SAWFLIES (Hymenoptera: Symphyta)
Henri Goulet
DECLINES IN ABUNDANCE
Most population decreases have been recorded for native species. However, three introduced forest pests have become uncommon in the past 10 to 15 years in our region. They are: the birch leaf miner (Fenusa pusilla (Lepeletier)), the larch sawfly (Pristiphora ericksonii (Hartig)) and the mountain ash sawfly (Pristiphora geniculata (Hartig)). The population of Fenusa pusilla was brought under control by Lathrolestes nigricollis (Thomson), an ichneumonid parasite, successfully introduced near Quebec City and in Pennsylvania in the mid 1970's (Quednau, 1984a); that of P. ericksonii by Olesicampe benefactor Hinz in the 1960's (Ives and Muldrew, 1984); and that of P. geniculata by O. geniculatae Chutney and Lim in the 1970's (Chutney, 1984b). The most remarkable drop in populations among native sawfiles is among species of Tenthredo and to a lesser extent those of Macrophya. Both genera are sister tribes. It appears that the populations of almost all common species have crashed since the 1970's and more so in the past 15 years. Table 11 is an example of an exceptionally common and widespread eastern species of Tenthredo, T. basilaris Say, found over all of our area. Because I did not closely monitor populations of Macrophya as I did for those of Tenthredo, a genus which I am presently revising, the following discussion is based on species of Tenthredo.
Table 6. Number of specimens seen from all North American collections of Tenthredo basilaris Say by 10 year periods from 1850 to 1995. "X" = 1-5 specimens. "WW I" & "WW II" = World War I & II. The number of specimens before 1900 may not reflect their true numbers because many were destroyed by fungi and dermestid beetles and there were fewer collections.
| Decade | Number of Specimens |
| 1850-1859 | 0 |
| 1860-1869 | 1 |
| 1870-1889 | 73 |
| 1890-1899 | 101 |
| 1990-1909 | 131 |
| 1910-1919 | 253 (WW I) |
| 1920-1929 | 248 |
| 1930-1939 | 227 |
| 1940-1949 | 151 (WW II) |
| 1950-1959 | 285 |
| 1960-1969 | 291 |
| 1970-1979 | 102 |
| 1980-1989 | 108 |
| 1990-1995 | 42 |
| Total | 2015 |
The results above are similar for several other common species Tenthredo of our region. Several years ago, I thought that I was inefficient at locating these insects, but my marked success in finding half of our species in Cape Breton Highlands National Park and other species especially in western Canada supports the theory that the population of common species in the MWPE has crashed.
The biology of Tenthredo species is unusual for sawflies. Based mostly on European host data, larvae feed over an extremely wide range of plants, from ferns to Asteraceae and Fabaceae. Generally, most species are recorded from one host species. The most common types of host plants are herbaceous. Moreover, adults are exceptional among sawflies as they are predators and usually pollinators. The predatory behaviour is marked and similar to that of tiger beetles. Adults of many species actively pollinate many plants species and feed on nectar. Adults of both genera oviposit generally later than most tenthredinid sawflies in June to late August. Finally, adults are large and remarkably showy Hymenoptera, and thus, have been regularly collected by entomologists as shown by about 45,000 specimens I assembled for a systematic revision of the genus.
I have two hypotheses for the crashes observed in Tenthredo: the disappearance of larval host plants and the contamination of adults by pesticides.