A Galling Problem

Is your lemon tree a bit too lumpy and it’s not just the lemons?

The back yard lemon tree is a much loved institution in many suburban backyards.

But an insidious pest has been making its mark. It starts slowly with small oblong shaped lumps or tumours appearing on branches. If left unchecked eventually these swell to take over large sections of otherwise healthy branches. While unsightly the tree still fruits and many people leave it that way. Unfortunately like any invasion, if left too long the tree weakens and your much prized lemons are not as juicy or sweet  until the point where there are no more lemons – there goes the lemon meringue!

The culprit is the Gall Wasp (Bruchophagus fellis) a native insect pest that used to infect only the Australian native finger lime but with the popularity in citrus trees has flourished in our backyards.

The Gall Wasp has a 12 month lifecycle starting each spring when the female flies around looking for tender stems on trees. The female Gall Wasp pierces these new stems inserting up to 100 eggs at a time. The larve hatch after two to four weeks and begin to feed on the fresh stem over 10 months. The tree’s defence is to form the prominent ugly tumours we see. Eventually the young adult wasps break out of their holes leaving the branch marked with pitted tumours fly around and begin the cycle again often in the same already damaged tree.

The good news is gall wasps don’t like to fly very far, the bad news is in the modern back yard with our smaller and smaller gardens this is perfect for them.

I was confronted with this problem when I brought an old suburban home and with it a lemon tree itself so riddled with tumours that I had to take drastic measures and cut it to a stump. It also didn’t help that both sides of the fence were breeding grounds for the wasp with two gall ridden lemon trees.

The stump has since sprung back and is producing lemons but I have not been able to completely eradicate the gall wasp mainly due to many of the neighbours around me not taking any action to stall or halt its progress.

But I found there were actions I could take.

  • I pruned in autumn when the wasp was not as active. Juicy green stems in spring are too much of an enticement.
  • I didn’t use a sprinkler under the tree as a humid under story is appealing to the gall wasp.
  • I used yellow fly tapes in spring, these can be brought from nurseries or home improvement stores. They hang from the branches and attract the wasp as it flies around. Yellow also being an attractive colour to insects (think of how much yellow flowers there are!) I found they were somewhat effective but needed to be used in conjunction with other methods.
  • As much as it pained me to do it particularly if a juvenile lemon is forming I pruned off the stem at least three centimetres down from the tumour and still try to do this.
  • While not quite as effective as the full prune I also shave off the first layer of the tumour exposing the tiny holes. The gall wasp larvae hate this exposure and eventually die.
  • I also put any cut stems into waste bags, tie them up and dispose of them in the garbage not in the green waste bin. The larvae are survivors and can grow to maturity in a cast off stem spreading the problem anew.

And nature has also come up with its own defence, it’s the parasitic wasp Magastigmus brevivalvus these wasps insert their eggs into the gall wasp killing it. Some places sell the wasp but this seemed to be only in Queensland and living in Melbourne this wasn’t an option for me. Also while these wasps are effective there is no guarantee that these good wasps are in your vicinity so human action is usually needed and pesticides avoided just in case.

So prevention and action is better than losing your lemons. And while I don’t think I will be able to completely eradicate the gall wasp I know if I keep up my efforts and talk to my neighbours and maybe entice them with a lemon meringue my battler tree will continue to provide me with delicious lemons for many years to come.
This article featured in Backyard farmer (issue 12)

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