Cabbage moth

Mamestra brassicae

Crops: Cole crops

Why is it a problem? Cabbage moth larvae are large and feed individually. They feed within cabbage heads, and even the presence of a few larvae within a head can completely spoil it for market. Once inside the cabbage they can be difficult to find and control.

Where and when is it a problem? This pest is poorly studied in Bhutan, but is expected to be common across the Cole-crop growing regions.

IDENTIFICATION

Adults are large, with a 34-50 mm wingspan. Forewings are greyish-brown mottled with dark brown. There are kidney-shaped markings with a white outline on the forewing. Larvae are up to 40 mm long and fat. They vary in colour from greyish-green to dark brown. They have a diffuse dorsal line which is dark brown with a pale spot on each segment. The body sides have pale stripes. The head is pale to dark brown. Pupae are reddish-brown, within a fragile cocoon. Eggs are hemispherical, pale brown and laid in batches.

Cabbage moth larva and feeding damage
Adult cabbage moth

SYMPTOMS

Small larvae feed on the underside of the external leaves where they make small perforations. Feeding holes become larger as larvae grow. Severe infestations of small larvae may rapidly skeletonize the leaves, and can sometimes destroy small plants. Older larvae tunnel into the heart of the plants, completely spoiling them for commercial use. While feeding in the heads they leave considerable amounts of faeces, which also encourages the growth of decaying bacteria and fungi. In cauliflower and broccoli, the larvae also feed on the inflorescence where they can chew deep holes.

Confusion with other pests: There are several moth species commonly found on Cole crops. Cabbage moth larvae are the largest and the stoutest. They can be readily identified by their features and the size and nature of feeding damage and frass. They are also solitary.

BIOLOGY

Lifecycle: This pest is little-studied in Bhutan. It is likely to go through at least a few generations a year. Eggs batches of 20 to 100 are laid on the underside of leaves. Young larvae feed on outer leaves. As they become older they tunnel into the plant. Larvae leave the plant to pupate, burrowing about 5 cm into the ground to build a cocoon. Overwintering is as pupae in the soil. Adults and larva are nocturnal, so can be difficult to find during full daylight.

Dispersal: Adults fly.

When can damage be expected? Early season Cole crops may escape significant damage as moth populations may not yet have started building up.

Hosts: This pest can utilize a wide range of legume and brassica hosts. In Bhutan it has been reported on cabbage, cauliflower and asparagus, although it is only a significant problem on cabbage.

MANAGEMENT PRACTICES

The size of the larvae, and the nature of the damage, means that even low numbers of larvae (one or a few per cabbage) can destroy its marketability. Careful, regular monitoring and physical control is therefore necessary to limit their effect, especially prior to larvae entering the heads.

Monitoring

Inspection for cabbage moth presence and damage needs to be part of regular (twice-weekly) crop monitoring for a wide range of Cole crop insect pests. Aim to detect larvae before they enter the heads. Larvae can be hard to find so search during the early morning or evening, including among leaves.

Effect of variety

Differences in level of susceptibility have been found between varieties elsewhere, but has not been studied in Bhutan.

Non-chemical management

  • Physical control: search plants for larvae and feeding damage. Remove and destroy larvae as they are found. It is best to find and control younger larvae as locating them in older plants requires searching in amongst cabbage heads.
  • Autumn ploughing has been found to be effective in Japan, probably because it exposes pupae in the soil to predators and the weather.

Chemical management

Chemicals are not recommended. They are unlikely to be effective once larvae enter the head.

 

Version: NPPC 2017. Cabbage moth V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact: nppcsemtokha@gmail.com

Image acknowledgements: NPPC & University of Minnesota, Extension

 

 

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