Various insect pests – such as the bagrada bug, diamond back moth, greater cabbage moth, army worm, bollworms and aphids – attack cabbages. The diamondback moth is the most destructive of them all.

The larvae of the diamondback moth (Plutella xylostella) are a serious pest on most plants in the cabbage family (Brassicaceae). The moth comes from Europe, but is today found worldwide.

It multiplies quickly and moves without effort over long distances, aided by the wind. As a result, infestations in remote cabbage fields happen easily.

Adult diamondback moths are small and greyish, and between 6 mm and 8 mm in length. Identifying characteristics include the antennae that always point straight ahead when it is resting, and the diamond-shaped pattern that is displayed on the upper side of its wings when they are folded.

LIFE CYCLE

• Moths live for about 2 weeks (depending on the season), but during this time they can lay up to 300 eggs, in groups of 2 to 8, on plants.
• The eggs are smaller than half a millimetre, the first-stage larvae are 2 mm long and the adult larvae are about 12 mm long.
• Larvae can complete their cycle within 9 days, after which pupae (in silk cocoons) are attached to the underside of leaves or in leaf debris on the ground.
• A full cycle (egg to moth) is completed within 2 to 3 weeks in summer, but takes much longer during the cold winter months.

DAMAGE

• Damage comes only from the larvae.
• Sometimes the small first-stage larvae tunnel between the upper and lower leaf surfaces.
• This damage is almost impossible to see, so infested cabbage plants can serve as the source of new infestations in fields.
• From their second stage (after the first moulting, or casting off of their skins) they feed only on the underside of the leaf.

Larval damage is distinctive:

• The thin upper layer (thin skin) of the leaf is usually not eaten, which makes the eaten part look like a window.
• The thin skin breaks as the plant grows, however, so damage is usually a mixture of holes and little windows.
• Damage is less if larvae start appearing in fields after the cabbage heads have formed and set.
• Larvae of the diamondback moth do not usually tunnel into already-formed cabbage heads, but young cabbage plants can be seriously damaged if the larvae reach the heads while they are still forming.
• 2 or 3 larvae per developing head are all that’s needed to stop that plant producing a crop.
• It’s not only cabbage plants that suffer serious losses; crops such as broccoli, Brussels sprouts, Chinese cabbage, kale, collards, mustard and radishes can also be severely damaged by diamondback moths.

INSECTICIDES

Several insecticides are registered for use against diamondback moths.

Most brassicas have a waxy layer on the leaf surface, so farmers should ensure that the products they choose will stick to the leaves. If not, the right additives should be used to make the insecticide adhere to the leaves.

The focus of control should be to protect young plants, so it is essential that seedlings be inspected for larvae and, if any are found, the whole consignment should be sprayed with insecticide.

SOFTER MEASURES

• Many insecticides no longer have any effect on diamondback moths, because they have built up a resistance.
• In such cases, alternative control strategies should be adopted.
• Using “softer” agents may also attract natural enemies (read the label on insecticides; it will say whether the insecticide is detrimental to natural enemies).
• It has been shown that using overhead irrigation at night reduces the number of diamondback moths significantly.
• Because the moth multiplies so quickly and aggressively, insecticides are currently the only measure that can provide effective control – as long as the moth has not been found to be resistant.
• Fortunately, various new insecticides that do not fall within older categories have been registered to counter diamondback moths.
• Although they are more expensive than older products, the chance of resistance is lower.
• Confirm which pesticides are registered in your country before taking action.

Diamondback Moth, Plutella xylostella (L.) in Southern Africa Research Trends, Challenges and Insights on Sustainable Management Options
* Research by Honest Machekano, Brighton M. Mvumi and Casper Nyamukondiwa, 1 Department of Biological and Biotechnological Sciences, Botswana International University of Science and Technology.

Click here to read more about the geographical distribution of the diamondback moth in Africa.

Also read: Vegetable production: Easy-to-grow cabbage is king

• This article was written by Diedrich Visser from the South African Agricultural Research Council and first appeared in Farming SA.