Eiablage-Strategien bei gallenbildenden Arten der Blattwespen-Gattungen Pontania, Euura und Phyllocolpa (Hymenoptera: Tenthredinidae: Nematinae)

Abstract

The galling habit within the Tenthredinidae has evolved especially in three genera of the subfamily Nematinae: Pontania Costa 1859, the leaf lamina gallers, Euura Newman 1837, the leaf midrip-, petiole-, bud-, and stem-gallers, and Phyllocolpa Benson 1960, the leaf folders. The oviposition behavior of these gall-making sawflies on willows (Salix Linnaeus 1754) shows a stereotyped search pattern for oviposition sites. The females [FF] use their antennae and oviposition sheath in searching the suitable willow species. They continuously palpated older and younger leaves with their antennae or touched the leaf surface with the oviposition sheath, probably obtaining either tactile or olfactory informations. Pontania FF form closed galls, which are attached to the midrip or subsidiary veins of the leaf. They oviposit into developing willow leaves in spring. FF can oviposit more than once into the same leaf, and each oviposition causes a gall, containing just one larva. Pontania species make different gall types: paired, elongated sausage-shaped galls produced on the upper surface of the leaf blade, bean- or kidney-shaped galls transected by the leaf blade, and pea-shaped galls on the underside of the leaf. The larvae leave the gall and pupate in the soil in late summer. Euura form bud- (subgenus Gemmura Smith 1968), petiole-, midrip-, and stem-galls (subgenus Euura). The FF of bud-galling sawflies oviposit never into vegetative buds but only into new flower buds (= catkins) for the next year, arising in the axils of young leaves. During summer, the larvae develop inside the galls produced within the bud scales by rampant growing of the meristematical tissue so that galled buds usually look bigger than ungalled ones. Fully grown galls are often covered by a thin layer of silky or woolly hairs, the existence of which in galled buds of some willow species can be completely reduced. The larvae often emerge in late autumn, spin cocoons and overwinter on the ground between litter. In some species they pupate within the gall, often cutting holes in the bud scale and cleaning the gall room from woolly hairs. The FF of petiole-galling Euura oviposit into petioles, or as midrip-gallers, into midrips of unfolded new leaves. The last instar larvae leave the galls and pupate in the soil in late autumn. The F of stem-galling sawflies probes into the bundle of unfolded new leaves several times before she oviposits through the newly emerging leaves. She inserts her long, saw-like ovipositor into the young top of the stem. Larvae develop and make cocoons inside the galls. In some species the last instar larva cuts an exit hole in the gall in late summer, allowing unimpeded emergence of the adult in the following spring. Phyllocolpa species cause open leaf folds down the edge of willow leaves, resulting from induced swelling of the young leaf or retarded longitudinal growth on the roll side through numerous repeated ovipositor insertions into the leave blade. The egg is laid into the tissue on the edge of the leaf blade or near the midvein. Young larvae feed within the fold, late instar ones supplementary on the edge of the leaf. In some species only one margin is galled but in other species the leaf rolls are paired and the whole leaf becomes spirally twisted throughout its length. Pupation of all species is in the soil. Dissections of very young galls show that a complete gall development can also occur, if the FF of these genera (especially of Phyllocolpa) injected the gall-inducing substance but did not lay an egg.