Gall induction requires an efficient host plant finding by the sawfly female. Due to her high sensitivity to variations of the host plant quality, the female is able to locate very reliably the plant tissue for oviposition. Gall induction and oviposition occur according to the following pattern: 1. selection of the host plant species, 2. selection of the host plant individual, 3. localization of the plant tissue for oviposition, 4. injection of the cecidogenous fluid, 5. egg deposition (sometimes no egg was laid).
Gall induction and oviposition are not separated temporally from each other in Euura and most Pontania species, occurring shortly after the female has inserted her saw-like ovipositor into the plant tissue. In contrast, females of the Pontania dolichura-group and particularly of the genus Phyllocolpa, perform a more or less complex procedure of gall initiation before oviposition.
Females of Pontania oviposit exclusively on longitudinally folded leaves of the apical bundle of a growing shoot, inserting their saw once through the midrib of the leaf (often side rib in the proxima-group). In general only a single egg is laid into the mesophyll tissue by a simultaneous injection of the cecidogenous fluid. Pontania species induce four different types of closed galls, exclusively on the leaves of their host plants: thick-walled, elongated sausage-shaped galls on the upper surface of the leaf; thin-walled, kidney-shaped spacious galls transected horizontally by the leaf blade; thick-walled, bean-shaped galls with a narrow cavity, transected horizontally by the leaf blade; and thin to thick-walled, pea-shaped galls at the underside of the leaf.
Oviposition of Euura occurs at different organs of their host plants, dependent on the particular gall-type of the species. Bud-gallers oviposit only into new flower buds of the next year, arising in the axis of young leaves. Similar to Pontania leaf-gallers, oviposition of stem gallers occurs at the apical bundle of longitudinally folded leaves. However, the females of this guild insert their long ovipositor through the base of unfolded leaves into the young, very succulent stem. The females of gall-formers which induce petiole- or midrib-galls insert their ovipositor into the suitable plant tissue of one of the longitudinally folded leaves of the apical bundle.
In contrast, the species of the genus Phyllocolpa exhibit the most complex behaviour of gall induction. Oviposition occurs at one of the still folded leaves of the apical bundle or at one of the freshly unfolded young leaves below the apical bundle, depending on the species. The females induce simple or twisted leaf folds along the edge of young willow leaves, resulting from a swelling which is caused by multiple ovipositor insertions.
Gall formations can be induced only at viable plant parts. They can develop either during the entire vegetation period or only in temporally limited phases, depending on the generation sequence of the inducer or his inclination for a certain developmental stage of the plant. Gall formation is indispensable for a successful reproduction, demonstrating the particularly close relationship of the inducer to his host plant.
Host plant specificity
The species of Pontania, Euura, and Phyllocolpa always select specific, viable plant parts for oviposition, initiating gall formation by secreting the cecidogenous fluid of the accessory glands through the ovipositor. A prerequisite for an unimpaired gall formation is a precise placing and dosage of the cecidogenous fluid by the female, causing specific physiological procedures in the plant which stimulate and control the growth of the gall. The success of the female is primarily based on her ability to locate exactly the plant tissue for gall induction and oviposition. The oviposition mode as well as the composition of the cecidogenous fluid may have a relevant influence on the shape of the gall.
Gall formation of sawflies is normally initiated by the females during (Pontania, Euura) or before oviposition (Phyllocolpa). The sawfly female injects the cecidogenous fluid of its accessory glands into the meristem tissue, in most species inducing a complete development of galls.
Sometimes hybrids are accepted by the gall formers. This was documented by galls of Pontania samolad
and those of P. arcticornis
next to each other on the leaves of the hybrid S. lapponum x phylicifolia
The ability to induce galls presupposes a functioning interaction of the cecidogenous secretion of the gall-former and the pysiological characteristics of the host plant, i.e. only with suitable host plants an unimpaired gall formation may occur which ensures the development of the sawfly larva. Therefore, the acceptance of the host plant depends on various factors, e.g. certain plant defence reactions, genetic changes, effects of hybridisation, and last but not least the high variabilities of biochemical characteristics (secondary compounds, in particular phenol glycosides).
A distinct host plant specificity can be understood as a prerequisite for successful reproduction, permitting only the attack of a single host plant species and, occasionally, of its hybrids. The latter was documented by galls of Pontania samolad and those of P. arcticornis next to each other on the leaves of the hybrid S. lapponum x phylicifolia. The galls differed clearly by their specific shape which appeared unchanged on the hairy leaves of this willow hybrid: the galls of P. samolad were typically pea-shaped and hairy, whereas the galls of P. arcticornis on the same leaf were often exceptionally bizarre and completely glabrous.