Sawflies

Identification

The larvae strongly resemble the caterpillars of butterflies and moths, but have more pairs of abdominal prolegs (six and more) and single ocelli on the sides of the head, by which they can be told apart. They often feed gregariously, skeletonising the leaves or eating them down to the veins, and can strip a gooseberry or currant bush bare within a few days. The host plants are gooseberry, currant, rose, plum and pine. Symptoms: besides the defoliation of leaves, the larvae of fruit-dwelling species (e.g. on plum) bore into the young fruit set and cause it to drop. Adult sawflies are small (a few millimetres) hymenopterans without a distinct 'wasp waist' — the abdomen is broadly joined to the thorax, which distinguishes them from wasps and bees; the females have a saw-like ovipositor (hence the name 'sawflies'), with which they cut into the tissue of a leaf or fruit set and lay their eggs inside it. The larvae are most often green, matching the colour of the leaf, in some species with dark warts or spots; their prolegs lack the grasping crochets that the caterpillars of moths have, and when disturbed the larvae of many species raise the rear end of the body into a characteristic 'S' shape. The most damaging groups are: the gooseberry sawfly (Nematus ribesii) on gooseberry and currant, stripping the bush from the inside out; the slug sawflies (genus Caliroa) on pear, cherry and plum, whose slimy, slug-like larvae graze away the upper skin of the leaf (windowing), so that the leaf turns brown as if scorched; the fruit sawflies of the genus Hoplocampa on plum and apple, tunnelling galleries into the young fruit set and leaving moist, foul-smelling droppings; and the conifer sawflies (Diprion, Neodiprion) on pine, feeding gregariously on the needles. Life cycle: the adults emerge in spring, the larvae feed for several weeks, after which they descend into the soil or leaf litter, where they pupate in a cocoon and overwinter as a larva (prepupa); depending on the species, 1–3 generations develop in a season.

Treatment

Effective measures are hand-collecting and shaking off the larvae, which drop from the plant in a defensive reflex, combined with inspecting the undersides of the leaves, and spraying with spinosad or potassium soap at the early feeding stage. Birds and predatory insects should be protected. Note: Bacillus thuringiensis, effective against the caterpillars of moths, does not work against sawfly larvae, because these are hymenopterans and not moths. The most important thing is controlling young larvae — the older ones feed most intensively and are less sensitive to contact products; that is why regular inspection (especially of the leaf undersides and the inside of the bush) from spring onwards lets you hit the early feeding stage. Potassium soap and other contact products act only at the point of direct contact and briefly, so they require thorough coverage of the plant and repeat applications. Spinosad (the spinosyn group) is a biological insecticide genuinely effective against sawfly larvae, but toxic to bees and other pollinators — it should be applied in the evening, outside the flowering period and the time bees are on the wing. Under heavy pressure or on plantations, contact products from the pyrethroid group are used, targeting the young larvae; these are equally dangerous to beneficial insects, so they must not be used during flowering. Cultivation measures support protection: loosening the soil beneath the bushes in autumn and spring destroys the overwintering cocoons, removing fallen, infested fruit set limits the fruit sawflies (Hoplocampa), and white sticky traps help to monitor and catch the adult plum and apple sawflies attracted by the white colour. Supporting natural enemies — parasitic ichneumon wasps, predatory insects and birds — is a supplementary method: on its own it will not eliminate a mass population outbreak, but it limits it and works best when broad-spectrum sprays are given up.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.