Sawflies are a group of insects belonging to the suborder Symphyta within the order Hymenoptera — the same order that includes bees, wasps, and ants. There are roughly 500 species in the United Kingdom and over 8,000 worldwide. Despite being hymenopterans, sawflies differ from their better-known relatives in several fundamental ways, and their plant-feeding larvae are frequently mistaken for caterpillars.
Why “Sawfly”?
The common name refers to the female’s ovipositor (egg-laying organ), which is saw-like in structure. Females use this serrated ovipositor to cut slits in plant tissue — leaves, stems, or wood — into which they insert their eggs. This saw-toothed ovipositor is the feature that gives the group its English name and distinguishes it from the needle-like ovipositors (modified into stings) found in wasps, bees, and ants.
Sawflies vs True Wasps
| Feature | Sawflies (Symphyta) | True Wasps (Apocrita) |
|---|---|---|
| Waist (petiole) | Absent — broad connection between thorax and abdomen | Present — narrow “wasp waist” |
| Ovipositor | Saw-like; used to cut plant tissue for egg-laying | Modified into a sting (or thin needle-like ovipositor) |
| Sting | Cannot sting | Many species can sting |
| Larval diet | Herbivorous (plant tissue) | Variable: predatory, parasitoid, or herbivorous |
| Larval form | Caterpillar-like (eruciform) with thoracic and abdominal legs | Grub-like (legless or with thoracic legs only) |
Sawfly Larvae vs Caterpillars
Sawfly larvae are frequently confused with the caterpillars of butterflies and moths (Lepidoptera). However, they can be distinguished by counting the number of prolegs (the fleshy, leg-like appendages on the abdomen):
How to Tell Sawfly Larvae from Caterpillars
- Sawfly larvae: Have six or more pairs of abdominal prolegs (plus three pairs of thoracic true legs)
- Caterpillars (Lepidoptera): Have five or fewer pairs of abdominal prolegs (plus three pairs of thoracic true legs)
- Eye spots: Sawfly larvae typically have a single pair of simple eyes (stemmata) on each side of the head; caterpillars have multiple stemmata arranged in a semicircle
- Behaviour: When disturbed, many sawfly larvae curl the posterior end of their body upward in a characteristic S-shape or cling in a ring formation around a twig
Did you know? If you find a “caterpillar” with more than five pairs of prolegs, it is almost certainly a sawfly larva, not a true caterpillar. This simple counting trick is one of the most useful identification skills in entomology.
Common UK Sawfly Species
Gooseberry Sawfly (Nematus ribesii)
One of the most familiar garden sawflies. The pale green, black-spotted larvae can completely defoliate gooseberry and redcurrant bushes in a matter of days. There are typically two to three generations per year.
Solomon’s Seal Sawfly (Phymatocera aterrima)
The grey, slug-like larvae strip the leaves of Solomon’s seal plants in late spring. Adults are small, black, wasp-like insects.
Rose Slug Sawfly (Endelomyia aethiops)
The pale green, slug-like larvae skeletonise rose leaves, leaving a network of veins. They feed on the upper surface of the leaf, creating translucent windows.
Birch Sawfly (Cimbex femoratus)
One of Britain’s largest sawflies (up to 25 mm). The large, green, caterpillar-like larvae feed on birch leaves. When threatened, the larvae can squirt a jet of fluid from glands above their legs.
Large Pine Sawfly (Diprion pini)
A significant forestry pest. Gregarious larvae feed on Scots pine needles and can cause severe defoliation during outbreak years.
Life Cycle
- Egg-laying: The female uses her saw-like ovipositor to cut a slit in leaf tissue and inserts an egg (or row of eggs) inside.
- Larval feeding: The caterpillar-like larvae feed externally on leaves (most species), sometimes gregariously. They moult through 4–6 instars.
- Pupation: Mature larvae typically drop to the ground and pupate in the soil in a cocoon, though some species pupate in plant stems or leaf litter.
- Adult emergence: Adults emerge in spring or summer. Most adults are short-lived and do not feed much, though some visit flowers for nectar.
- Reproduction: Many sawfly species can reproduce by parthenogenesis (unfertilised females producing viable offspring).
Ecological Significance
While some sawflies are garden and forestry pests, they also play positive ecological roles. Sawfly larvae are an important food source for birds, parasitoid wasps, and other predatory insects. Some species contribute to nutrient cycling by feeding on and breaking down plant material. Wood wasps (Siricidae), a family of sawflies, bore into wood and introduce symbiotic fungi that help decompose dead timber.
Key Takeaway
Sawflies are plant-feeding hymenopterans whose caterpillar-like larvae are distinguished from true caterpillars by having six or more pairs of abdominal prolegs. They lack the narrow waist and sting of true wasps. Several species are significant garden and forestry pests, but they also serve as food for birds and contribute to nutrient cycling in ecosystems.