Plant Doctor Archive

Brittle robinia - 2

SOME of the branches of my Robinia 'Frisia' are quite brittle and there are holes riddled throughout the trunk, indicating borer. The roots of the tree also seem a little unstable - I can lean against the trunk and the whole tree moves slightly, roots and all. Is there anything I can do to save the tree - any treatments available for the borer? Also, is it possible the borer from outdoors will migrate indoors into my furniture?

IT sounds as if your robinia has suffered severe borer damage. If enough of the stems are damaged, roots will also die off, causing the movement you describe. It's a native insect which can affect quite a wide range of trees and shrubs, and while the adult only lives for two months or so, the larva, which causes all the damage, can spend almost two years boring tunnels causing major damage to otherwise healthy trees.

Unfortunately, there's little you can do to control this type of borer unless you're lucky enough to catch it before the larvae has tunnelled very far into the stems. During summer, look out for signs of fresh borer activity indicated by fresh droppings, which look like blobs of fine sawdust exuding from holes in the stems.

You could then try pushing a thin wire into the holes, which may pierce the larva, or injecting kerosene into the holes, but neither method has a high likelihood of success. You could prune out dead and dying branches and encourage fresh growth by watering and feeding, but if the borer has invaded the main trunk there's not a lot you can do. Some robinias survive for years with quite major borer damage, but they're unlikely to reach their full potential, so it might be best to remove the tree and replace with something similar, like Gleditsia 'Sunburst', which in my experience seems less susceptible to borer.

The one piece of good news is that this particular borer only feeds on living trees and shrubs and isn't interested in feeding on your furniture. The borer that does affect house framing and furniture is the larvae of an entirely different fly.

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