Why Are My Cherry Leaves Being Skeletonised?

Leaves that have had all their green tissue removed, leaving only the pale, papery network of veins behind, are a distinctive form of damage with a very specific cause on cherry trees: the cherry slug sawfly (Caliroa cerasi). This pest has nothing to do with true slugs — it is the larva of a small wasp-like insect — but the larvae look superficially slug-like, which gives the pest its common name. The damage is unmistakable and, in heavy infestations, can leave large sections of the tree looking bleached and tattered.

Identifying cherry slug sawfly

The larvae are small (up to 12 mm), dark olive-green to black, with a shiny, slug-like appearance due to a slimy coating. They feed on the upper leaf surface, rasping away the green tissue without penetrating through to the underside. The result is areas of leaf turning pale or white-brown as the tissue dries, while the vein network remains intact — hence the "skeletonised" description. Affected leaves eventually turn brown and die. There are typically two generations per year: the first in June and July, and a second in August and September.

Why it matters

A single cherry slug sawfly larva causes relatively little harm, but populations can build rapidly. Heavy infestations in summer, particularly the second generation in August and September, can defoliate significant portions of the canopy. While this rarely kills a mature, established tree, it weakens it by reducing the photosynthesis available to build carbohydrate reserves for the following season's flowering and fruiting. Young trees are more vulnerable than established ones.

Non-chemical control

The simplest and most effective control for small trees is dusting the larvae with a fine layer of kaolin clay, wood ash or talc. The dry powder desiccates the slimy larvae within hours. Apply on a dry day when no rain is forecast for 24 hours. Alternatively, wipe the larvae off by hand wearing rubber gloves, or spray with a forceful jet of water to knock them from the leaf surface, where they cannot easily return.

Insecticidal controls

For larger trees or heavy infestations, insecticidal soap or pyrethrin-based sprays give effective control when applied to the leaf surface where larvae are feeding. Apply in the evening to minimise impact on daytime pollinators. Avoid broad-spectrum persistent insecticides as they have a disproportionate impact on beneficial insects.

Soil cultivation and winter management

Sawfly pupae overwinter in the soil beneath the tree. Cultivating the soil shallowly around the root zone in autumn exposes pupae to birds and frost, reducing the following year's population. Do not cultivate deeply, which damages surface roots.