Plant Doctor Archive

Tree borer

I'VE just found a pile of sawdust at the base of one of my best-loved roses, where a weevil has bored into the bush through a dead branch. I know my lemon tree, two metres away, has this weevil and we've lost a camellia the same way. Can I save this rose and lemon tree, or is it too late? The hole goes in some way on a downward angle.

THIS sounds like a simple case of lemon tree borer, a native beetle that attacks quite a wide range of trees and shrubs. The female beetles lays eggs in spring and the larvae hatch out and bore into branches then spend up to the next two years boring tunnels, feeding and growing larger before pupating and finally emerging as adult beetles. The sawdust you spotted are remains of the wood that has passed through the borer and been discarded from its tunnel.

Borer can be difficult to control as they're quite well protected inside their tunnel. Rose expert Doug Bone suggests you cut back the affected branches until you reach solid wood with no sign of tunnelling. You can cut every shoot off a badly affected rose bush and it will usually recover.

If you spot damage on the lemon tree early enough you can sometimes kill the borer by poking a thin wire into the tunnel or squirting kerosene or insecticide into it with a syringe, otherwise cut back the damaged branches to healthy wood. Unless they're severely attacked, lemon trees can often last for years with some borer damage.

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