Plant Doctor Archive

Grass grubs

THIS is our second spring living in a rural area and in late October we suffer a plague of what we believe to be grass grubs in their flying stage. They are decimating the new foliage on stone fruit trees, myrtles and some roses. We do not want to use insecticides and have been using organic natural sprays, but this is becoming expensive. Can you suggest a homemade remedy?

IT sounds like either the common grass grub beetle (also known as brown beetle) or bronze beetle, or both. Bronze beetle is a native insect which has happily moved from feeding on native plants to exotics and horticultural crops. Both beetles can cause significant damage to a wide range of trees and shrubs in spring, chewing leaf edges as well as eating holes through leaves. They are difficult to control, even with conventional pesticides, as by the time you realise you have a problem it's usually too late - the damage is done and the beetles have generally moved on. And if you do find them on plants and try to spray, they usually just fly away. In some seasons damage can be worse than in others.

If you want to attempt some sort of control without using insecticides, I can only suggest you try spraying susceptible plants with something like garlic or perhaps chilli spray which may deter the beetles from feeding on them. But you'd have to apply it in anticipation of the beetles flying, which isn't easy to time right, as weather conditions have a great influence. To make matters worse, the effectiveness of these sprays is also greatly reduced by rain.

The larvae live in the soil, so cultivating around susceptible plants in winter is said to reduce bronze beetle numbers but, like grass grub beetles, they can fly in from elsewhere. Perhaps other readers have advice they could pass on.

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