Lemon tree, very pretty…

“And the lemon’s flower is sweet…

But the Aussie Citrus Gall wasp

Is impossible to beat…”

I love to use lemons in my cooking, and I understand the Melbournian obsession with the tree. Unfortunately, our old enemy the Citrus Gall Wasp has shared the love. This native pest has had its way with many a fine citrus tree, most noticeably the iconic backyard lemon tree. Originating in Queensland, the wasp has migrated its way south over the last twenty years and found its way to Victoria. Thankfully, the infestation won’t usually kill the tree unless it gets control of a very young or a very old and diseased specimen.

The wasps mate in early to late spring. The female then flys or more commonly is blown in – they are poor flyers – and lays its eggs under the bark, usually in areas of soft new growth. The larvae hatch in 3 or 4 weks and then mature and grow under the bark for 9 to 12 months.  An adult wasp then emerges and the whole process starts again. The ‘gall’ is the rounded lump the little pest leaves behind, the tree’s response to the presence of the larvae.

Treatment is difficult. Although unsightly, old galls – those with small exit holes – can be left, because the wasp has literally flown the coop. Fresh galls can be pruned off, but because you’ll need to remove the whole stem from its junction point, you will also lose the fruit growing along the stem. Try not to remove more than a third of the tree at a time, as this may over-stress the tree. Prunings should be burnt or bagged.

Fertilise in late autumn or winter rather than spring. High nitrogen fertilisers applied in spring promote rapid sappy growth, which the wasps crave.

From mid-August to October, those sticky yellow strips can be used to attract and trap the little terrors. Make sure you take them down before November as they also attract beneficial insects.

Let the battle begin, and good luck. You’ll need it!