Diamondback moth

What you are looking at

Small, irregular pinholes scattered across brassica leaves, often thickest near the growing point and on the youngest leaves, sometimes widening into larger patches where the lower leaf surface has been eaten out and only a papery, translucent “windowpane” of upper surface is left behind. On a seedling or young transplant the growing point itself can be damaged directly, which kills or badly checks the plant rather than just marking outer leaves.

The caterpillar responsible is small, pale green and spindle-shaped, tapering at both ends, rarely longer than about 12 mm. Touch one and it wriggles violently and drops off the leaf on a fine silk thread, hanging in mid-air until the disturbance passes and then climbing straight back up, a habit that is often the fastest way to identify it in the field. Small, loosely-woven, lattice-like cocoons on the underside of leaves or tucked among outer wrapper leaves mark where larvae have pupated.

The world’s most economically damaging brassica pest

Believed to have originated in Europe or the Mediterranean, this moth is now reported from more than 80 countries and is present everywhere brassica crops are grown, making it one of the most widely distributed insects on earth. In tropical and subtropical regions across Asia, Africa and the Americas it breeds continuously through the year, and it is there that it does the most damage and drives the resistance problem described below. In temperate regions, including Britain, it overwinters as an adult or in sheltered plant debris and builds through several generations between spring and autumn instead.

What sets this moth apart from other brassica caterpillars is its capacity to evolve resistance. It was the first insect recorded to develop resistance to DDT, and populations somewhere in the world have since developed resistance to essentially every class of insecticide brought against it, including organophosphates, pyrethroids, insect growth regulators and, in parts of Asia and the Americas where it has been used intensively, Bacillus thuringiensis itself. Global crop losses and control costs from this one species are estimated in the billions of dollars a year, most of it falling on vegetable-growing regions in the tropics and subtropics rather than on gardens in temperate climates, where damage is real but rarely as severe.

What to do about it

Cover the crop with fine insect mesh from the day of planting, fitted before the first eggs are laid and kept clear of the foliage.

Rotate brassicas away from the bed for at least a year, and clear and destroy old plants and wrapper leaves straight after harvest rather than leaving them to break down in place.

Where pressure is high, plant a border of a highly attractive brassica such as collards or Indian mustard as a trap crop, and monitor or treat that border rather than the main crop.

Avoid routine spraying and conserve the parasitoid wasps that provide substantial natural control on their own; where a treatment is needed, use Bacillus thuringiensis or spinosad and rotate between different modes of action rather than relying on the same product repeatedly, since resistance to almost every insecticide class has already been recorded somewhere in the world.

What it gets mistaken for

Large and small white caterpillars are bigger, feed openly in groups or bore into the heart of the plant, and do not drop on a silk thread when disturbed, unlike this moth’s caterpillars, which wriggle hard and drop off the instant they are touched.