Have you noticed strange lumps appearing on the branches of your citrus trees? This is usually a tell-tale sign of Citrus Gall Wasp. This is an Australian native insect from NSW and Queensland that affects all citrus. It is now an established pest in some Perth suburbs.

The adult wasp lays its eggs inside the stem of the tree. After they hatch, a woody gall forms around the larvae usually on young green branches. The growing insect lives off the sap of the tree. Although an infestation of gall wasp may not kill a citrus tree, the damage it causes is unsightly and can weaken the tree. Repeated attacks can lead to leaf drop, spindly growth and make the tree unproductive.

The gall starts to be visible in February and are easy to see by June. The best time to check and remove the galls is from April to June, before the adult wasps emerge. Inspect the new young growth on all citrus trees and look for galls forming on young, green twigs.

If the galls are present on older branches, this may be from the previous year. Look closely at the older galls for small holes which show that the adult has already emerged.

Pruning the galls is essential for controlling this pest. Prune off new galls before the end of June. The larvae inside the gall will not survive without food and the gall wasp will not mature. Pruned material should be disposed of in green waste or general rubbish.

Galls removed after June may have more mature larvae that will mature and emerge as adult wasps. Treat these galls by placing them in a sealed black plastic bag and putting the bag in full sun for at least four weeks. The heat from this process of solarising will kill the adult.

Avoid heavy fertilising in winter and spring. The new growth is particularly attractive to the adult wasp and can encourage infestation.

Encourage neighbours with citrus trees to inspect regularly for galls. The wasp can’t fly very far so if you have an outbreak it is likely to have come from a local source.

There are no chemical controls of gall wasp registered for use in domestic gardens. Prevention of infestation is one of the best ways to control this pest.

One method of prevention is Vasili’s Citrus Gall and Wasp Spray. This is a natural and effective way to control a range of citrus problems including the gall wasp. The powder is mixed with water and spayed on the tree to form a natural barrier that also protects against sun damage. It has concentrated seaweed added for strong healthy plants.

You can also apply Green Life Soil Co. Clay Spray to treat the Gall Wasp.

There is a useful article on Citrus Gall Wasp identification and control at https://www.agric.wa.gov.au/plant-biosecurity/citrus-gall-wasp-control

This is intended as general information only. Please ask one of our horticulturists for specific advice for your situation.