Diagnosing diamondback moth
The diamondback moth (DBM) caterpillar is a serious pest of brassicas that is difficult to control. They are a problem when abundant early rains and mild winters allow them to multiply on volunteer canola plants and radish.
What to look for
- Chewed leaves, buds, and flowers.
- Leaf chewing ranges from irregular holes in leaves to extensive leaf damage with only bleached leaf veins left.
- Surface of stems and pods grazed (no yield effect).
- Pale green, cigar-shaped, caterpillars up to 12mm in length, that wriggle violently when disturbed and can hang down on a fine thread.
- Pupae are green to dark brown and have an open-mesh cocoon casing that is often stuck to the underside of leaves.
- Moths are 10mm long and are grey-brown, with a white stripe of uneven width down the centre of the back.
- When the moth is at rest and the wings are folded over the body the stripe forms three diamond shapes, hence the name ‘diamond back’.
What else could it be
Where did it come from?
- DBM breed up on volunteer canola, radish and other native plants in the off season.
- Adult moths disperse when green plant material has dried off. They may be carried long distances on prevailing winds.
- Several generations may occur each year. Under warm conditions, they may complete their life cycle in a fortnight.
- DBM is difficult to control because they breed prolifically and insecticide sprays have limited effect. Thresholds for control are:
- Pre-flowering (stressed crop) - 30 or more grubs per 10 sweeps
- Pre-flowering (no stress) - 50 or more grubs per 10 sweeps
- Mid to late flowering (no stress) - 100 or more grubs per 10 sweeps
- Pod maturation - 200 or more grubs per 10 sweep
- A single insecticide spray application often gives poor results. Two sprays applied at 5-7 days apart ensures DBM eggs and larvae that survived the first application are controlled.
- It is important to monitor numbers a few days after each spray application to determine the effectiveness of the chemical.
- Withholding periods before swathing and harvest are the same, because swathing is considered to be a harvest event.
How can it be monitored?
- If crops are moisture stressed or DBM are noticed in crops pre-flowering start monitoring using a sweep-net, otherwise monitor crops from flowering.
- Monitor at least four separate locations within a crop by taking two sets of 10 sweep samples. If no or low numbers of DBM are found, monitor again in two weeks.
- Monitor 3-5 days after each spray to check spray effectiveness.
Where to go for expert help
Page last updated: Monday, 11 September 2017 - 9:52am