Elm zig-zag sawfly. Left: characteristic damage from young larvae. Centre: late instar larva. Older larvae still eat between major veins, but don't leave the zig-zag pattern. Right: pupal stage in silk basket.
Sawflies complete their life cycle within one year, with some species having two or three generations during the summer. The females deposit eggs on or in their host plants and these hatch a few days or weeks later. Female sawflies have an ovipositer formed from a pair of blade-like structures, sometimes with a saw-tooth edge, that they use to make incisions in leaf blades, petioles or stems when they are inserting eggs in the larval food plants.
Some species when laying eggs also secrete chemicals into the plant that induce abnormal growth in the plant. Rose leaf-rolling sawfly females cause the leaf margins of expanded rose leaflets to roll downwards to form tubes in which the larvae will subsequently feed. Similarly Pontania species secrete chemicals that induce the growth of galls, shaped like baked beans, in willow leaves within which the larvae will feed.
Those of sawflies generally go into the soil but some bore into rotten wood or spin cocoons in sheltered places above ground level. Most sawflies overwinter as non-feeding prepupal larvae that will pupate a few weeks before the adult insects emerge in spring or early summer. A few sawflies overwinter in the egg stage.
Role of sawflies and woodwasps in gardens
Woodwasps are generally not a problem in gardens, although adults sometimes lay eggs in or emerge from pine stems used to make rustic pergolas and bird feeders.
The majority of sawflies are not garden pests. Their larvae feed as individuals on grasses, sedges, rushes, ferns, horsetails, wild flowers, forest trees and shrubs. Sawfly larvae provide food for some birds and other insect feeders; they are also attacked by some parasitic wasps and flies.
Some sawflies can be damaging to garden plants, particularly those with larvae that can cause extensive defoliation. Fortunately, defoliated plants will recover, although they may be less vigorous in the following year. Heavy infestations do not occur every year. A sawfly that caused extensive damage one year may be completely absent in the following year, possibly because the larvae devoured all the leaves before they could complete their feeding.
It would be fair to say that the biology of sawflies in gardens is poorly known, and this is an areas where keen amateurs could make a significant contribution to science.
Other sources of information
Checklist of British and Irish Hymenoptera: Symphyta
Benson, R. B. (1951) Handbooks for the Identification of British Insects Vol 6 part 2a. Hymenoptera – Symphyta
Benson, R. B. (1952) Handbooks for the Identification of British Insects Vol 6 part 2b. Hymenoptera – Symphyta
Benson, R. B. (1958) Handbooks for the Identification of British Insects Vol 6 part 2c. Hymenoptera – Symphyta
All the above are out of print but can be downloaded from here.
Wright, A. (1990) British Sawflies (Hymenoptera: Symphyta) A key to adults of the genera occurring in Britain. A Field Studies Council AIDGAP key
Page drafted by Andrew Halstead, reviewed by Andrew Salisbury edited by Steve Head