They are not true caterpillars – as close inspection will reveal. Rose Slugs or Slugworms will adequately describe them with their slimy shiny skin which looks like the common garden slug.
Rose Slugworms are the larvae of the Rose leaf Sawfly – which in spite of its name rarely does any damage to the rose leaves. It simply lays its eggs – sometimes three broods in a year – on the underside of rose leaves. The eggs then hatch out into the slug-like larvae which eat into the top surface of the rose leaf, leaving the veins and underside of the leaf looking like a partial skeleton. The damaged leaf eventually dies.
As its name suggests this is a pest of only rose bushes, though there are several other types of sawfly slugworms.
Because of its elusive habit, it generally means that no pest is seen until the leaves develop areas of partial vein skeletons as the larvae eat into the underside of the leaf – leaving the veins in preference for the soft foliage. They rarely feed on the upper surface, unless the rose is in a well shaded place. As with common slugs, they prefer shade rather than full sunshine.
The larvae - grub - of the sawfly are yellowish green caterpillar look-alikes known as slugworms.
Image by Missouri Botanical Gardens ©
These slugworms are voracious feeders – as will soon be seen by their size in a short span of time. Their food (the rose leaf) can normally be seen inside the semi translucent body – showing up in the linear route between mouth and rear end.
The larvae eventually falls to the ground, pupates and overwinters in the soil, ready to emerge as an adult sawfly in early spring.
If just a few leaves are affected, there is little need for action – unless you want the roses for exhibition or decoration purposes. The slugworms can be stopped if found in time, with most garden insecticides. Contact with the underside of the rose foliage is necessary unless you use a systemic insecticide.
The other alternative if a slight infestation, is to pick the offending grubs off by hand and disposing.
Bifenthrin is the spray if they are visible - otherwise Provado as a systemic insecticide.