Quick answer: Cabbage caterpillars — larvae of several butterfly and moth species — chew holes through brassica leaves from late spring to autumn. The best protection combines fine netting or horticultural fleece with regular checks, hand-picking, and growing nasturtiums as a sacrificial crop.
What are cabbage caterpillars?
Cabbages and other brassicas are a food source for the caterpillars of several species of moth and butterfly.1 The two most familiar garden offenders are the large white and small white butterflies, known scientifically as Pieris brassicae and Pieris rapae.2
Small white caterpillars are pale green with a body up to 2.5cm long, while large white caterpillars reach 4cm and are yellow with black markings.2 Both can strip brassica leaves quickly if you let numbers build.
When do they appear?
Adult butterflies lay eggs from May to June and then again from July to September.2 That gives you two distinct waves of caterpillar pressure across the growing season, from late spring right through to autumn.2
The damage shows as holes eaten through the outer and inner leaves of brassicas, and nasturtiums are also targeted by these caterpillars.2 That last fact is actually useful — nasturtiums can be turned into a deliberate sacrificial crop, with eggs and caterpillars moved from brassicas onto the nasturtium leaves.2
Step 1: Net your plants
Netting is your number one defence against cabbage caterpillars.4 Covering plants with horticultural fleece or fine netting stops butterflies from reaching the leaves to lay their eggs.4
With two egg-laying windows across the season, getting netting on early and keeping it on pays off. Make sure edges are firmly weighted or pegged down so butterflies cannot find a gap underneath.
Step 2: Spot and remove eggs
Egg spotting and removal is one of the most practical early-season steps you can take.4 Walk your brassica rows regularly and check the undersides of leaves for small clusters of eggs — catching them before they hatch stops the problem at its source.
Removing any caterpillars that appear can prevent further damage to the rest of the plant. Make checks part of your routine garden walk, not just an occasional task.
Step 3: Hand-pick caterpillars
Hand-picking caterpillars is a reliable, chemical-free approach.4 Focus on leaf undersides and along the central rib of leaves — both are spots where caterpillars tend to rest, especially earlier in the day.
Make checks part of your routine garden walk through both egg-laying periods to catch problems early.
Step 4: Grow nasturtiums as a decoy
Grow nasturtiums as a sacrificial crop and move any eggs and caterpillars you find on brassicas over to the nasturtium leaves.2 Nasturtiums are willingly fed on by the same caterpillars that attack your cabbages, so they pull pests away without you needing to destroy anything.
Decoys and diversions are a well-established tactic for protecting brassicas.4 Plant nasturtiums close enough to be attractive to butterflies, but not so close that caterpillars simply cross straight back onto your crop.
Companion planting — what helps and what doesn't
Camouflage with companion plants can make it harder for egg-laying butterflies to locate your brassicas.4 Mixing other plant species in among your cabbages and kale disrupts the visual cues butterflies rely on when searching for a host.
One persistent belief is that onions keep cabbage caterpillars away — but onions do not repel caterpillars of the European cabbage moth.3 You may want to plan brassica plantings to include other companion varieties nearby,3 but treat companion planting as support for netting and hand-picking, never a replacement.
Protecting brassicas all season long
Brassica crops are often grown over a long season, and with two egg-laying periods running from May through September, protection cannot stop after the first summer flush.2 Stay watchful from late spring right through to autumn.2
Steady moisture from a sub-irrigation reservoir — the design GardenStack planters use — supports consistent plant health and reduces the stress that leaves brassicas more vulnerable to pest damage.
A layered approach works best: netting as the physical barrier, regular egg and caterpillar checks, and a nasturtium sacrificial patch.24 Together these three tactics keep cabbage caterpillar pressure manageable across the whole season without reaching for sprays.
Frequently asked questions
How do I know if my brassicas have cabbage caterpillars?
Look for ragged holes in the outer and inner leaves of your plants. Check the undersides of leaves carefully for small egg clusters or the caterpillars themselves — pale green for small whites, or yellow with black markings for large whites.2 Both hide well against leaf surfaces.
When should I put netting on brassicas to protect them?
Get netting in place before the first egg-laying period, which runs from May to June. Butterflies lay in two distinct waves — May to June and then July to September2 — so keeping netting on throughout both periods gives the strongest protection.
Do nasturtiums actually protect brassicas from caterpillars?
Yes, nasturtiums work well as a sacrificial crop.2 Grow them alongside brassicas and move any eggs and caterpillars you find on cabbages or kale directly onto the nasturtium leaves.2
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