Diamondback moth (DBM)

Cabbage damaged by diamondback moth

© A. M. Varela, icipe

Throughout the world diamondback moth is considered the main insect pest of brassica crops, particularly cabbages, kales, broccoli and cauliflowers. The economic impact of diamondback moth is difficult to assess since it occurs in diverse small scale and large-scale production areas, but it has been known to completely destroy cabbage and kale crops. It is considered a major pest in all countries of the eastern and southern African region.
 

Broccoli, cabbage, cauliflowers, and other brassica crops. In Kenya, diamondback moth has also been found feeding on peas. 
 

Diamondback moth - Young caterpillars and their damage

© A.M. Varela, icipe

Newly hatched DBM caterpillars feed as leafminers inside the leaf tissue. Older caterpillars feed on all plant parts. They feed on the leaf tissue leaving the upper leaf surface intact. This type of damage is called "windowing", since it gives the appearance of translucent windows on the leaf. In cases of severe infestation entire leaves could be damaged. Caterpillars and pupae are found on damaged leaves. Older caterpillars are often found around the growing bud of young plants. Their feeding can deform the plant. DBM caterpillars also feed on stems and pods. Heavy damage results in the marketable parts contaminated with excrement, which makes the produce unsaleable. 
 

Diamondback moth caterpillars feeding on kales

© A. M. Varela, icipe

 

Affected plant stages

Seedling stage, vegetative growing stage, flowering stage and fruiting stage. 

Affected plant parts:

Fruits/pods, growing points, inflorescence, leaves and stems. 
 

Eggs of the diamondback moth

© F. Haas, icipe

Eggs are tiny (less than 1 mm), flat and oval in shape, and yellowish in colour. They are laid singly or in groups of 2 to 3 along the veins on the upper and lower leaf surfaces. The eggs hatch in 3 to 8 days depending on the environmental conditions. 
 

Diamondback moth caterpillars feeding on kales

© A. M. Varela, icipe

Caterpillars are pale yellowish-green to green covered with fine, tiny scattered, erect hairs. Mature caterpillars are cigar-shaped and about 12 mm long. They have chewing mouth parts. 

The caterpillars go through four instars and complete their development and pupate in 10 to 28 days. Diamondback moth (DBM) caterpillars are easily identified because they wriggle violently when disturbed, drop from the plant suspended by a silken thread and finally climb their way back up and continue feeding.
 

Diamondback moth pupal colour changes to brown before adult emergence. The developing moth can be seen through the cocoon. The pupa is 5 to 6 mm long.

© A. M. Varela, icipe

Pupae are 5 to 6 mm long. Pupae are initially light green and turn brown as the adult moths become visible through the cocoon. They are covered with a loosely spun net-like cocoon that is attached to the leaves, stems or seed pods of the host plant. Cocoons are about 9 cm long. 

The moths emerge 3 to 15 days after pupation depending on the environmental conditions. 
 

Diamondback moth adult on cabbage leaf. The adult is greyish brown with a nine mm long body and a wings- pan of about 1.2 to 1.5 cm

© Alton N. Sparks, Jr., The University of Georgia,www.insectimages.org

The adult is a small greyish-brown moth, approximately 8 to 9 mm long with a wingspan of 12 to 15 mm. It has diamond-shaped markings on the back when the wings are folded, which gives the common name to this insect. The moth folds its wings over the abdomen in a tent-like manner when resting. The wing tips are fringed with long hairs. Adult females can lay an average of 160 eggs during their lifespan of about 16 days. Moths lay eggs at night. The greatest number of eggs is laid the first nights after emergence, egg laying continues for about 10 days. In the field, moths will fly up out of the plant canopy when disturbed.

Diamondback moth infestations tend to be serious in the dry months. Heavy rains may reduce populations dramatically, thus this pest is less likely to be a problem in wet years and during rainy seasons. Diamondback moth populations can increase rapidly at temperatures above 26°C. 
 

Lifecycle of Diamondback moth

© A. M. Varela. icipe

This illustration shows the methods promoted on infonet-biovision. The methods shown at the top have a long-term effect, while methods shown at the bottom have a short-term effect. In organic farming systems, methods with a long-term effect are the basis of crop production and should be of preference. On the other hand methods with a short-term effect should be used in emergencies only. On infonet we do not promote synthetic pesticides. 

Further below you find concrete preventive and curative methods against Diamondback moth (DBM).

Cultural practices

Inspect the crop regularly. Diamondback moth populations can increase rapidly in warm conditions. Therefore, it is important to scout for DBM moth regularly, at least twice a week. Diamondback moth caterpillars are detected by visual observations of the plant. (Adults can also be detected by the use of pheromone traps though they are not yet available in East Africa.) 

Scouting should begin when the plants are young; the earlier the pest is discovered, the easier it is to control. Plants should be checked thoroughly. Growing points should be carefully examined. Caterpillars that are inside the cabbage head are difficult to detect unless outer leaves are pulled back. When scouting, it is important to record presence of parasitic wasps and parasitised caterpillars. Please also refer to section on natural enemies under Biological Pest Control further down on this page. 
 

Economic thresholds for the diamondback moth have been developed in several countries. For example, in small cabbage plots (0.25 ha) in Honduras, it is recommended to sample at least 60 plants and the action threshold is one caterpillar per plant. Broccoli and cauliflower at the vegetative stage can support 30% defoliation. At harvest time, an infestation level of one caterpillar per head is the action threshold (Rueda and Shelton, 1995). In the Midwest (USA), the treatment threshold for caterpillars (including DBM) attacking cabbage is given as 10% of infested pants in the seedbed, 30% infested plants from transplant to cupping stage, 20% infested plants from cupping to early heading, and 10% infested plants at early heading to mature head stages. For processing cabbage, which will be trimmed and shredded, more injury is tolerable; treatment is advised at 75% infestation. The treatment thresholds for broccoli and cauliflower are: 10% plant infestation in the seedbed, 50% plant infestation from transplant to first flower, and 10% infestation from first flower to maturity (Foster and Flood, 1995).

