SAWFLIES (Hymenoptera: Symphyta)
Henri Goulet
DISAPPEARANCE OF LARVAL HOST PLANTS
I think that larval host plants have become rare under the strong competitive drive of European plants. Though we know very little about the larval host plants of North American Tenthredo, I assume, based on the European species larval host data, that each species is restricted to a narrow range of host plant species. Most species of Tenthredo favour meadows. In the Mixedwood Plains Ecozone, these habitats are generally pastures and old fields, and consist mainly of European plants. Therefore, it is likely that larval host plants have become rather uncommon, and would be an important cause of low population levels. Data from two field seasons support this hypothesis. In the summer of 1994, I sampled a natural meadow near Almonte. This habitat is an alvar and consists almost entirely of native plants. I was successful in getting only two species in numbers. In the summer of 1995, in a wet and slightly acid meadow consisting only of native plants, I collected many specimens and about 20% of the species expected near Ottawa. The nearby area was basically free of European weeds and there was no agriculture for 2 or 3 kilometres.