An outbreak of the European spruce sawfly, Diprion hercyniae (Htg.), was discovered in the Gaspé Peninsula in 1930. By 1938, heavy infestations had developed west of the Gaspé Peninsula in Quebec, throughout New Brunswick and northern Maine, and in parts of Vermont and New Hampshire. Moderate to light infestations occurred through all other parts of the spruce forests of this region and extended from Nova Scotia, to the north shore of the St. Lawrence River, and west to Ontario.

The European pine sawfly, Neodiprion sertifer (Geoffr.), is a serious defoliator of pine in many parts of Europe and Asia. Infestations of this insect have been controlled by weather, and frequently high percentages are destroyed by parasitic and predacinus insects, by small mammals, and by birds. Infectious diseases are most frequently reported as having controlled outbreaks, namely: virus disease (4, 5), bacterial disease (12). fungus disease (8), bacterial and fungus diseases (7, 11), a disease not diagnosed (9).

Although a virus disease of the larch sawfly, Pristiphora erichsonii (Htg.), has not been discovered, polyhedrosis viruses of several other Tenthredinids are known. They have been used to control infestations of two introduced species: the European spruce sawfly, Diprion hercyniae (Htg.), (Bird, 1954) and the European pine sawfly, Neodiprion sertifer (Geoffr.), (Bird, 1950, 1952, 1953; Dowden, 1953). Unfortunately, the viruses of these, as well as the viruses of several other species tested, are not pathogenic to the larch sawfly. Some viruses, however, appear to he pathogenic to more than one species. J. M. Burk of this laboratory found, for example, that a polyhedrosis virus affecting the native jackpine sawfly, Neudiprion americanus banksianae Roh., is pathogenic to N. sertifer, N. nanulus Schedl, and D. hercyniae.

An outbreak of the European spruce sawfly, Diprion hercyniae (Htg.), occurring in Eastern Canada between 1930 and 1942, was controlled by a virus disease (Balch and Bird, 1944). The sawfly has been kept at a low level by the disease and by parasitic insects introduced from Europe (Bird and Elgee, 1957).An infestation of the sawfly was discovered in 1947 near Sault Ste. Marie, Ontario, by the Forest Insect Survey Section of the Forest Biology Laboratory at Sault Ste. Marie. This infestation was about 100 miles beyond the previously known western distribution of the insect. Intensive larval sampling in 1949 showed that it was free from virus and practically free from introduced parasites. Thus, an excellent opportunity was provided to introduce the virus into a disease-free population, to study its establishment and spread, and to compare the long term effects of virus alone on population trends with the effects that virus plus introduced parasites were shown to have on population trends in New Brunswick (Bird and Elgee, 1957). The virus was introduced into the infestation in 1950 and studies were carried out each year from 1950 to 1959.

Escherich (7) was the first to report a polyhedral virus disease affecting the European pine sawfly, Neodiprion sertifer (Geoffr.). Later Forsslund (8) observed that populations of this insect in Sweden were controlled by a virus disease. In 1948, H. S. Hanson. Entomologist of the Forestry Commission, England, observed mortality among N. sertifer in England which was due to a polyhedral virus disease (10). In 1949 virus-killed larvae collected in Sweden by Forsslund, were sent to the Laboratory of Insect Pathology, Sault Ste. Marie by G. R. Wyatt of this laboratory and the virus from these insects was propagated and used in the biological control of N. sertifer in southern Ontario (3, 5). This paper describes some aspects of the laboratory studies of the disease, namely: the infection process in cells susceptible to the virus, incubation period of the disease, and the isolation and electron microscope study of the causal agent.