The eucalyptus gall wasp is a minute (<1mm) black insect whose larvae develop inside raised galls that form on eucalyptus leaves during spring.

Gall wasps are a group of the bees ants and wasps (Hymenoptera) that as larvae feed within plant material. They feed within altered plant growths (galls) that are a result of their presence. Eucalyptus gall wasp is unusual for a chalcid wasp as most wasps in this family are parasitoids of other insects. There are many other species of gall wasp but most of these (over 70) are in the family Cecidae.

There are more than 900 plant gall forming animals in the UK, including mites, beetles, flies, gall wasps, plant sucking bugs (psyllids), aphids and sawflies.

The British Plant Gall Society encourages and co-ordinates the study of plant galls in the British Isles.

• Slightly raised swellings, about 1mm in diameter, develop on either side of eucalyptus leaves
• These pinkish-brown galls can be mistaken for a physiological disorder known as oedema, but the galls are hollow and in spring and early summer each contains a tiny white grub. The galls are of uniform size and shape 
• Oedema growths are solid and more irregular in size and shape
• The galls are most noticeable in early spring when it can occasionally cause heavy leaf fall

The gall wasp does not affect the long term health or vigour of the tree, but can affect its appearance. Its presence must be tolerated and insecticides are unlikely to give control. 

• The black adult wasps are 1mm long and they emerge in late May to June
• Females lay eggs on new eucalyptus foliage in spring, and the larvae develop within the leaves, although the raised galls do not become particularly obvious until the following spring, when the mature larvae pupate within the galls