Rose slug sawfly or slugworm

The grazing activities of the slug-like larvae of the rose slugworm sawfly can cause leaves on roses to turn brown and dry up.

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Rose slug sawfly (<EM>Endelomyia aethiops</EM>) on rose
Rose slug sawfly (Endelomyia aethiops) on rose

Quick facts

Common name Rose slug sawfly or slugworm
Scientific name Endelomyia aethiops
Plants affected Roses
Main symptoms The surface of leaves is grazed away and the grazed areas dry up and turn brown.
Most active June to October

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What is rose slugworm?

Rose slugworms are the larvae of a sawfly. Sawflies are a group of insects in the suborder (Symphyta) of the Hymenoptera (bees, ants and wasps). There are about 500 species of

sawfly in Britain. They have caterpillar like larvae that feed on plant material and are named after the saw like egg laying organ used by females to lay eggs in plant material. Adults can come in a range of colours many are black, green orange or striped yellow and black. Most are small (<1 cm) but some species such as the Birch sawfly (Cimbex femoratus) can be over 2 cm long. Several species can be found in gardens and are part of the biodiversity a healthy garden will support. More information can be found at The Sawflies of Britain and Ireland webpages.

 Adult rose slugworm have black bodies about 4-5 mm long and two pairs of dusky transparent wings, they emerge during mid May to mid June.

Eggs are laid on the foliage and they hatch into larvae that are black with a slug-like appearance. They are pale yellowish-green with light brown heads. Their bodies are translucent and the gut contents can often be seen as a dark green line inside the body. The larvae grow to a length of about 10mm and are somewhat club-shaped with the head end being swollen. When fully fed the larvae go into the soil to pupate.

There are two or sometimes three generations a year between June and October. The second generation in July and August can be the most abundant. Larvae that complete their feeding late in the year overwinter in cocoons in the soil before pupating in April to May of the following year.

Symptoms

This sawfly does not cause any distortion of the foliage and the caterpillars feed exposed on the leaf surface. They are normally found on the underside of leaves but will feed on the upper surface if the plant is shaded.

The larvae do not make holes in the leaves but graze away the leaf surface until only the epidermis remains. Affected areas dry up and become white or pale brown; this type of damage is known as window paining.

Other species of slugworm feed on pear, cherry, lime, oak and willows, these insects are not found on roses.

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Rose slug sawfly (Endelomyia aethiops) on Rose (Rosa sp.)
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Rose slug sawfly (Endelomyia aethiops) on Rose (Rosa sp.)
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Rose slug sawfly (Endelomyia aethiops) on Rose (Rosa sp.)
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Rose slug sawfly (Endelomyia aethiops) on Rose (Rosa sp.)
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Management

Rose slugworm can usually be accepted as part of the biodiversity roses support as the damage to a few leaves will not affect the vigour of plants.

Check roses frequently from June onwards so action can be taken before a damaging population has developed. When choosing management options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section and avoiding pesticides. Within pesticides the shorter persistence products (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.

Non-pesticide

  • Where possible tolerate populations of rose slugworm, plants usually recover from defoliation with no effect on flowering 
  • Encourage predators and other natural enemies of sawfly in the garden, such as birds and ground beetles.
  • Check plants regularly from June for the presence of larvae and remove by hand where practical

Pesticides

The RHS recommends that you don't use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.
 
Where you cannot tolerate sawflies, manage them using the information above as your first course of action.
 
Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.

If you do decide to use pesticides, the shorter persistence products (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
 
The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.
 
Always follow the instructions on the products. For edible plants, make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.

Homemade products are not recommended as they are unregulated and usually untested.
Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.

Plants in flower must not be sprayed due to the danger to bees and other pollinating insects.
  • If numbers of larvae are too high for hand picking, control may be achieved by spraying with pesticides. Spraying at dusk is likely to be more effective
  • Organic contact insecticides containing natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) are available and broad spectrum so will kill a wide range of insects
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer). These products have long lasting action against insects including those that are beneficial
  • Pesticides, with both systemic (absorbed and transported through plant tissues) and contact action, are available. These include Flupyradifurone (Provanto Smart Bug Killer) for use on ornamentals and selected edibles and the neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra). These pesticides are widely considered to be the most environmentally damaging, remain active for a long time and will kill beneficial invertebrates

Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Download

Pesticides for gardeners (pdf document outlining pesticides available to home gardeners)

Biology

The adult sawfly is 4-5 mm long, black in colour with two pairs of transparent wings. The adults fly between May and June. The adult female rose slug sawfly lays eggs individually along the edges of leaves in pockets created by inserting their ovipositor (egg laying organ).

The eggs hatch into, pale green, caterpillar like larvae. These grow up to 12-13 mm (about 1/2in) long as they feed, characteristically skeletonising leaves. When mature the larvae go down into the soil where they overwinter as non-feeding larvae before pupating the following spring. There is one generation per year.
 

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