Plant Doctor Archive

Stemming borer scourge

MY albizzia has grubs in the branches. They bore through the centre and the branches appear to be bleeding. What can I do?

I HATE to say it but it sounds like the native insect known as lemon-tree borer has attacked your poor albizzia. It's a beetle that evolved among native plants but has taken a liking to many of the exotics we now grow in our gardens. Over the years it has caused extensive damage to citrus trees, hence its common name, but it can attack a wide range of ornamentals.

The adult beetle is quite common in spring and summer, flies at night and lays its eggs in the bark of susceptible trees. The larva hatches, bores into the branch and burrows along its length, creating tunnels that can weaken the branch and may eventually kill it. Badly affected trees can lose so many branches that the tree is weakened and total removal is the only option. The good news is many trees can survive quite happily with some borer damage and may recover to live long, fruitful lives.

It's virtually impossible to control borer with sprays. Some claim you can kill the grubs by poking fine wires into the holes or injecting insecticide in with a syringe, but I've always found that harder than it sounds - just finding the entry points is a challenge. I suggest as soon as you see signs of borer damage, you prune off as many of the affected branches as practicable. Make sure to cut well past where the borer has travelled - if you see borer tunnels in the branches cut back into solid healthy wood.

The female beetle may be attracted to fresh bark wounds, so an application of pruning paste to the cut surfaces could help. Try using Bacseal Pruning Paste which may create a physical barrier to deter the beetle from laying eggs around fresh pruning wounds.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.