The white butterfly fluttering around a brassica bed on a warm day looks harmless enough, but it's laying the eggs behind one of the most common and most damaging pests in a New Zealand vegetable garden. Cabbage, broccoli, cauliflower and kale are all targets, and by the time the damage is obvious, ragged holes chewed through leaves, sometimes whole leaves reduced to skeleton veins, the caterpillars responsible are usually well established.
The eggs themselves are the earliest and most reliable sign, small, pale yellow, laid singly rather than in clusters on the undersides of leaves. They're easy to miss on a casual glance but straightforward to spot with a deliberate check every few days through the warmer months. Once hatched, the caterpillars are a soft, velvety green that blends in remarkably well with brassica leaves, which is why the damage often looks more sudden than it actually is: the caterpillars have usually been feeding for a while before they're noticed directly.
Netting is the most reliable prevention available to a home gardener, and it works by stopping the problem before it starts rather than treating it after the fact. A fine insect mesh laid over brassicas from the moment they're planted keeps adult butterflies from reaching the leaves to lay eggs at all, and it needs no ongoing spraying or monitoring once it's in place, just checking that it stays secured at the edges.
Netting is the most reliable prevention available to a home gardener, and it works by stopping the problem before it starts rather than treating it after the fact.
Where netting isn't practical, or damage is already underway, Bacillus thuringiensis, sold under names like Yates Success or Dipel, is the standard organic treatment. It's a naturally occurring bacterium that specifically targets caterpillars without harming bees, ladybirds or other beneficial insects, applied as a spray directly onto the leaves the caterpillars are feeding on. It needs to be eaten to work, so coverage matters more than concentration, and it breaks down in sunlight within a few days, which means reapplying after each fresh batch of eggs hatches through the season rather than treating it as a single fix.
Checking leaf undersides regularly, before the caterpillars are already grown, is what separates an easy catch from a stripped plant, and it's exactly the kind of small, easy-to-forget habit that a scheduled check-in handles better than memory alone.