Read the damage first: ragged edges, pinholes, see-through patches and black droppings each point to a different caterpillar. Once you know the likely culprit, the usual home-garden answer is the same: check often, pick them off, and cover young plants with fine net before moths and butterflies lay eggs.

Not every caterpillar is a pest, so work out what is eating the plant before you act.

What does the damage pattern tell you?

Start with the leaf, then look for the animal. Check the undersides too, because many caterpillars feed there.

Frass is the other clue. These are the small dark pellets on or under a damaged leaf. Fresh, moist frass suggests the caterpillar is close by, probably above the pellets.

Treat any single table row as a lead, not a verdict. Many species look alike, and damage can come from snails, slugs or beetles instead. If the cause is still unclear, look at the bigger picture in the overview of common Australian garden pests.

How do you find caterpillars that hide?

Camouflage is the main problem. Many caterpillars match the colour of the stems and leaf veins they sit on.

1. Look at the right time. Night-feeding caterpillars are easiest to find after dark. Take a torch an hour or two after sunset and check the plants showing fresh damage.
2. Follow the frass. Look on the leaf below the pellets, and on the stem above.
3. Check undersides and centres. Turn leaves over. Part the heart of cabbage-family plants, where webbing and group feeding show up.
4. Look for eggs. Egg masses on leaf undersides are easier to remove than grown caterpillars. The Pacific Pests page lists removing egg masses and larvae as a control step.
5. Repeat every few days. One search rarely catches everything, and new eggs hatch.

What is the simplest way to remove them?

Hand removal is the method sources keep returning to. Professional Pest Manager describes physical removal in the home garden as generally the best course for cluster caterpillar, and as a viable option for cabbage white and loopers.

Drop the caterpillars into a container of soapy water, or relocate them well away from the vegetable patch if you would rather not kill them. Wear gloves when hand-picking, and leave hairy or spiny caterpillars you cannot identify for a gloved hand or a stick rather than bare fingers.

Squash or scrape off egg masses when you see them. Cut off and bin a badly webbed leaf head rather than trying to pick every tiny larva from it.

Does netting actually stop them?

Netting stops the egg laying, which is the step you need to prevent. Butterfly and moth adults cannot lay on plants they cannot reach.

• Use a fine insect-proof net over hoops, so it does not rest on the leaves. Moths can lay through a net touching the foliage.
• Fit the net before plants are attacked, ideally at planting.
• Seal or weigh down the edges completely with soil, pegs or boards.
• Check under the net now and then. If a caterpillar or pupa was already on the plant, it is now protected from predators too.
• Rotate where you grow brassicas. The Pacific Pests page advises planting new crops away from old ones to reduce infestation.

Netting also keeps out bees, so it suits leafy crops that do not need pollination. Leave it off flowering crops such as pumpkins and tomatoes. For more on pests and pollinators together, see the guide to dealing with aphids while protecting bees.

Can natural enemies do some of the work?

Yes, within limits. Spiders, lacewings, ladybirds and parasitic wasps all feed on or attack pest insects. The evidence in the sources used here is firmest for cabbage white, where introduced parasitic wasps are credited with some help. Professional Pest Manager says two species introduced in the 1940s appear to have helped keep numbers down.

The Castlemaine Field Naturalists describe how the wasp Cotesia glomerata develops inside a cabbage white caterpillar and then spins small cocoons beside it. Leaving those cocoons in place helps control the following season’s caterpillars. If you find a caterpillar surrounded by small yellow-white cocoons, do not remove them.

To encourage natural enemies:

• Keep a few flowering herbs or flowers growing near the vegetable patch, such as alyssum, dill or coriander left to flower.
• Provide mulch, ground cover and shade so predators have shelter.
• Avoid spraying where you can. Broad-spectrum products can kill the beneficial insects as well as the pest. If you decide a spray is justified, check the product label and your state agriculture department first.

Natural enemies work slowly. They reduce numbers over weeks; they will not rescue a seedling being eaten tonight.

When should you leave the caterpillar alone?

Some leaf damage is the cost of having butterflies. Many caterpillars are the young of native butterflies and moths, and each one eats little. The City of Kingston says native caterpillars eat only one or two leaves each, and recommends a mix of host plants, particularly native grasses.

A practical rule:

• Tolerate caterpillars on ornamental and native plants if the plant is healthy and the damage is cosmetic.
• Act when they are on food crops, on young seedlings, or in numbers large enough to strip a plant.
• Do not guess. If you cannot tell a pest from a native butterfly caterpillar, relocate it onto a similar plant away from the vegetables rather than killing it.

The same logic applies to cabbage white. HerbiGuide says control is not economic in most years, which fits a home garden where a few eaten outer leaves rarely cost a harvest.

What mistakes make it worse?

• Treating before identifying. Chewed leaves can come from snails, slugs, beetles or grasshoppers, which need different responses.
• Searching only in daylight. Night feeders hide on the soil, under pots or low on the stem.
• Netting plants that already carry eggs. Check first.
• Weakening the plant. A stressed plant tolerates damage poorly. Poor drainage can make this worse, especially in containers; see the guide to root rot in pots if a chewed plant is also wilting or yellowing.
• Removing parasitised caterpillars and cocoons. You lose the natural control.
• Expecting zero damage. Aim for a harvest, not a flawless leaf.

What depends on your climate?

Timing varies by region, so use your own observations over a calendar. In the tropics and subtropics, caterpillars can be active across much of the year and brassicas are often grown in the cooler, drier months. HerbiGuide records most cabbage white damage from January to late autumn, but that is a general description rather than a rule for every location. In temperate and cool areas, activity usually tails off in winter. Cool Tasmanian and highland gardens typically see the main pressure in the warmer months. In arid regions, irrigated vegetable patches can still attract egg-laying adults.

What is well established: hand removal, netting and avoiding broad-spectrum sprays all support control. What varies: how many generations occur each year and which species dominate. For a specific local pest, ask your state agriculture department or a local nursery.