Willow Sawflies
Few, if any, plants are more liable than Willows to be injured by Sawflies. By far the larger number of the species that feed on Willows and Sallows belong to the genus Nematus, but a few belong to two or three other genera. It would be useless to attempt to describe, or even to enumerate, the many kinds that have been found on the different species of Salix; suffice it to say that Cameron, in his "Monograph of British Phytophagous Hymenoptera," enumerates as British thirty-three species of Nematus, and twenty species under other genera, found on Salix; and that yet others have been recorded from the Continent of Europe as found on Willows. We cannot, therefore, do more than indicate briefly the chief modes in which these insects may prove hurtful.
The larvae of many species feed, exposed to view, on the edges or the lower surface of the leaves, often keeping the hinder part of their bodies coiled in a spiral. Others live between folded or rolled leaves; and a few live in mines in the leaf-blades. Many species are Gall-makers; and of these the habits and the galls vary greatly. Several species, chiefly of Euura, produce swellings of the branches, mostly in those of the cinerea section of Salix. Each swelling is occupied by one larva. Others, mostly species of Nematus, produce galls of two or three forms on the leaves. One common kind of gall resembles peas in form and size, and is usually adherent to the lower surface; though on some Willows (e.g., S. purpurea) the galls are more often fixed to the upper surface. The Pea Galls vary in surface-appendages according to the species of Willow, being smooth on some, downy on others. They are formed by Nematus pedunculi, and by several other species of this genus of Sawflies. Another very common form of gall on Willow leaves-often distinguished as "Bean Galls," from their shape-sometimes resembles small haricot beans in pairs, side by side, in the tissue of the leaf, projecting about equally from both of its surfaces. They are usually about 1/3in. long by 1/5in. broad. They are green or red on the upper surface, pale green below, and may be smooth or downy, according to the kind of Willow on which they are formed. One of the commonest Gall-makers of this group is Nematus gallicola, which makes the galls so often seen on the leaves of Salix alba, S. Caprea, S. fragilis, and other Willows; other species form somewhat similar galls on several of the smaller Willows, e.g., Salix Lapponum, S. nigricans, S. phylicifolia, &c.; but it is impossible, in the brief space at disposal, to enter into a fuller discussion of these galls and their makers.
Remedies are scarcely called for unless the larvae of Willow Sawflies are very plentiful. If so numerous as to endanger the health of the plants, those larvae that feed exposed on the leaves may be dislodged by jarring or shaking the branches. They may be shaken into a vessel, and destroyed by having boiling water poured over them; or a ring of tar and cart-grease painted round the base of each tree will prevent them from crawling up again. The only reliable remedy against larvae in rolled or mined leaves, or in galls, is hand-picking; but the damage is seldom so great as to call for so tedious and costly a remedy. As by far the greater number of the species burrow into the soil to become pupae, any means to prevent their doing so tends to lessen the numbers of the insects. Rolling the ground below the trees, so as to render it compact, and applications of gas-lime, soot, or other substances poisonous or disagreeable to the insects, will prove useful in this way.