this species is essentially the
same as that reported for o. benefactor Hinz by Pschorn-Walcher & Zinnert (1971).
O. geniculatae may have a partial second generation each year in the field.
Whereas the colonization of Rhorus sp. No.3 was unsuccessful, O. geniculatae became
firmly established at Beaumont nurseries, and within 4 years parasitized a large proportion of
hosts. A brief history of this event follows.
At the beginning of massive implantations in 1m, population densities of P. geniculata
were medium to heavy, and 1 year later were light to medium. In the fall of 1978 it was
found from dissections of sawfly cocoons that the percentage of parasitism at the release
centre had dropped to 25-30%, compared to 60-75% in 1977, as a result ofthe dilution
of the parasitoid population in the area. It was estimated that within 1 year O. geniculatae
spread about 500 m from the original release centre. In 1979 infestations by the mountainash
sawfly were only light at Beaumont. Of30soil samples of 0.1 m l each, taken near the
original release point of the parasitoid, only 10 contained living cocoons (range 1-5) and
parasitism was about 73%, much higher than in 1978. Other sampling plots about 500 m
from the release point yielded 30-35% parasitism in 1979. In 1980, in the same locality,
dissections of mountain-ash sawfly larvae that were still in colonies (about third instar)
were made. The infestation level was from 700 to 1 250 young colonies of the sawfly per
hectare. Percentage parasitism obtained from these dissections was 88.7-93.5% in all the
plots containing mountain-ash in the nursery (up to 500 m radius from the release centre).
No second generation of P. geniculata was observed in 1980. From an evaluation of host
cocoons collected in the fall of 1980 it was found that the number of P. geniculata cocoons
had further diminished as compared with 1979 and few were unparasitized.
Preliminary indications for 1981 are that O. geniculatae has spread about 30 km from
the original release site and could be recovered at Ste-Foy, Charlesbourg, La Durantaye,
St-Charles de Bellechasse, St-Michel de Bellechasse, St-Etienne de Lauzon, and Chutes
Mt. Ste-Anne where parasitism was 81, 24, SO, 30, 82,70, and 4% respectively. A quantitative
evaluation of the impact of the parasitoid on its host population has not been carried out
so far. However, preliminary measures of pest numbers in consecutive years in the
original release site at Beaumont indicate a drastic decline of the host populaton (from
500 colonieslha in 1980 to an estimated 30 colonieslha in 1981), which must be attributed
primarily to the action of the parasitoid. Comparative population counts of P. geniculata
at St-Edouard de Frampton, where the environmental structure is very similar to that at
Beaumont, but is situated outside the action radius of o. geniculatae, indicate very high
levels of infestation in 1981.
The cost of chemical insecticide treatment against P. geniculala would be at least
$2OOlha. For this reason most owners of property with mountain-ash do not use chemical
treatments. The mass production of O. geniculatae would need extensive manual labour
and costs cannot be calculated accurately at present. However, as the parasitoid spreads
on its own, its implantation could be highly beneficial.
During the evaluation work in summer 1980, a large number of the larvae of
O. geniculatae found in the hosts were dead or appeared sick. At this stage it was decided
to rear adults from P. geniculata cocoons collected from Beaumont in order to determine
the percentage parasitism by O. geniculatae and to find out whether a hyperparasitoid was
present. Emergence of the insects in 1981 revealed up to 90% total parasitism, but about
60% of the parasitoids obtained were Mesochorus globu/alorThnb. This latter species is a
hyperparasitoid with a wide distribution in Canada. It attacks various other species of
Olesicampe associated with nematine sawflies.