Caterpillar identification starts with two checks: what the larva looks like and what it is feeding on. A caterpillar is the larval stage of a butterfly or moth, but a caterpillar on a plant is not automatically causing meaningful harm. Other larvae, including sawfly larvae, can look similar, and leaf damage alone rarely identifies a species. Before treating, photograph the insect, inspect the affected plant and look for signs of continuing feeding. If you cannot identify it, it has irritating hairs, or damage is getting worse, avoid handling it and seek reliable local advice.
Start by observing the plant rather than removing the insect immediately. Note which plant is affected, where the damage is concentrated and whether you can find a larva feeding. Check leaf undersides, folded leaves, new shoots, stems and nearby fruit. Small droppings, often called frass, may collect below a feeding site and can support the case for an actively feeding larva, though they do not identify its species.
Look at how much damage is occurring and whether it is continuing. A few chewed leaves with no fresh damage call for a different response from repeated feeding on new growth or damage that is spreading across several plants. The plant’s condition matters too: a small seedling may be more affected by leaf loss than an established plant.
Do not rely on a single feature such as a green body, stripes or a particular size. Caterpillars change as they grow, and colour varies between species and developmental stages. A useful identification combines appearance, host plant, feeding pattern and location.
When asking for help, send the photos along with the plant name, location, date of observation and a short description of the damage. A picture of the larva alone may not show enough to distinguish it from a lookalike.
Feeding patterns can narrow down the possibilities, but different insects may cause similar damage. Some caterpillars eat holes through leaves; others feed along the edges, strip tissue between veins, roll or web leaves together, or attack fruit. Wind, disease and other plant-feeding insects can also mark leaves, so inspect the plant for the cause rather than assuming every hole came from a caterpillar.
| What you notice | What it may suggest | What to check next |
|---|---|---|
| Chewed leaves on cabbage, broccoli or other brassicas | A leaf-feeding caterpillar is one possibility; white butterfly larvae are among the caterpillars found on brassicas in New Zealand. | Inspect both sides of leaves and the plant’s inner growth. Look for larvae and fresh feeding rather than relying on holes alone. |
| Leaves folded, rolled or held together with silk | A leaf-rolling or webbing larva may be present, although other small organisms can also shelter in plant folds. | Open a damaged fold carefully and look for a larva, feeding marks and droppings. |
| Damage to apples or pears, or a larva inside the fruit | An internal-feeding fruit pest is possible. Codling moth is one potential cause, but fruit symptoms do not confirm the species. | Check the fruit and any entry or feeding tunnel. Save clear images and seek identification if the problem recurs. |
| Many leg-like structures along the underside of a larva | A sawfly larva may be mistaken for a caterpillar. | Compare the leg pattern and plant damage, then confirm the identification before choosing a treatment. |
| Chewing marks with no larva visible | The feeder may be hidden, active at another time, or may not be a caterpillar. | Check at different times and inspect sheltered areas, the soil surface and nearby leaves. |
On brassicas, a caterpillar is a reasonable possibility when you find both feeding damage and a larva on the plant, but the plant alone does not establish the species. The same principle applies to fruit: a larva inside an apple or pear warrants investigation, not an automatic assumption that it is codling moth.
The word “caterpillar” is often used informally for any small, soft-bodied larva. That can lead to the wrong treatment. Sawfly larvae are a particularly relevant lookalike because they also feed on plants. Their leg pattern may help distinguish them, but close identification can require a good photograph or examination by someone familiar with the group.
Other clues can help rule out a caterpillar. Slugs and snails do not have the paired jointed legs of an insect larva, and slime may be visible nearby. Beetle grubs are often found in soil or other sheltered places rather than sitting on a leaf and chewing it. These are broad clues, not substitutes for identifying the organism causing the damage.
Hair is a reason for caution, not a reliable way to name a species. Some caterpillars have hairs or spines that can irritate skin or eyes, while many others do not. Keep children and pets away from an unfamiliar hairy larva, do not pick it up, and avoid rubbing your eyes after working near it.
Choose a response that matches the confirmed pest and the scale of the damage. If you find only a small amount of damage and no further feeding, monitor the plant and check it again rather than applying a treatment. If an identified plant pest is continuing to feed, consider proportionate options first.
A broad spray is rarely a sound first step when the insect is unidentified. It can affect non-target insects while leaving the actual cause of the damage untouched. If the label does not clearly cover the pest and plant, do not assume the product is suitable.
Seek reliable advice if you cannot distinguish a caterpillar from a lookalike, if the larva is hairy or otherwise unsafe to handle, or if fresh damage continues despite your response. Professional help is also sensible when several plants are affected, a valuable tree or crop is at risk, or the problem returns year after year.
Ask the adviser to identify the organism and explain the evidence behind the identification before agreeing to treatment. They should be able to connect the pest to the plant damage and explain the proposed method, its limitations and any precautions. If an unfamiliar species appears to raise a biosecurity concern, consult current reporting advice from Biosecurity New Zealand (MPI).
A clear photo can help, especially when it shows the larva’s side, markings and host plant. Add a wider image of the damage and record your location, since similar species may look alike and not all identification can be confirmed from one photograph.
No. Many unrelated butterfly and moth larvae are green, and their appearance can change as they grow. Use markings, body shape, host plant and feeding pattern together rather than treating colour as a species identification.
Both can have a soft, segmented body and feed on leaves. Sawfly larvae often have more pairs of fleshy abdominal prolegs than caterpillars, but that feature may be difficult to see; compare clear images or ask for identification before treating.
No. A larva may be a harmless visitor, and a caterpillar’s presence does not prove it is causing significant damage. Identify it, check whether feeding is continuing and use a targeted response only if the plant is at risk.
Get advice when the identification is uncertain, damage is spreading, or the larva may be unsafe to handle. Bring photos and details of the affected plant so the adviser can assess both the insect and the damage before recommending treatment.
Good caterpillar identification combines the larva, its host plant and the damage—not just its colour. Check those clues first, monitor the plant, and choose treatment only when the pest and the need for action are clear.
