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Inspecting Brassica Crops For Diamondback Moth

Now that the weather is warming up and the hard work of sowing summer forage crops is coming to an end, it is important to remember to monitor your crops for insect damage, especially as they become water stressed.

By Department of Primary Industries - 12th December 2005 - Back to News

The main pest of Brassica forage crops such as turnips, rape or pasja causing most damage in Gippsland is diamondback moth (DBM). Natural predators such as wasps and spiders are usually able to keep the DBM population under control. At times, however, the population reaches a stage where it may become necessary to take action to prevent economic losses from occurring. Monitoring crops on a weekly basis for presence of larvae, the amount of associated damage and the activity of beneficial insects will help determine the most appropriate course of action.

DBM eggs and larvae can usually be found on the underside of the Brassica leaves. The larvae can vary in size from approximately 1 mm to 12 mm and are very sensitive to leaf disturbance and movement. On warm days they move at such rapid speeds they are able to drop off the leaf on silken threads and wriggle out of sight before the leaf has been turned over. Therefore, it is important to check right down into the crown of the Brassica plant and the surrounding area around the base of the plant for larvae trying to escape detection.

Insecticide sprays are most effective in killing the second and third stage larvae, which represent approximately 25% of the moth’s lifecycle. Insecticides are not effective in killing the moths, eggs or first stage leaf miner larvae when they are in their mines. It is important to monitor the presence of moths, eggs or leaf mines on a weekly basis, as this provides clues as to the extent of the second wave of larvae damage.

To avoid missing "hot spot" areas of DBM infestation in a crop, the general recommendation is to follow a "figure 8" pattern that covers the whole crop from end to end.

The acceptable level of DBM damage can vary according to the various growth stages of the plant. DBM larva can consume approximately an area of five square centimetres of Brassica leaf. Hence during the establishment phase of the crops, brassicas are particularly vulnerable to insect attack and extra vigilance is required. Regular monitoring of crops is crucial in determining when DBM larvae reach a level which necessitates action.

The Brassica forage crop industry does not have any published threshold levels which are suitable for dairy farmers to use as a guide to spraying DBM. At this stage, dairy farmers growing Brassica forage crops have to determine for themselves whether pest levels have been reached which will result in economic losses if action is not taken.

Crops should only be sprayed with an approved insecticide when the pest population has reached a level where it becomes economic necessity to treat the crop. Natural predators of the DBM are more sensitive to the sprays than the larvae and will be destroyed. Once this natural line of defence has been removed, it is likely the crop will have to be continually sprayed every 10 to 14 days until it is grazed.

Suitable spraying techniques

Once the decision has been made to spray it is vital that crops are sprayed properly to ensure maximum effectiveness of the spray. The following steps are necessary for good spraying technique.

1. Read the chemical label and only use the recommended rate.
2. Use a minimum of 200 litres of high quality water (preferably rain water) per hectare to spray crops.
3. Use a surfactant or wetting agent to the spray mix so that spray droplets will stick to the leaf.
4. Consider using cone type nozzles on your spray equipment to improve the coverage of the plant.
5. Regularly calibrate your spray equipment to ensure that insecticides are applied at the correct rate.
6. If possible spray late in the day or in the evening when larvae are feeding most actively on Brassica leaves and are therefore more likely to come in direct contact with the insecticide sprays. This will result in increased kill rates. UV degradation of the insecticide will also be minimised.
7. Continue to monitor crops regularly, to determine whether further management strategies will be necessary. When inspecting a crop after it has been sprayed, check the youngest leaves (the inside leaves closest to the crown) for any new feeding damage. Whilst damaged leaves will continue to grow, the holes will remain. Therefore it is important to view the youngest leaves to gauge to how "fresh" the damage from larvae may be.

Source: http://gippsland.com/

Published by: news@gippsland.com