Crop: Cole crops
Why is it a problem? Diamondback moth (DBM) larvae feed on the foliage of Cole crops. Although larvae are small, outbreaks can destroy entire crops if not recognized and managed in time. Larvae can be evenly distributed across a field, so can cause significant losses if high densities are reached.
Where and when is it a problem? This pest is probably present throughout Cole-crop growing areas in Bhutan. However, only one serious outbreak has been recorded, during 2016 in early season Cole crops across several farms in Bjemina Valley (c 2,500 m asl).
Adults are 12-16 mm long, brown or grey in colour, with conspicuous white spots on their forewings. When wings are folded on its body a diamond-shaped median dorsal patch is seen, giving the pest its name. Eggs are whitish-yellow and minute (0.5 mm). Newly hatched larvae are pale white with a pale brown head. Fully grown larvae are only about 12 mm long. They are pale yellow-green with fine black hairs all over the body. The central part of the body is comparatively bigger than the ends. Larvae characteristically wriggle and drop off on silk threads if disturbed. They pupate in greenish, cottony cocoons about 10 mm long. The silk mesh is attached to the surface of leaves or other substrates, making them difficult to remove.
Early larval stages leaf-mine. Larvae feed on the underside of leaves, leaving veins, and often the upper leaf surface, intact. This results in characteristic whitish patches on damaged leaves. Older, larger larvae make bite holes in the leaves, making them unfit for sale and consumption. Severe infestations result in undersized cauliflower, and head formation may not take place in cabbages. All plant stages are attacked.
Confusion with other pests: Several moth species are commonly found on Cole crops. Diamondback moth caterpillars are the smallest. They can be readily identified by their appearance, behaviour (they produce silky threads when disturbed) and feeding damage.
Lifecycle: First instars feed as leaf-miners. The remaining three stages feed on the underside of leaves. They pupate on plant stalks, stubble and debris. Their lifecycle can be completed within a few weeks, depending on the temperature, which means there can be many generations in a year. They overwinter as pupae.
Dispersal: Adults are known to be excellent long distance fliers. Individuals can survive as pupae in crop residues, dispersing on emergence.
When can damage be expected? Experiences in other countries suggest that rainfall is an important mortality factor, and as a result diamondback moth is largely a problem during the dry season. This is supported by recent observations in Bhutan. The only recorded outbreak In Bhutan was observed in the first Cole crop of the season in Bjemina Valley, near Thimphu (2016). Larvae were hard to find by July of the same year by which time the weather was warm and the summer rains had started. The outbreak was studied to help understand the cause. Relatively few farms in one part of the valley were affected, but some of those lost their entire crop. No definitive cause of the outbreak was found, or any reason for it being contained to such a small area in the valley. It was their first reported outbreak of this pest.
Hosts: Diamondback moth is a worldwide pest of Cole crops and oilseed crops (e.g. mustard and canola). In Bhutan it has been reported off Cole crops and mustard.
Diamondback moth outbreaks appear to be rare, but when they do occur entire crops can be lost. Early detection, through regular monitoring, and rapid response is therefore necessary to avoid this.
Inspection for diamondback moth presence and damage needs to be part of regular (twice-weekly) crop monitoring for a wide range of Cole crop insect pests. Monitoring of pre-monsoon crops is particularly important for diamondback moth.
Effect of variety
Resistant cabbage cultivars have been developed elsewhere but are not currently thought to be grown in Bhutan.
- Practice crop hygiene: collect and destroy all stalks and stubble when harvesting the crop to prevent the build-up of a second generation, especially if planting a second pre-monsoon crop.
- Collect and physically destroy larvae during the early stage of infestation. This will also help in monitoring the pest.
- Overhead irrigation can reduce populations, especially if applied at dusk, by disrupting mating and egg-laying, and washing off larvae. However, it could also encourage disease development as a negative side-effect.
- Neem and bio-pesticides are recommended in Nepal, but have been not tested in Bhutan.
- Parasitoids are used as biocontrol agents elsewhere, but are not currently available in Bhutan
- Natural enemies are important for regulating diamondback moth populations, and populations of other moth pests of Cole crops. The use of pesticides should therefore be kept to a minimum.
- This pest is especially prone to rapidly developing resistance to pesticide. Their use in Cole crops should therefore be kept to a minimum.
- Insecticides are rarely required, and then normally only on the first crop of the year.
- Consider insecticide application when there are 20 larvae per 10 plants.
- Spray Cypermethrin (1 ml per 1 litre water) as a last option to reduce the damage.
Image acknowledgements: NPPC.