Diamondback moth

Diamondback moth (DBM), Plutella xylostella, also referred to as cabbage moth or Plutella, is potentially the most damaging and difficult to manage pest of brassica vegetable crops.

Scientific name

Plutella xylostella

Description of adult

The moths are greyish-brown with a wingspan of about 10mm and a body length of 10mm. When the female moth is at rest the folded wings form a row of roughly diamond-shaped marks where they join. In male moths the diamond pattern is less distinct and some male moths are just brown. Moths are most active in the early evening but can also be seen flying out of the crop when disturbed during the day.

Immature stages

The eggs are pale yellow, flat, oval-shaped and about 0.5mm long. Eggs are usually found stuck singly or in small groups beside the veins on the upper-side of the leaves or between the veins on the lower side, but can be laid anywhere on the plant. As the eggs mature they become deep yellow and a day or 2 before hatching, 2 dark eye spots develop at 1 end giving the eggs a brownish appearance.

After hatching, the young caterpillars burrow into the leaf to feed. After 1 or more days, caterpillars emerge to feed externally on the underside of the leaves. When fully grown, caterpillars are about 10mm long, plump and bright green.

Cocoons are generally stuck to the underside of a leaf but can be anywhere on the plant, including broccoli and cauliflower florets.

Life history

The eggs hatch after about 3 days in hot weather but can take up to eight days to hatch in cool conditions. After 1 to 2 weeks, the caterpillar pupates inside a gauze-like silken cocoon. The adult moth emerges from the cocoon after 5 to 10 days.

Distribution

Caterpillars are abundant through spring until autumn, especially during dry summers following a warm winter. Populations often persist through winter. In the warmer areas of south-eastern Queensland, DBM can cause damage throughout the season but autumn and spring crops are most at risk. Avoid summer production in warm districts. In colder districts around Stanthorpe, mid to late summer crops sustain more damage than early summer crops.

DBM adults are weak fliers but can move over long distances with the wind. However, recent research suggests that moths are not very mobile once they have settled in a crop, preferring to move short distances within the crop but tending not to leave it.

Host range

Broccoli, cabbage, cauliflower, Chinese cabbage and brussels sprouts.

Damage

Very small caterpillars cause leaf mining (pinholing) damage, then as caterpillars mature, they cause 'windowing' damage, leaving just the epidermis. These 'windows' tear as the plant tissue grows, leaving holes in the leaves. High populations of DBM cause leaf tattering and can check growth during early plant development.Caterpillars may also tunnel into cabbage heads and brussels sprouts or feed inside broccoli and cauliflower florets. Once the caterpillars are within the developing head, sprouts or florets they are almost impossible to control with pesticide sprays. The presence of caterpillars and cocoons on and inside heads, sprouts and florets, the fouling caused by caterpillar frass (droppings) and feeding damage to the heart tissue or wrapper leaves makes the product unfit for sale.

Control

DBM can quickly reach damaging levels. This is partly due to its short life cycle (less than 20 days in hot dry weather) and also the ability of the female moth to lay numerous eggs. The feeding habit of the caterpillars also makes them difficult to target effectively with pesticide sprays. In Queensland, DBM has developed resistance to insecticides from the synthetic pyrethroid, organophosphate, carbamate and organochlorine groups of chemicals.

Your best chance of managing DBM successfully is to use an integrated pest management (IPM) approach. This approach relies on a combination of cultural strategies, biological controls, regular crop monitoring and strategic use of narrow-spectrum insecticides. There is some evidence that overuse of broad-spectrum insecticides tends to kill off DBM's natural enemies that usually help keep the pest in check. These problems are magnified if attempting to grow brassica vegetables during hot dry weather.

Crop monitoring

Crop monitoring or crop scouting is the regular systematic checking of crops for pests and diseases. Either check the crop yourself, train 1 of your staff to do it or employ a professional crop consultant. The use of phermone traps and coloured sticky traps can augment (but not replace) crop monitoring.

Cultural

• A production break – this helps to break the DBM life cycle. In areas with hot summers, such as the Lockyer Valley, do not grow brassicas over summer.
• Farm layout – avoid paddocks with a history of DBM problems for early season plantings. Plant new crops upwind of older crops and separate blocks of plantings to reduce risk of DBM spreading to new plantings.
• Crop hygiene – plant clean, healthy seedlings. Slash and turn in crop residues once harvesting is completed and keep the farm clean of brassica weeds. Where DBM has become unmanageable, immediately destroy crops.
• Grow healthy plants – healthy plants are less likely to suffer from pests and diseases.

Biological

A range of predators and parasitoids attack DBM and can give good control of the pest in mild weather. In Queensland the most significant of these is Diadegma semiclausum, a small, black, 7mm long wasp which lays its eggs into young DBM caterpillars. To protect and enhance the activity of natural enemies do not use broad-spectrum insecticides and minimise the use of narrow-spectrum insecticides.

Bacillus thuringiensis (Bt), a bacterium, provides the active ingredient for a biological insecticide effective against DBM, heliothis and most other caterpillar pests of brassicas. Caterpillars must eat enough Bt to be killed. After eating a fatal dose, larvae will stop feeding but it may take a few days for them to die. Time the spray application to target small caterpillars and ensure spray equipment is capable of achieving good crop coverage. Bt affects only caterpillars and so fits well into an IPM program. It does not harm the adult stages of natural enemies and is relatively soft on their immature stages. DBM resistance to Bt has been reported overseas, so Bt should not be the only strategy for managing DBM.

Chemical

• Insecticides. Apply only when necessary, based on monitoring results. Choose the insecticide that is least disruptive to natural enemies and spray only those plantings where DBM or other pests are likely to cause economic damage (see the natural enemies impact table). Rotate between different chemical groups to reduce the risk of insecticide resistance developing.
• Other pest problems. These can have a negative impact on DBM management. Avoid using broad-spectrum insecticides to control pests such as centre grub, cluster caterpillar species, heliothis, thrips and silverleaf whitefly. One option is to delay planting until these pests are less active, or to accept higher levels of damage early or late in the season.
• Spray equipment. Well-maintained and regularly calibrated spray equipment will give the best spray results. Make sure that spray equipment is set up so it is capable of achieving good plant coverage. This includes the underside of leaves where DBM caterpillars feed.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.