Protect your cabbage family crops
from the diamondback moth
Crop damage from this moth is common in Japan, but can be prevented with a number of natural and organic methods.

By Pat Michalak

The diamondback moth (Plutella xylostella), a small green caterpillar with a voracious appetite for cabbage, has made itself at home wherever crucifers (members of the cabbage family) are grown, including Japan’s cool subtropical climate. In its caterpillar stage, the diamondback moth happily chews its way through leaves of cabbage, broccoli, cauliflower, collards, kohlrabi, kale, mustard and Chinese cabbage, earning it’s reputation as the most serious pest of cabbage worldwide. In Japan and elsewhere, this pest has developed resistance to every pesticide used against it, including the bacterial insecticide Bacillus thuringiensis, commonly known as "BT". Both organic and conventional cabbage growers have few control options for diamondback moth, and are eager for ways to prevent its damage.

Why is the diamondback moth a significant problem?
The diamondback moth originated in the Mediterranean area. Prior to the 1940s–when synthetic insecticides were introduced–the diamondback moth was not a major pest of crucifers. However, once the application of synthetic insecticides such as DDT became common practice, important natural enemies of the diamondback moth were eliminated. Soon after, the diamondback moth became the first crop pest in the world to develop resistance to DDT. Without its natural enemies, such as ground beetles, spiders and minute parasitic wasps, the diamondback moth is content to munch away on a never-ending supply of cabbage worldwide.

The moth has continued to develop resistance as new insecticides are created. Natural enemies have not been able to keep up with the moth’s quick spread. Unfortunately, the moth can migrate long distances more quickly than its enemies, and can remain in flight for several days and cover distances of 1000 km per day. In Japan, the moths are known to migrate from southwesterly islands to the temperate climate at Honshu and Hokkaido.

How can I tell when diamondback moth caterpillars are damaging my cabbage?
Where crucifers are grown throughout the year, all stages of the diamondback moth can be present at any time. You can find the adult moth on the undersides of leaves. Look for the diamond shapes in the center of the back. Adults feed on water drops or dew and are short-lived. After mating, female moths lay small, yellow eggs under the leaves near the center of the leaf, or close to the leaf veins. Caterpillars hatch in 5 to 6 days and reach a length of 8 to 12 mm. While still small, caterpillars may burrow into leaves. Older caterpillars feed on the underside of leaves, often leaving behind the veins and the thin, upper skin of the leaf–like a clear "window".

In broccoli and cauliflower, the damage is indirect because caterpillars eat the leaves and not the commercial flower head. If you disturb the caterpillars, they’ll wriggle away quickly and drop from the leaf on a silk thread before returning. Sometimes, you can find large caterpillars or cocoons hidden in the heads. Look for cocoons–dark green with a white, silky mesh and 10-12 mm long–among cabbage leaves, broccoli florets, or on the soil surface under plants. Once the adult moths emerge from cocoons, their entire life cycle will require one to two weeks.

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