Euura petiolaridis

Pontania petiolaridis New Species

An almost spherical, sawfly gall is produced on the leaves of Salix petiolaris Sm., a native willow that is not uncommon in low ground near the city. The deformity closely resembles the conspicuous apple-like gall of Pontania pomum Walsh, but differs from this species in some details of structure. The host plants of the two galls differ as Pontania pomum is restricted almost, if not entirely, to Salix cordata [eriocephala] Muhl. In their attachment to the leaves of their hosts they also vary as the species on Salix petiolaris is almost equally divided by the blade, while Pontania pomum projects, only very slightly, from the upper side of the leaf. Further, the former species is hollow from the earliest stages, but the latter only becomes so when eaten out by the larvae. A number of the galls from Salix petiolaris Sm. were collected in 1915, just before the fall of the leaves, and were kept, out-of-doors, in jars containing earth, during the winter. The adults began to emerge April 15th, and were sent to Mr. S. A. Rohwer, Washington, D.C., who has kindly replied as follows: "I have made a preliminary examination of the species and find that it may easily be distinguished from Pontania pomum and that it comes near to P. pisum. I think the species is undoubtedly new." 

An interesting observation was made concerning this new species of sawfiy, namely that the aperture of exit is prepared a considerable length of time before the larvfe leave the galls. Just what conditions finally prompt their departure and why their means of escape have to be ready, are points not yet cleared up. Since all sawfly galls are well advanced in development before the larvae are hatched, it is safe to conclude that the chief stimulus to abnormal growth must emanate from the ovipositor of the insect. The sawflies, when depositing eggs, clasp the opening buds with 'their legs and insert their sawlike ovipositors into the young leaves from the under side. Into each of the incisions, thus made, an egg is injected. The larvae, as soon as hatched, commence to feed upon the substance in the interior of the gall, but leave the rind uninjured. Since the pear-slug, Eriocampa cerasi Peck makes a similar incision in the leaf, without causing an abnormal production of tissue, it is highly probable that in the case of the sawfly gall-producers, the stimulus is not due to the mechanical effect of the cutting, but to a chemical action arising from the introduction of some substance by the ovipositor of the insect. 

Range: Toronto, Ontario

Reports on insects of the year, Division No. 3, Toronto District. Forty-Seventh Annual Report of the Entomological Society of Ontario, 1916