Caterpillars

(numerous butterfly and moth species)

The caterpillars of numerous species of butterflies and moths attack and eat the leaves and stems of a wide range of garden plants, often causing extensive damage.

1. Caterpillars on leaves, stems and flowers – sometimes individually, sometimes in large numbers
2. Large areas of leaves eaten away with ragged edges
3. Dark droppings on leaves being eaten or on those below
4. Sometimes fine webbing or cocoons
5. Soil-dwelling caterpillars cause plants to wilt

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.

Widespread on numerous outdoor and even indoor plants – especially in greenhouses

Most caterpillars are hungry feeders with voracious appetites, devouring large amounts of leaves and other above-ground plant tissues, including fruit and stems. Some, such as cutworms (moth caterpillars) and swift moth caterpillars, feed on the roots causing so much damage that the plants then wilt and die.

Codling moth caterpillars feed on the fruit of apples and pears, hidden away within the fruit, while those of the plum moth (called pink plum maggots) eat away the inside of plum, damson and gage fruit. Pea moth caterpillars feed on the peas within the pea pod.

Most small caterpillars that feed individually rarely do significant damage, whereas larger ones that feed in congregated groups, such as those of the large cabbage white butterfly, can quickly strip away much if not all of the foliage. The caterpillar-like larvae of sawflies strip plants literally overnight.

As they feed, caterpillars produce large amounts of excrement that drops onto the plant below where they’re feeding, spoiling them.

Once caterpillar numbers build up, control becomes more difficult. When large numbers are present, more damage is done to the plants, and this will affect your plants more severely. So, it is important to check plants regularly, especially those known to be favourite food sources, and deal with any you see – as soon as you see them.

You can easily control eggs and a small numbers of caterpillars by either squashing them or scraping them off with kitchen tissue or similar. Be careful touching those with toxic hairs

Remove and destroy the plant parts where large numbers of caterpillars are congregating and feeding.

Caterpillars, including cutworms, can be controlled using the biological control based on a mixture of nematodes.

Pheromone traps, which are designed to monitor moth activity, may also give some degree of control. There are pheromone traps for leek moth, tortrix moth, plum moth (pink plum maggots) and box tree moth. These trap the adult males, resulting in the females laying fewer fertile eggs. They should be hung in the tree early in the year, usually spring, when the adult moths are active. While using pheromone traps on their own rarely completely control the target moths, they usually significantly reduce the damage caused.

Careful hoeing around plants, so as not to damage the roots, to the extent of the branch or stem spread may kill overwintering pupae, or expose them to foraging birds and other animals.

There are numerous chemical insecticides available that will kill soft-bodied insect pests, such as caterpillars. The majority of these are what are known as “contact” insecticides. That is, you have to spray the actual insect (make contact with it) to kill it. If you miss some, or others come on the plant after spraying, these will not be affected and will carry on feeding. Also, most are based on “organic” or “naturally-occurring” materials and have very little persistence once sprayed on to the plant. This means you have to spray regularly to control them.

A systemic insecticide which also kills on contact – is absorbed by the leaves and moves all around inside the plant, so will control future attacks from insect pests, for up to two months.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to the danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

Not all insecticides have approval for use on all edible crops. Check the label before spraying.

You can protect plants from attack by covering them with a physical barrier – such as insect-proof, fine-mesh netting or horticultural fleece. Just bear in mind that plants must be pest-free before covering – otherwise you’re providing a protected environment for the caterpillars to carry on feeding  – and they can get through any breaks in the barrier. While this may not be possible for large plants or ornamental plants, where the covering will spoil their appearance or prevent pollinating insects getting to the flowers, it is perfect for the veg garden.

• Horticultural fleece
• Hoes
• Contact insecticides
• BugClear Ultra

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