SAWFLIES (Hymenoptera: Symphyta)
Henri Goulet
SUMMARY
The Symphyta fauna in the MWPE is impressively diverse in species (450 species recorded) and rich in genera (108 out of 119 Canadian genera). Most of the species are native (89%) with ranges mainly east of the 100th meridian, and are centred in temperate regions. About 27% of the fauna is found nowhere else in Canada. It is the region of the greatest diversity in Canada.
Most of the species consume plant tissue (usually leaves) as larvae, and therefore, many species have an important economic impact in agricultural, forest, and the ornamental industries. Some species are beneficial as they contribute to the control of noxious weeds. As adults, the economic impact is minimal except for the markedly predatory species of the genus Tenthredo.
Within this fauna two groups stands out: the native and the introduced species. The introduced species though less diverse (11% of the species) are responsible for most of the damage by sawflies to the Canadian agriculture, especially in fruit and tree cultures. These species are mainly encountered in man-made habitats. Most of the native species have mainly no economic impact, but some species are important defoliator of forest or ornamental trees. These species are mainly encountered in natural habitats. The alien fauna, though less diverse, is dominant generally in the MWPE because their favourite man-made habitats dominate the landscape. The origin of the native fauna is mainly from a south to north mouvement following the ice retreat and the climatic improvements. The origin of the alien fauna is mainly through the nursery trade especially following the Second World War, and in colonial times through the boat ballasts. Introductions are still discovered despite excellent quarantine efforts.
Populations of most sawfly species are generally stable. However, populations of common species of Tenthredo have crashed probably due to widespread use of insecticides in agricultural regions, and that of few introduced species have decreased dramatically in the past 30 years due to the discovery of natural enemies for some species, and environmental factors for other species. About ten species, all from Europe, are presently in full expansion.