Plant Doctor Archive

Pesky weevils

A FEW weeks ago you described how to control fungus gnats on polyanthus, but mine have larger worm-like bugs that seem to have eaten away almost all the base of the plants. Any ideas?

IT SOUNDS as if the problem is weevil larvae, sometimes called vine weevils. There are many different types of weevils, most of which are grey or black beetle-like insects but with more prominent "snouts" than real beetles.

Adult weevils can feed on a wide range of plants, often causing distinctive holes in leaves, and the larvae can be destructive root feeders on many annuals and perennials.

Adult weevils are relatively easy to control with insecticides like Mavrik and Confidor, but, protected by the soil, the larvae are a more difficult target. The most widely recommended insecticide for soil use is diazinon, which is available in liquid and granular forms from most garden centres. Follow the label instructions carefully and try to restrict use to the affected areas only as it can kill most soil insects - the good as well as the bad.

Some gardeners claim to have had success reducing weevil larvae damage by cultivating the soil thoroughly, to destroy pupating larvae, immediately after removing affected plants.

Avoid planting the same type of plants in that area for at least a year or two in an attempt to break the cycle. You could sow a green cover crop like oats or mustard and leave it to grow over winter, then dig it in, before it sets seed in spring, to improve the soil. Then plant vegetables or unrelated ornamentals for the next year or two.

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