💎 Diamondback Moth
Plutella xylostella
Tiny moth, diamond pattern, world-class pesticide resistance.
How to identify
Adults are small (1/3 inch) gray-brown moths with a diamond-shaped pattern of pale spots down the back when the wings fold — visible with a hand lens. Larvae are small pale-green caterpillars that wiggle violently and drop on silk threads when disturbed.
Damage signs
Larvae chew small holes and 'windowpane' the leaves, eating everything but the upper epidermis; heavy feeding skeletonizes brassica leaves. They contaminate broccoli and cauliflower heads with frass and silk.
Plants it attacks
Cabbage, broccoli, cauliflower, kale, collards, Brussels sprouts, canola
Non-chemical controls
- Floating row cover from transplant — the moth is tiny but the mesh still stops her.
- Apply Bt (Bacillus thuringiensis) to leaf undersides — a caterpillar-specific bacterium, not a chemical.
- Release the parasitic wasp Diadegma insulare, a specialist natural enemy of diamondback larvae.
- Interplant with tomato or garlic, which confuse the egg-laying moths, and destroy crop residue after harvest.
- Handpick larvae — they're small, but the wiggle-and-drop behavior makes them easy to spot once you know the trick.
- Use overhead irrigation to physically dislodge small larvae from leaves.
Nerdy fact: The diamondback moth was the first insect ever documented evolving resistance to Bt sprays in the field, and it holds the record for resistance to the most insecticide classes. Row covers don't care about resistance, though.
Still not sure this is your culprit? What’s Eating My Plant? narrows the suspects from what you actually see on the plant.