Caterpillars are the larval stage of butterflies and moths. They range in size and colouring. Some – such as the oak processionary moth – are covered in fine hairs, which contain toxins, so be careful not to touch these.
While many adult moths and butterflies, and even some caterpillars like the hawk moths, are highly attractive and should be encouraged in the garden, the caterpillars of many species are highly destructive plant pests.
Depending on species, the female adult lays eggs in small to large batches or singly. These are laid very close to or on the food source, often on the undersides of leaves or in other places hidden from predators – and human eyes. Some species have more than one generation of adults per year, so eggs can be laid at different times throughout the year. The cabbage moth, for instance, has up to three overlapping generations.
The eggs hatch into caterpillars, which immediately start to devour their food source.
Some caterpillars are very large, brightly coloured – to warn off predators – and highly visible, while others may be very small, green and tend to hide on the plant, making them exceedingly difficult to find.
While most adult moths and caterpillars are present in spring and summer, those of winter moths, as their name suggests, emerge and lay eggs between late autumn and spring. These include the winter moth, the March moth and the mottled umber moth. The females are wingless, emerge from pupae in the soil and then crawl up trunks and branches to lay eggs. The caterpillars hatch in spring and attack most types of tree fruit and many deciduous trees and shrubs.
Some species – including box tree caterpillars, ermine moths and cotoneaster webber caterpillars – further protect themselves, by spinning webbing or even extensive cocoons, while others – such as tortrix moth caterpillars – use a silky thread or webbing to bring opposite leaf margins together and live inside the tunnel created.
When fully grown, the caterpillars stop feeding and pupate. The pupa may stay on the plant or drop onto the soil, or the caterpillars go and find a suitable, protected place to pupate. The adult butterflies or moths may emerge fairly soon afterwards or, especially later in summer, overwinter as pupae, emerging the following year.