Plant Doctor Archive

Brittle robinia - 1

I PLANTED a Robinia 'Frisia' about 10 years ago which grew really well and looked a picture every summer until a couple of years ago. Last summer several branches hardly grew any leaves. Now they look dead and brittle. From what I've heard, these symptoms sound a bit like root disease. Have you any tips that might help me?

IT'S more likely your robinia has been attacked by lemon tree borer. The borer is a native beetle that lays its eggs in quite a range of trees, including robinias and citrus. The female beetle lays eggs in spring and the larvae hatches out and bores into branches, then spends up to the next two years boring tunnels, feeding and growing larger before pupating and finally emerging as an adult beetle. Some tunnels open to the outside. If you look carefully you may find these openings with tell-tale droppings coming out.

The damage the borer causes can result in whole branches gradually dying. Unfortunately, there's no easy way to control them. Prune out all the branches that look as if they're affected, cutting back to healthy wood. Burn them or put them through a chipper to kill the larvae. You can try poking thin wire down the tunnels in an attempt to kill them or squirt kerosene into the holes with a syringe, but neither method has a high likelihood of success.

Fertilise the tree well and water in summer to encourage new growth which, with luck, will stay free of borer.

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