What is rose slugworm?

Rose slugworms are the larvae of a sawfly. Sawflies are in the same group of insects as bees, ants and wasps (the Hymenoptera). They have caterpillar like larvae that feed on plants the adults are winged insects that can appear fly-like. Adult rose slugworm have black bodies about 4-5mm 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 damaging. 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. Damaged areas dry up and become white or pale brown; this type of damage is known as window paining.

Other species of slugworm cause similar damage on pear, cherry, lime, oak and willows, but these insects are not found on roses.

Control

Light infestations of rose slugworm can be tolerated 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 infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (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.

Non-pesticide control

• Where possible tolerate populations of rose slugworm
• 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

• If numbers of larvae are too high for hand picking, control may be achieved by spraying with pesticides. Spraying at dusk is likely to give the best results
• Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
• More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Download

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