Oviposition Stimulant for a Gall-Inducing Sawfly, Euura lasiolepis, on Willow is a Phenolic Glucoside

Abstract

The ovipositional stimulation for the shoot galling sawfly, Euura lasiolepis, whose natural host is the North American willow, Salix lasiolepis, was studied in response to its original host, Finnish willows, poplar species, and individual phenolic glucosides and fractions. The major phenolic glucosides in the natural host were salicortin and tremulacin, which were also the major components in the Finnish S. purpurea and S. rosmarinifolia. The natural host, S. lasiolepis, was chosen over six Finnish willow species in multiple-choice tests. Multiple-choice tests with six Finnish willow species and one Finnish popular species showed that S. rosmarinifolia was significantly more attractive than any of the other species by at least 16-fold. No-choice testing with phenolic glucosides showed the tremulacin fraction to be the significant ovipositional stimulant, which elicited probes with the ovipositor at least 19 times more frequently than other glucosides. A tremulacin fraction at 1.5% elicited significantly more probes than a tremulacin fraction tested at 0.4%. In addition, tremulacin was the only glucoside to induce prolonged and repeated probing with the ovipositor, lasting a mean of 6.4 sec per bout per female. Only single probes lasting a fraction of a second were observed in response to other glucosides. This is the second study showing a direct effect of a phenolic glucoside as an ovipositional stimulant for a sawfly, and it also suggests the existence of a mechanism that enables females to respond positively in a graded way to shoot length heterogeneity in host plants and populations.