It is common these days for people to come into the garden centre either describing or bringing in branches of Citrus trees (commonly lemons) which have thick knobbly sections or rounded bumps on them wondering what it is and how to treat it.
These galls or cankers are caused by a native pest called the citrus gall wasp which usually parasites native plants in its home areas of northern NSW or southern Queensland. It has unfortunately taken a liking to introduced citrus especially lemons and grapefruit but also oranges and other citrus to a limited extent.
Its natural distribution is far north of here around the NSW and Queensland border. Unfortunately over the last decade or so it has become more and more common in the gardens around Victoria and Melbourne.
The adult gall wasp lays eggs on the stems of the host citrus plant from late spring on new young stems. The eggs hatch and the larvae burrow into the stems and cause the plant to form cankers around them as they develop inside. When they are mature in late spring they come out of the gall by burrowing leaving pin sized holes.
A severe infestation can lead to a loss of vigour and dieback of the tree. Therefore it is essential to reduce the infestation by treating the tree as in a couple of years it can lead to very reduced vigour and cropping of the tree.
There is no recommended insecticide or similar treatment for this pest and the recommended treatment is to remove the galls with larvae in early spring before the next generation of wasps appears.
Another treatment is the use of Sticky Traps which we stock in the nursery hung in the plants from early spring onwards. These long yellow tubes attract the emerging adult wasps and they get stuck on the exposed glue.
My suggestion for treatment is to remove as many galls as possible around August and hang the sticky traps in the tree around this time too. Feed generously in spring to ensure that good growth develops to replace the one year old growth that has been removed and the tree remains in good health.