Holes in kale and broccoli leaves usually mean one of two caterpillars, and they are not the same problem. Cabbage white butterfly caterpillars are the big, obvious ones that chew large ragged holes and can strip a plant fast. Diamondback moth caterpillars are smaller, harder to spot, and breed so quickly that a garden can carry several overlapping generations at once.
Both show up on kale, broccoli and the rest of the brassica family — cabbage, cauliflower, Brussels sprouts, bok choy. Knowing which one you have changes how urgently you need to act and what actually works.
Quick Identification
| Sign | More likely pest | What it means |
|---|---|---|
| Large, plain green caterpillar, smooth-bodied, chewing big ragged holes | Cabbage white butterfly | Handpick and net; can defoliate quickly if ignored |
| Small pale-to-bright green caterpillar with a dark head, wriggles violently and drops on a silk thread when disturbed | Diamondback moth | Check leaf undersides; multiple generations can be present together |
| Tiny "window" patches or blister-like marks on the underside of young leaves | Diamondback moth, early stage | Caterpillars are feeding inside the leaf tissue before holes appear |
| White butterflies fluttering over the bed during the day | Adult cabbage white | Eggs are likely already laid — start checking leaves |
| Small, dusty brown moths active at night, easy to miss | Adult diamondback moth | Damage often noticed before the moth is |
| Broccoli head chewed into, not just the leaves | Cabbage white butterfly caterpillar (larger, later instars) | Check inside the head, not just the outer leaves |
The two pests are easy to conflate because both leave chewed brassica leaves, but the caterpillars themselves look and behave differently, and that difference matters for control.
Cabbage White Butterfly
The adult is the familiar white butterfly seen flying over vegetable beds in daylight. Its caterpillar is a smooth, plain green grub that chews voraciously through leaves — on broccoli, damage extends beyond the leaves into the developing head itself if the caterpillar is left to grow.
Because the butterfly is active and visible by day, egg-laying is often easy to spot before caterpillars do much damage: pale, elongated eggs on the underside of leaves, usually laid singly. Checking leaf undersides every few days through spring and summer catches most infestations before they get serious.
Diamondback Moth
Diamondback moth caterpillars are green, ranging from pale to bright, with a darker head, and noticeably smaller than cabbage white caterpillars. The giveaway behaviour is how they react when disturbed — they wriggle violently and often drop off the leaf on a silk thread rather than staying put.
Young caterpillars hatch on the underside of leaves and burrow in, feeding on the internal leaf tissue first. This produces small "blister" or "window" patches on the leaf surface before any hole is visible from above. As the caterpillars grow, they move to chewing numerous small holes through the leaf.
The moth breeds fast — as many as seven generations in a single season — which is why diamondback pressure can build up quickly and why a bed can carry caterpillars at every stage at once, rather than one clear wave like cabbage white tends to produce.
What to Do About Either One
Start with physical controls, since both pests respond to the same basic approach.
- Net young plants. Fine insect mesh with a weave small enough to stop moths and butterflies laying eggs is the single most effective step, especially over seedlings and young transplants.
- Check leaf undersides regularly, not just the tops. This is where eggs are laid and where diamondback caterpillars do their early feeding.
- Handpick and break the life cycle early. Removing leaves carrying eggs or small (early instar) caterpillars is effective while numbers are low — much more effective than waiting until a plant is visibly chewed.
- Remove badly damaged outer leaves on kale so the plant can keep producing new growth from the centre.
If numbers get ahead of netting and handpicking, sprays fall into two broad groups worth knowing apart:
- Bacillus thuringiensis (Bt)-based products act only on caterpillars (moth and butterfly larvae) and leave bees and other beneficial insects unaffected — a good first choice for a vegetable garden, but timing matters since it needs to be eaten by the caterpillar to work.
- Pyrethrum-based sprays and other insecticides labelled for caterpillars on vegetables act faster and more broadly, which also means more caution around timing and pollinators.
As with any spray on a food crop, use only a product currently labelled for the crop and pest in NZ, and follow the withholding period on the label. A generic online recommendation is not a substitute for the label.
Why Broccoli and Kale Are Both Targets
Both crops sit in the same plant family (brassicas), which is exactly why the same two caterpillars turn up on both. Kale's soft, tender leaves are attractive to cabbage white caterpillars from seedling stage onward, and because it is usually harvested leaf-by-leaf over a long season, low-level ongoing pressure is normal — the goal is keeping enough clean leaf coming, not achieving zero holes.
Broccoli is more exacting because the plant is grown for one main head. Leaf damage on the way to heading is a cosmetic nuisance, but a caterpillar that reaches the forming head is a real loss. Step up checking frequency once heads start to form, and prioritise netting over broccoli specifically if you only have enough mesh for part of the garden.
Prevention for Next Season
Brassica caterpillar pressure is largely a function of how exposed the crop is, not luck.
- Net brassica beds from transplant, not after damage appears — both pests lay eggs early and often.
- Remove old brassica stalks and leaves at the end of the crop rather than leaving them to host late-season caterpillars or pupae.
- Rotate brassicas to a different bed where practical; concentrated brassica plantings in the same spot season after season make it easier for both pests to find the next crop.
- If white butterflies or small night-flying moths are consistently active over the vegetable garden, treat that as a signal to check leaf undersides that week, not next month.
The Bottom Line
Big ragged holes and a visible green caterpillar point to cabbage white butterfly. Small "window" marks on young leaves, tiny holes, and a caterpillar that thrashes and drops on a thread when touched point to diamondback moth. Both respond to the same core defence — netting, regular underside checks, and early handpicking — but diamondback's speed of reproduction means a bed left unchecked can go from clean to heavily damaged faster than cabbage white alone would explain.
Growing kale, broccoli or other brassicas this season? Check timing for your area: NZ Temperate Planting Calendar or NZ Cool/Mountain Planting Calendar.