Caterpillars generally live on vegetable matter - ie leaves, flowers, stems, roots, tubers etc. So for practical considerations, caterpillars are seen as severe garden pests that have to be controlled.
However, against that, bear in mind that some caterpillars are the young of beautiful butterflies and moths. This being the case, it is a little hypocritical to plant insect attracting plants and shrubs such as the Buddleia - butterfly bush - in order to attract butterflies to your garden, then destroy the offspring - caterpillar.
The other side of the argument would come from a vegetable gardener who has had Brassica crops decimated – almost overnight – by the caterpillars of the common Cabbage White Butterfly. Damage to Brassicas – the Cabbage family – is probably the greatest threat. This is certainly the case in commercial crops.
Get to know your butterflies basically and be happy to see the wandering red Admiral Butterfly, whose young – caterpillars – feed almost exclusively on stinging nettles!
A good balance can normally be attained by recognising the caterpillars/butterflies that are the real garden pests - the cabbage White for instance, and perhaps those that you can tolerate for their beauty - such as the Elephant Hawk Moth.
Other than a sighting of the fluttering Butterflies, the first signs of damage will normally be large holes taken out of the foliage. Delay will often mean that you will be left with little more than the bare ‘ribs’ of the plant leaf.
Butterfly on Asclepia flowers - seeking the nectar
Most caterpillars can be controlled, simply by picking them off the damaged plant, others will need a little bit more severe treatment by chemical spray. If the infestation has already started, then a contact insecticide will be needed. This type if insecticide will need to be sprayed on the caterpillars direct. Be sure not to spray when plants are in flower and attracting beneficial insects. Spray in early evening when such insects are less active. Otherwise a systemic spray – absorbed into plant tissue – will give good control
Sprays containing Pyrethrum are suited for vegetable infestations, oy Provado vegetable and Fruit Spray.
Damage is normally to the edges of leaves, but also sometimes to the inner parts of the leaves. There are several moths and butterflies which are a severe problem, and will need chemical treatment.