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Diamondback moth: how to recognise and treat it

Pest

Diamondback moth — Photo: Please report references to olei@despammed.com ., CC BY-SA 2.5, Wikimedia Commons

Symptoms

Small "windows" on the undersides of leaves where young caterpillars eat the tissue but leave the upper skin intact.
Small green caterpillars, up to about 1 cm, tapered at both ends; when touched they wriggle violently or drop and hang on a silk thread.
Older caterpillars chew small holes and attack growing points, flower buds and broccoli or cauliflower heads, which can stunt young plants.

Organic treatment

In small plantings, hand-pick caterpillars, checking the undersides of leaves every few days.
Spray Bacillus thuringiensis (Bt) while the caterpillars are small; follow the label.
Encourage natural enemies such as parasitic wasps, ground beetles, spiders and hoverfly larvae with flowering insectary plants.

Chemical treatment

Spinosad, following the label dose and pre-harvest interval; diamondback moth resists many insecticides, including pyrethroids, so alternate products with different modes of action and avoid broad-spectrum sprays that kill its natural enemies.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Prevention

Cover brassicas with fine mesh netting so the moths cannot lay eggs on the plants.
Remove and destroy crop residues as soon as harvest is complete, and keep weeds down around the garden.
Do not grow cabbage-family crops in the same spot in consecutive years.
Check the undersides of leaves regularly, especially at the seedling stage and just before heading.

Plants it affects

Sources

Figures checked against these sources.