Sunday Sawfly: The Willow Red Bean-Gall Sawfly

A gall is the result of the interaction between a plant and a consumer of the plant that causes aberrant, tumor-like growth of plant tissues. In some cases galling results in a net benefit for the plant, e.g. the nodules with the nitrogen-fixing rhizobia bacteria that form on the roots of legumes. Some galls, especially those that deform reproductive tissues or suppress elongation of shoots, are clearly detrimental to the plant. Most of the time, though, the nature of the galling interaction is more mysterious. In the insect-plant systems that have been investigated, galls are induced by substances injected into the plant by an insect. The insects (or at least their offspring) then go on to eat the plant tissues that develop around them. So, it is tempting to think of galls as bad for plants, but the evidence to test such a hypothesis is usually just not known. Galls are often brightly coloured, red is common, and one wonders if the plants aren’t trying to attract the attention of some gleaner vertebrate (red does attract birds to fruit) or insect (many galling insects are heavily parasitized) to ‘scratch their itch’.

 Among the numerous kinds of arthropods that are known to induce galls are several hundred species of sawflies in the subfamily Nematinae (Family Tenthredinidae) that pick on species of willow (Salix) and poplar (Populus). Full-blown gallers seem to have evolved from species that cause leaves to fold or curl around their grubs. These grubs feed externally in the sense that they eat from the surface down, but are well protected inside the deformed leaves. The more derived types of galls are consistently formed by particular lineages of sawflies, and the most derived type of gallers seem to be the sawflies that induce deformities in stems, buds, or leaf petioles where the grubs feed internally. 

Members of the genus Pontania (in the broad sense) are intermediate and form blister-like structures on the leaf blade inside which their grubs feed (see Nyman et al. 2000* for all the details). Pontania proxima (Lepeletier) (a complex of similar species in Europe and possibly here) form Red Bean-Galls on White Willow. Actually, our form is a Golden or Orange-stemmed Willow, Salix alba 'vitellina'. The galls came with the willow, and in spite of a fairly heavy and continuing infestation of Red-Bean Sawfly, the 2’ potted willow we planted 6 years ago is now about 20’ tall, shading out a good portion of our backyard, and has to be constantly chopped back, away from the telephone line into the house. If the sawfly (and the chopping) has had any effect on this monster’s growth, then we haven’t noticed it). I suppose this sawfly is another accidental introduction into North America and it also has been introduced into Australia and New Zealand. One variety of its host, Salix alba 'Caerulea', is called Cricket-bat Willow, so the Aussies and Kiwis may have a better handle on any damage this sawfly may do.

 The larvae of the Willow Red-Bean Gall Sawfly are grub-like and feed in a gallery inside the bean gall. The gall can be anything from pale to green to red (usually most intense on the upper side of the leaf). The adults are small black sawflies – we have pictures of lots of these that are not identifiable, but we offer one that may look more or less like the adult (remember, take any identification based solely on a picture with a grain of salt).

 *Tommi Nyman, Alex Widmer, & Heikki Roininen. Evolution of Gall Morphology and Host-Plant Relationships in Willow-Feeding Sawflies (Hymenoptera: Tenthredinidae). Evolution 54(2), 2000, pp. 526–533