Beating the Diamond-backed Moth
This year we have seen relentless attacks on our crops from Diamond -backed moths. Seasonal conditions have been perfect this year for the moth.The traditional use of sprays to control this moth by can be unrewarding. There are a number of reasons for this and it is likely that this problem will be with us more commonly in the future.
The Diamond-backed moth is about 1.5 cm long and brown/fawn colour. It has a 33-day life cycle and is active from early spring to late autumn, permitting it to have up to 7 lifecycles per year. Eggs are laid on the brassica leaves and hatch after 7 days. The grubs go through 4 stages of development before cocooning in preparation for adult life.
The initial damage is evidenced by whitening of the leaf as immature grubs bore into the leaf. This is followed within days by holes appearing. It is during this period that they are most effectively controlled by sprays. It is important to observe the plants so that spraying can be timed to coincide with the early post hatch period before they start eating the plants. Spraying eggs is not successful since the sprays registered for use have little residual effect.
The commonly available sprays are effective at killing these grubs. Resistance is possibly building up due to poor use of the sprays over several life cycles.
Key points to remember when spraying are:
- Time the spraying to hit grubs soon after hatching
- Use an effective dose rate
- Use liberal volumes of water to improve chemical plant contact (at least 300 litres/ha)
- Always use a surfactant to improve contact
- Ideally spray in the evenings and in cooler weather
Infestation of crops is unlikely to be in waves such that they are all at a similar stage, therefore the spray interval can be as short as two weeks instead of monthly to match the life cycle. Crops need to be monitored regularly for eggs.
Sprays used for controlling moths
| Product Name | class of compound | application rate per Ha | Cost per Ha |
|---|---|---|---|
| Fastac | synthetic pyrethroid | 400ml | $25 |
| Astound | synthetic pyrethroid | 400ml | $7.50 |
| Follidol | organophosphate | 700ml | $10 |
| Lorsban | chlorpyrifos | 1.5L | $28 |
There is a chemical called 'mobate' that has claims for improving the effectiveness of sprays such as fastac if added. The product is mollasses based and probably acts as a surfactant as well as supposedly attracting the pests. There is insufficient data available to support this claim. Some biological control methods have been trialed but again there is not sufficient evidence to endorse their use.
Click here to view the rest of the 2002 February newsletter.