Oak slugworm

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

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Oak slug sawfly (Caliroa annulipes) on Oak (Quercus sp.)

Quick facts

Common name Oak slug sawfly or slugworm
Scientific name Caliroa annulipes
Plants affected Oak (Quercus) and lime (Tilia) trees
Main symptoms The surface of leaves is grazed away these areas dry up and turn brown
Most active Summer

What is oak slugworm?

Oak slugworms are the larvae of a sawfly. The adult sawflies have black bodies about 7-8mm long and two pairs of blackish transparent wings, they emerge during spring. They are part of the biodiversity healthy trees support. 

Eggs are laid on the foliage and they hatch into larvae that are pale green with a slimy slug-like appearance. 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 12mm 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 generations of larvae a year between May and August. The second generation in mid to late summer can be the most noticable. In some years there is a third generation of caterpillars in the autumn. Larvae that complete their feeding late in the year overwinter in cocoons in the soil and adults emerge in spring the following year.

Symptoms

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 sometimes known as window paining.

This species is primarily found on oak (Quercus) and lime (Tilia) trees. Occasionally birch (Betula) and willow (Salix) trees are also hosts.

Other species of slugworm cause similar effects on pear, cherry, and roses.

Control

Oak slugworm is part of the biodiversity healthy host trees support. Heavy populations are uncommon and although the damage to trees may appear alarming, the tree will survive and in the year following a heavy damage, they will usually produce normal leaves. There is unlikely to be any long term effect on the tree's health.

Natural enemies play an important part in the natural balance of a garden and in most years will keep slugworms at a relatively low level. These natural enemies are very susceptible to insecticides. Host trees may have suffered a check in growth as a result of a heavy defoliation but it will only be badly affected if severe it occurs early in the growing season and in successive years. 

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