THE EUURINA GALLERS (Hymenoptera: Tenthredinidae: Nematinae: Nematini)

The willow-galling sawflies in the subtribe Euurina constitute one of the most species-rich groups in the subfamily Nematinae. The subtribe includes three main genera: Phyllocolpa species induce leaf folds or rolls, Pontania species form various closed galls on leaves, and Euura species induce bud, petiole, and shoot galls. Gall formation is initiated by an unknown secretion that the sawfly females inject into the oviposition wound along with the egg, but in some galler groups additional stimulus provided by the feeding larva is needed for normal gall growth. Each gall is usually inhabited by a single larva.

The willow gallers are distributed throughout the Northern Hemisphere, and like their host plants they are particularly common in many arctic and subarctic habitats. Euurina sawflies are very specialized in their host-plant use, and most species attack only a single willow species. Apparently this strict specialization has led to the extremely rapid radiation of the Euurina gallers on willows, and current estimates place the number of species around 400-500. Many undescribed species probably exist especially in North America and in the mountains of the Far East.

The evolutionary history of the Euurina gallers is well known as a result of phylogenetic studies based on molecular genetic data. The studies have shown that the gallers evolved from nematine species having larvae that feed externally on plants. Apparently leaf folders and rollers were the first ones to appear, and they gave rise to species inducing closed galls on leaves. The Euura gall types were the last ones to evolve. Because of this complex phylogenetic history, both Phyllocolpa and Pontania are paraphyletic genera. Gall morphology is apparently mainly determined by the gallers, and the galler fauna on a given willow species is a collection of galler species (inducing different galls) that have colonized the host lineage at different times. This cross-replication of gall types and host species makes the Euurina an ideal model group for testing many coevolutionary hypotheses.

Phyllocolpa leaf folds and rolls

Phyllocolpa species induce leaf folds or various leaf rolls on willows (Salix spp.), but a few North American species attack local Populus species. In some leaf rollers, larval feeding is initiated inside a small gall which is formed at the oviposition site (see P. purpureae below), but the larvae exit the galls and continue feeding inside the leaf roll once the gall tissue has been consumed.

Phyllocolpa excavata on Salix pentandra.

Phyllocolpa purpureae on Salix purpurea. Note the small light-coloured gall at the oviposition site in the middle of the roll.

Phyllocolpa sp. on Populus grandidentata.

Pontania leaf blade sausage galls

Galls induced by species in the Pontania dolichura -group resemble long paired sausages parallel to the leaf midrib. The hollow "sausages" are formed as a result of repeated injections by the ovipositing females, and they protrude more on the upper side of the leaf. The galled tissues are greenish.

Pontania virilis (and unknown external-feeding sawfly larva) on Salix purpurea.

Cross section of Pontania dolichura gall on Salix phylicifolia showing larval chambers.

Pontania lapponicola on Salix lapponum.

Pontania leaf blade bean galls

Leaf blade bean galls are located more or less away from the midrib of the leaf. Internal gall tissues are typically greenish. Older larvae make a hole on the lower surface of the galls in order to remove frass from the gall chamber.

Pontania bridgmanii on Salix aurita.

Pontania proxima on Salix fragilis.

Microscopic cross section showing abnormal cell growth in Pontania proxima gall on Salix fragilis (leaf blade to the left, gall chamber on the right-hand side)(Photo: Anneli Kauppi).

Pontania leaf midrib bean galls

Leaf midrib bean galls are attached to the leaf midrib, and protrude equally on the lower and upper surfaces of the leaf. The galled tissues are usually pale, but the thickness of the gall wall varies widely among species.

Pontania polaris on the arctic-alpine dwarf willow Salix herbacea.

Pontania maculosa on Salix helvetica.

Larva of Pontania pustulator inside gall on Salix phylicifolia.

Pontania leaf midrib pea galls

Pea galls are attached to the leaf midribs, but they typically protrude only from the underside of the leaf. Gall tissues are whitish, and the thickness of the gall wall varies among species.

Pontania arcticornis on Salix phylicifolia.

Pontania glabrifrons on Salix lanata.

Pontania reticulatae on Salix reticulata.


Simultaneous heavy infestation of the bean galler Pontania vesicator and the pea galler Pontania viminalis on Salix purpurea.

Lepidopteran facultative inquiline larva feeding on pea gall of Pontania glabrifrons on Salix lanata.


Euura bud galls

Females of bud-galling species oviposit through petiole bases into bud primordia, and galled buds swell to an abnormal size. Bud gallers are sometimes extremely common in subarctic willow thickets, and the loss of buds during consecutive years leads to distinct changes in the growth patterns of the host individuals.

Euura mucronata on Salix aurita.

Euura boreoalpina on Salix glauca.

Euura lanatae female contemplating oviposition on Salix lanata.

Euura petiole galls

These Euura species cause spindle-shaped galls on leaf petioles, but galls of Euura testaceipes are frequently found also on leaf midribs. In these cases the galls can be identified on the basis of their pointy shape.

Euura testaceipes on Salix fragilis.

Euura testaceipes gall on midrib of a Salix fragilis leaf.

Euura venusta on Salix caprea.

Euura shoot galls

Euura shoot gallers oviposit through developing leaf bundles into the growing shoot. As the shoot grows lengthwise, it becomes thickened and thus provides food and shelter for the growing larva.


Euura lapponica on Salix lapponum.

Larva of Euura lapponica inside gall on Salix lapponum.

Euura multilarval shoot galls

This gall type is represented by a single European species, Euura amerinae, which occurs solely on Salix pentandra. The galls are large and contain multiple larvae that live in separate cavities.


Euura amerinae on Salix pentandra.

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