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 understudied in Bhutan, but it is expected to be widespread in cole-crop growing areas.
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 color from grayish-green to dark brown. They have a diffuse dorsal line that 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 eggs
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 hearts of the plants, completely spoiling them for commercial use. While feeding in the heads, they excrete a large amount of feces, which promotes 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 stoutest. They can be readily identified by their features and the size and nature of their feeding damage and frass. They are also solitary.
Lifecycle: This pest is understudied in Bhutan. It is likely to go through at least a few generations a year. Egg batches of 20 to 100 are laid on the underside of leaves. Young larvae feed on the 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 occurs as pupae in the soil. Adults and larvae are nocturnal, so they 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: In Bhutan, it has been reported on cabbage, cauliflower, and asparagus, although it is only a significant problem on cabbage.
The size of the larvae and the nature of the damage indicate that even low numbers of larvae (one or a few per cabbage) can destroy its marketability. Regular monitoring and physical control are therefore necessary to limit their damage, especially prior to larvae entering the heads.
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. Monitor 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 have not been studied in Bhutan.
- 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.
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: firstname.lastname@example.org
Image acknowledgements: NPPC & University of Minnesota, Extension