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The grazing activities of the slug-like larvae of the rose slugworm sawfly can cause leaves on roses to turn brown and dry up.
Rose slug sawfly (Endelomyia aethiops) on rose
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. The adult sawflies 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.
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.
Light infestations can be tolerated as the damage to a few leaves will not affect the vigour of plants.
Regularly check the plant from late May onwards for slugworm larvae and if feasible pick them off by hand.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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