Gardening – The dreaded Diamondback moth

Observation is the best garden tool, and it came in handy when disaster struck. Observing the defoliation of the Bok choy and kale in a friend’s garden, led me to realise that the lace-like leaves on my Chinese cabbage were not caused by slugs or snails.  Luckily as we were discussing the devastation of the brassicas, a tiny moth landed and a quick phone camera shot gave us an image that I was able to find online later.

It was an infestation of the dreaded Diamondback moth, Plutella xylostella, considered the worst pest of Brassica crops.

Diamondback moths are small greyish-brown moths around 8mm in length, elongated in shape and have a distinct diamond pattern down their backs. 

Female moths can produce up to 150 eggs throughout their life either singularly or in clusters along leaf veins, shortly after mating. Eggs are oval, pale yellow and about 0.5 mm in length. Larvae are pale yellow green, reaching 10-12 mm in length when mature. When disturbed the larvae often fall from their host plants, hanging on by a thin silk thread. Larvae go through four instar stages before they pupate.

Pupae are cream-green, darkening in colour prior to the adult moth emerging. They are present in New Zealand all year round, but the life cycle slows right down in winter.

Egg development depends on temperature, taking more than 100 days if the temperature is below 12°C which is why my winter cabbages were not attacked, but for those planted in late September, the eggs required only 14 days as the temperature rose. This allowed a rapid build up destroying the plants in just two weeks.

The larvae tunnel into leaf tissue and the damage progressively becomes more severe as the larvae grow. Interveinal tissue is usually eaten, resulting in a lace-work appearance on the leaves. Large infestations of Diamondback moth larvae chew holes known as windows when they feed on the underside of leaves resulting in complete skeletisation of plants. In brassica vegetable crops larvae can burrow into the heads of cabbage, broccoli or cauliflower. 

As well as brassicas radishes, turnips, stocks, wallflowers and sweet alyssum are all food for the caterpillars.

My cauliflowers have been so badly attacked that I will be pulling them all out, bagging them and delivering them to a friend who has a big flock of hens.

And for future proofing we will be building an enclosure covered with netting for growing next season’s spring crop of cabbages, broccoli and cauliflower outdoors!