These thresholds are given as examples. However, note that economic thresholds depend on many factors (crop stage, crop age, and economic and climatic conditions) and cannot be adopted without taking into consideration local conditions. 
 

• Start with a healthy crop. Place seedling beds away from production fields to minimise attack by the diamondback moth. Transplant only healthy seedlings, which are free of eggs, caterpillars and pupae of the diamondback moth and other pests.
• Remove and destroy or plough down crop residues in seedling beds and production fields.

Habitat Management

Managing the habitat or the way a crop is grown helps to prevent or reduce pest and disease. Mix cropping brassica crops with some other crops or plants (intercropping, trap crops, strip cropping) has been shown to reduce infestation by the diamondback moth. The plants to be grown together with the brassica crops need to be carefully selected. Oniona and tomatoes can be intercropped with brassicas.

Maintaining natural surroundings, including trees and shrubs help to conserve natural enemies by providing shelter and plenty of breeding places for them. Maintaining strips of local flowering plants in the vicinity of the brassica crops is useful for beneficial insects. Trap cropping with flowering mustard can also augment the number of beneficial insects in the trap crop and the neighbouring crops. 
 

Biological pest control

Natural enemies (local and imported) can help to keep the pest at acceptable levels if they are conserved and their activity encouraged. Habitat management and avoidance of broad-spectrum insecticides early in the season, when the diamondback moth is present in low numbers may preserve natural enemies that can help keep diamondback moth and aphid populations under control later in the season. 

Many natural enemies prey on the diamondback moth at different stages of its life cycle. Birds and spiders feed on moths; ants, lacewings, wasps, and parasitic wasps among others attack the caterpillars. 

Numerous parasitic wasps attack diamondback moth. The most common are wasps of the genus Cotesia, Diadegma, Diadromus and Oomyzus. These wasps are also known from Africa and some are reported to effect excellent control of the diamondback moth elsewhere. 

Unfortunately, the locally existing wasps do not provide satisfactory control of the diamondback moth in eastern and southern Africa. For this reason, two species of wasps (Diadegma semiclausum and Cotesia plutellae) were imported and released by ICIPE in Kenya, Uganda and Tanzania. The former has provided almost complete control of this pest in highland growing conditions while the second is specific to mid-altitude, semi-arid areas where it also provides good control. 

Diamondback moth parasitoid (Diadegma semiclausum). This parasitic wasp was introduced and is now established in East Africa highlands.

© A. M. Varela, icipe

Diamondback moth parasitoid (Cotesia plutellae )

© A. M. Varela, icipe

Coccon of the parasitic wasp Diadegma semiclausum. The wasp larva spins a brown, rounded cocoon within the silk cocoon of diamondback moth.

© A. M. Varela, icipe

It is important to distinguish parasitised diamondback moth caterpillars from healthy ones. Caterpillars parasitised by Diadegma semiclausum can be distinguished at the pupal stage. The larva of this parasitic wasp eats the diamondback moth caterpillar from inside and pupates inside the diamondback moth cocoon. The pupa of the parasitic wasps appears as a round elongated brown capsule within the diamondback moth cocoon. In contrast, it is possible to see the developing moth through the cocoon of a healthy pupa. 

The larva of Cotesia plutellae feeds inside the diamondback moth caterpillar and emerges from the caterpillar to pupate in a silky cocoon on the leaves near the dead diamondback moth caterpillar.

Diamondback moth pupal colour changes to brown before adult emergence. The developing moth can be seen through the cocoon. The pupa is 5 to 6 mm long.

© A. M. Varela, icipe

Diamondback moth caterpillar parasitised by Cotesia plutella. Note silky cocoon of the parasitoid near dead DBM caterpillar. The wasp larva emerges from the caterpillar and spins a white cocoon from which the adult wasp emerges.

© A. M. Varela, icipe

Pathogens including fungi, bacteria and viruses are naturally found causing diseases to the diamondback moth in the field. However, they generally occur during rainy seasons when problems with this pest are not very pronounced. There are some commercially available pesticides based on disease-causing microorganisms (microbiological pesticides). A well-known example is Bacillus thuringiensis (Bt). 

For more information on natural enemies click here.

Biopesticides and physical methods

Bacillus thuringiensis var. aizawai and Bt var. kurstaki are very effective in controlling infestations of the diamondback moth. Bt var. kurstaki is widely used at a weekly interval and a rate of 0.5/ha. This type of strategy provides effective control of this pest. However, continuous use of Bt can induce development of resistance. Bt kills the diamond back moth and does not harm beneficial insects. Bt insecticides should be applied when the newly hatch caterpillars appear. Sprays may need to be applied at intervals of 5 to 7 days when populations are high. Because Bt insecticides are UV-degraded treat crops in the late afternoon. For more information on Bt click here. 
 

Farmers in some countries produce their own homemade biopesticides by collecting diseased diamondback moth caterpillars (fat and white or yellowish or with fluffy mould on them), crushing them and mixing them with water in a blender. Large tissue clumps are filtered out and the liquid is sprayed onto the crop (Dobson et al, 2002).

Neem (Azadirachta indica)
Neem-based products give a good control of the diamondback moth and are relatively harmless to natural enemies and non-toxic to warm-blooded animals. Since the action of neem is relatively slow, caterpillars may survive for a few days after application, but their growth and feeding is inhibited and they do not cause further damage to the crop. For more information on Neem click here.