Plant Doctor Archive

Caterpillars on my plants

I have caterpillars on my eucalyptus tree. How can I get rid of them?

These are the caterpillars of the gum emperor moth (see photo above — middle). You usually find them, as the name suggests, on eucalyptus species, but sometimes they'll go for liquidambar or even silver birch. They came from Australia almost 100 years ago, and they're here to stay.

Here's the problem. When a little eucalyptus gets hammered by these beautiful caterpillars, they will have some detrimental effects, basically because all the green material has been removed.  

Before we do anything about control, we need to take a look at the life cycle of the insect.

It all starts with some sizeable eggs laid on the leaves (see photo above — far right) by the female. These hatch into small black hairy caterpillars. They don't eat much at this stage, but the caterpillars change their skins in order to grow, and the bigger they get, the more they eat.

Of course, caterpillars are the feeding-and-growing stage of the insect's life cycle. They can double their weight every 48 hours. When they're getting really sizeable, that's when a lot of foliage is removed, and the caterpillar starts to produce sizeable droppings, which can be as big as raisins, but I wouldn't try eating them.

When the caterpillars are fully grown they spin themselves a nice silken cocoon, in which they change into a pupa. After a month of metamorphosis, the pupa hatches and the moth makes a hole in it and emerges.

The adult moths are a beautiful fawn-brown with pink wing tips and conspicuous eye spots. These adults then start the whole life cycle all over again.

So, what can you do to control these caterpillars on your eucalyptus?

If you're fearing for the safety of a small tree, use any insecticides you like — Carbyral, Maldison. Caterpillars are susceptible to that.

If you go organic, you can use pyrethrum or Dipel, which contains BTK and gives those caterpillars a pretty lethal gut disease.

But hang on a minute, I've got a better idea. Why not pick off the easy-to-spot caterpillars and take them to a school for their nature table? They make beautiful life-cycle studies.

Ruud Kleinpaste

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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