Pest and Disease Spotlight – Tortrix Moth and Leaf Miners
August 1, 2023

By Kate and Len Lucas

These two pesky varmints have been grouped together because they are quite small, and each has a unique way of disfiguring the leaves, almost without you noticing until the damage has been done.

Let’s discuss the Tortrix moth first.

The adult insects are proper moths, and as is usually the case, it’s the caterpillars that do the damage. They are sneaky because they hide away by rolling themselves up in the leaves. The moths themselves are quite small, so the caterpillars are tiny as well, and by the time you realise you have an infestation, some damage has already been done. Once the eggs hatch, the caterpillars waste no time making a den by joining one or two leaves together with silken threads or rolling themselves up inside one leaf to munch away. These attacks are not fatal but do spoil the look of the plant. You may find them in large numbers, and you will often see the adult moths flitting about in your garden. Some have attractive markings, and don’t think for one minute that they are confined to garden plants; we have found them on house plants as well!

We regularly deal with infestations on several shrubs, especially our evergreen Viburnum davidii and newly planted Banksian roses. While there are hundreds of species of these moths, only a few cause trouble, but it’s essential to know why one of your garden plants is beginning to change its appearance.

Leaf miners are the caterpillars and grubs of quite a wide range of insects, including flies, moths, and some sawflies (which are related to bees and wasps and not flies at all), but the damage they cause looks the same. Once they hatch from the eggs, they burrow into the leaves and mine their way through the leaf by tunnelling in between the two leaf surfaces. You will know you have an attack because you will see the very clear and transparent tunnels as the grub eats the leaf from the inside. If you remove the leaf and hold it up to the light, you will see the grub inside the tunnel or the pupa if it is later in the year. Like the Tortrix moth, they are not fatal but are very frustrating as they spoil the look of the plant. Once the squiggly white lines have appeared, it’s too late. Our Aquilegia are a favourite target for leaf miners, and they will also attack house plants.

So what can you do? These pests will appear every year but will vary in their severity. You could always put up with them, and if you can ignore the damage, then there’s no problem. Like many pests, they cannot be eradicated, only kept under control. Regular inspection in the summer does work – usually with a glass of something in one hand – cut off the mined leaves from the plant and destroy them or unroll the leaves and take out the Tortrix caterpillar, very carefully, as some damage has already been done.

We have a good idea of which plants are going to be attacked every year by either pest, so we spray them with a broad-spectrum insecticide before any signs of damage, and that seems to keep them down.

The Royal Horticultural Society says that these insects in your garden are a reflection of the health of your biodiversity. In plain English, that means the more, the merrier. Well, we can report that these entomological bandits don’t bring much merriment to our garden, and what’s more, they have been eating garden plants for breakfast, dinner, and tea for millions of years, which is a long time for a killjoy.

Despite all that, you will find helpful information on the Royal Horticultural Society Website, so cheer up!