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.

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

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.

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 damaging. 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 attacked.

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

Control

Oak slugworm can be tolerated. Heavy infestations are uncommon and although the damage to trees may appear alarming, the tree will survive and in the year following a heavy infestation, they will usually produce normal leaves. There is unlikely to be any long term effect on the tree's health.

Natural enemies can play an important part in the control of this type of insect and in most years will keep it at a relatively low level. Host trees may have suffered a check in growth as a result of a heavy infestation but it will only be badly affected if severe defoliation occurs early in the growing season in successive years. In addition it can be impossible to treat large trees effectively.


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