Diamondback moth

Worst in winter and spring, June to November, and active through the cooler months on brassicas.

Diamondback moth is a small brassica pest often mistaken for white butterfly, since both attack cabbages, broccoli and kale at the same time. Its slim green caterpillars graze the leaf surface and wriggle violently or drop on a silk thread when disturbed, and it can build resistance to sprays fast.

How to identify

• Small grey-brown moths with a pale diamond pattern along the folded wings
• Slim green caterpillars under 12mm that thrash and drop on a silk thread when touched
• Window-pane patches where caterpillars graze the lower leaf surface but leave the upper skin
• Small ragged holes as feeding advances
• Damage worst in winter and spring on cabbages and other brassicas

How to prevent

• Cover brassicas with fine insect mesh from planting
• Rotate brassica crops and clear old plants that carry the moth over
• Remove weedy brassica relatives like wild turnip and cress nearby
• Encourage parasitic wasps by keeping flowering plants in the garden
• Avoid routine spraying, which speeds up resistance in this pest

How to control organically

• Hand-pick caterpillars, checking lower leaf surfaces and growing tips
• Spray a Btk product, alternating with neem so the moth does not build resistance
• Apply sprays late in the day and reapply every 7 days during heavy pressure
• Use a neem-based spray to deter feeding and egg-laying
• Protect the parasitic wasp Diadromus, an important natural control in New Zealand