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Pest Status
Background
Chapter 65
Pristiphora geniculata (Htg. ), Mountain­
Ash Sawfly (Hymenoptera: Tenthredinidae)
F.W. QUEDNAU
The mountain-ash sawfly, Pristiphora geniculata (Hartig), is a major defoliator of
mountain-ash, mainly Sorbus americana Marsh, and S. aucuparia L. These tree species
have little industrial importance but are planted widely as shade and ornamental trees.
The mountain-ash sawfly was accidentally introduced into the United States in 1926
(Schaffner 1936) and was first recorded in Canada in southern Quebec in 1934 (Petch
1935). Its present Canadian range is from Newfoundland to southwestern Ontario (Fig.
20). There has never been a serious outbreak. The sawfly causes noticeable and occasionaJly
complete defoliation of mountain-ash, but this seldom causes mortality of the
tree. To owners of such trees and to municipal park authorities, the insect is a considerable
nuisance and often requires localized control by chemical methods. An account of the
biology of mountain-ash sawfly in eastern Canada was given by Forbes & Daviault
(1964). P. geniculata usually has one generation a year, but there may be a partial second
generation depending on the climate.
The decision to embark on a biological control programme in Quebec against P. geniculata
was determined by the policy to reduce chemical control in urban forestry and by the
success of biological control in Manitoba against the closely related larch sawfly, P.
erichsonii (Htg.), on tamarack, Larix laricina (Du Roi) K. Koch (Muldrew 1967, 1973).
Data furnished by the CIBC European Station suggested that the ichneumonid wasp
Olesicampe geniculatae Quednau & Lim, would be a primary candidate for introduction
into Canada. Another parasitoid species under consideration was the ichneumonid wasp
Rhorus sp. No.3.
During the first years of attempts to colonize these parasitoid species in Quebec,
infestation levels of mountain-ash sawfly were very low. Furthermore, in 1972 O. geniculatae
was not found in sufficient numbers in Europe to warrant a shipment to Canada.
However, in 1973, heavy infestations of P. geniculata occurred in several localities near
Quebec City. Because of the scarcity in Europe of P. geniculata, laboratory massrearings
of the sawfly were carried out in Quebec in order to obtain host material that
could be sent to Europe to increase the host populations there. The rearing techniques
were essentially the same as those described for the larch sawfly by Heron & Drouin
(1969). In 1974 the Laurentian Forest Research Centre sent 6 000 cocoons of Canadian P.
geniculata to CIBC in Switzerland and another 30 000 cocoons in 1976. This insect
material was reared to the adult stage in Europe and the sawfly females were placed in
plastic bags tied over branches of mountain-ash trees in the Waldviertel area in Austria,
where O. geniculatae and other parasitoid species of the mountain-ash sawfly were known'
to occur in significant numbers during the previous years. As a result, in 1977 large
numbers of O. geniculatae and Rhorus sp. No.3 were received in Quebec.
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