Caterpillars
Also known as Moth and butterfly larvae, Armyworms, Loopers, Spodoptera litura (tobacco cutworm)
In short
Caterpillars are the larvae of moths and butterflies. They chew leaves, flowers, buds and fruit and can strip a plant quickly, but low numbers are usually tolerable, and many are food for birds and other wildlife and become pollinating adults.
How it looks
Soft, segmented, worm-like larvae with a hardened head capsule marked with an inverted Y or V, six simple eyes on each side of the head, three pairs of jointed true legs behind the head and up to five pairs of fleshy prolegs on the abdomen, each tipped with tiny hooks (crochets). Size ranges from about 1 mm on hatching to over 10 cm in the largest species. Colour and texture vary widely: smooth, striped, spotted or covered in hairs or spines. The tropical armyworm Spodoptera litura, common across South and Southeast Asia including Malaysia, starts pale green and becomes dark green to brown with a yellow stripe along the back, reaching about 3 cm. Many species feed at night and hide by day under leaves or in rolled or webbed leaves.
Signs
Holes chewed in leaves, ragged leaf edges, skeletonised leaves, eaten buds and flowers, rolled or webbed leaves, and dark pellets of droppings (frass) on leaves or below the plant — often seen before the caterpillars themselves.
Life cycle
Egg, several larval stages (instars), pupa and winged adult moth or butterfly. Eggs are laid on or near the food plant; the larval stage is the only one that feeds on plants. S. litura passes through six larval instars and its larvae feed at night and may move en masse to new plants when food runs out.
What favours it
Plants outdoors or on balconies and verandas where adult moths and butterflies can lay eggs; warm weather speeds development.
Prevention
- Check susceptible plants often, including leaf undersides, for eggs and young larvae.
- Cover vegetables and seedlings with fine insect mesh or row cover before egg-laying starts.
- Encourage birds, predatory wasps, ground beetles and parasitoid wasps, and avoid broad-spectrum sprays that kill them.
Treatment
Start with the first step and move down only if the problem persists.
- Tolerate light damage; most plants recover.
- Hand-pick caterpillars, checking at night for night feeders, and squeeze rolled or webbed leaves to crush hidden larvae.
- Spray Bacillus thuringiensis subsp. kurstaki (Btk) on young, actively feeding caterpillars; it only affects caterpillars (not sawfly larvae) and is most effective against young larvae.
- If still needed, spinosad or other insecticide classes labelled for caterpillars — follow the product label.
Pros and cons
Pros
- Most are food for birds, wasps and other wildlife.
- Adults of many species are pollinators.
Cons
- Can defoliate small plants quickly.
- Some species have irritating or venomous hairs or spines; handle hairy caterpillars with gloves.
Look-alikes
- Sawfly larvae
- Six or more pairs of abdominal prolegs without hooks, one pair of eyes and a smooth head; Btk does not kill them.
- Cutworms
- A group of caterpillars that live in the soil by day and cut through stems at soil level at night; curl into a C when disturbed.
- Slugs and snails
- Leave smooth-edged holes and silvery slime trails rather than frass.
Affected plants
Almost any plant, depending on the species; vegetables, brassicas, ornamentals and fruit trees are frequent hosts. S. litura has been recorded on over 100 plant species.
Related
Sources
- Caterpillar — Wikipedia
- Managing Caterpillar Pests — UConn IPM
- Spodoptera litura — Wikipedia
- Cutworms — UC IPM Home and Landscape
- Is it a Sawfly Larva or a Caterpillar? — Ohio State BYGL
Written in our own words from these sources. Check a product label before using any treatment.