We all love to see butterflies in our gardens, and moths can be attractive and fascinating, but we are not so keen on the caterpillars.
The larval stage of the butterfly and moth lifecycle that eats our precious plants. These seem to appear from nowhere, eat voraciously causing so much damage in a short space of time. So how can you control them, and if you do will you still get butterflies and moths in your garden.
Avoid growing host plants
The first thing to remember is that they are host specific. Caterpillars feed on pretty specific things. For example, the cabbage white butterflies and cabbage moths feed specifically on brassicas.
They are best known for the damage they do on the vegetable patch, although they will feed on ornamentals of the brassica family, for example Crambe cordifolia.
One way to prevent attack from these pests is to avoid growing the host plants. The butterflies will lay their eggs on brassicas elsewhere so that the larvae have food as soon as they emerge.
Vigilance is all important in caterpillar control. If you see butterflies around a likely host plant look for their eggs, remove the eggs and you won’t get the caterpillars. Butterfly eggs are generally round and yellow, grouped on leaves and stems.
Moth eggs are more egg shaped and white, but in similar clusters. Alternatively, as soon as you see caterpillars, pick them off. An old gardener friend of mine swore by the “finger and thumb” method of control of caterpillars.
He certainly wouldn’t have transferred them to another possible food source!
Use barrier mesh
Insect barrier mesh can be effective in preventing caterpillar damage if they are in place before the butterflies or moths are around looking for a place to lay their eggs.
This is only really practical to keep them away from food crops and is easiest to achieve when the fine mesh can be secured over a framework. Insect barrier tunnels are ideal for lower growing crops.
If using horticultural fleece or insect barrier mesh it must not be in contact with the foliage. Cabbage white butterflies, for example, are able to lay eggs through the fleece if it is touching the leaves. This type of barrier also works to prevent attack by gooseberry sawfly.
Biological control is when you introduce another organism that feeds on and destroys the pest.
These predator organisms are often microscopic and are introduced in a solution that is either sprayed or watered on to the host plants and caterpillars.
The biological control used for caterpillars is a pathogenic nematode. It’s most effective when the foliage and the caterpillars stay wet with the solution for as long as possible.
Using a pesticide
There are numerous pesticides that will kill caterpillars. Systemic chemical ones go inside the foliage and are toxic when the caterpillars feed.
These are best avoided on vegetable crops and by those that are anti the use of chemicals in the environment. Organic solutions based on natural pyrethrins are effective. These must actually wet the caterpillars because they work on contact.
They are generally available as ready-to-use hand sprays as well as concentrates that are diluted and sprayed onto the plants.
The most difficult caterpillars to control are those which roll the leaves around themselves for protection. They feed on the foliage and then have a nice safe haven to pupate. This is normally at the tips of the shoots.
Tortrix moth is a good example, a common pest of bay and Viburnum tinus. It is difficult to wet the caterpillars with contact insecticide or a biological control. Probably the best solution is to lightly prune, removing the rolled tips containing the caterpillars.