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1 January 2020
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Crop Protection

Controlling diamondback moth

Mild winters can result in higher insect pressure in spring and if these conditions turn into a hot and dry summer, then Diamondback Moth (DBM) thrives. Unfortunately, this can mean damage to the leaves of valuable winter feed crops.

DBM is a pest which feeds on brassica plants including forage crops. In a hot dry year, DBM can build to plague proportions and destroy leaf area of feed crops. Using what we know about its lifecycle and beneficial insects, we can make better decisions to reduce the impact of this pest.

The lifecycle of DBM consists of four stages:

  • Adult
  • Egg
  • Larvae (caterpillar)
  • Pupae

The damaging stage is stage three. The caterpillar has the job of eating as much leaf material as it can to allow it to pupate into the adult moth. Just like a monarch butterfly, the caterpillar builds a pupae around itself and transforms into the adult inside it. This eating results in all of the visible leaf damage that we see. 

Knowing which stage causes all the damage allows us to target control options and minimise these effects. The last few years have seen the introduction of two new insecticides which are safer for the applicator and on beneficial insects than previous chemistry. Beneficial insects help us with controlling crop pests by feeding on the pests themselves, so we need to protect them wherever possible. An example of this is the lacewing. The larval stage of the lacewing walks around the leaf feeding on aphids on the crop.

Monitor your crop for the first signs of DBM. If a selective pesticide needs to be applied, the timing should be based on identification of the DBM and lifecycle stage. Identification of this first incursion is hard because the eggs are small and the adult moth doesn’t sit still long enough to easily identify the distinctive diamond pattern on its back. An easier way to identify if the pest is present is visible damage to the crops foliage. DBM damage can be easily identified by the distinctive windows formed in the leaves and/or holes which are away from the edge of the leaf. This is when to time your application for maximum benefit. As the caterpillars ingest the plant material, they also ingest the insecticide. 

If dry conditions continue through late summer, more than one application may be required. Each adult can lay more than 100 eggs, so keep an eye out for another flight going through the crop then monitor for another batch of eggs hatching. 

To get the best control of DBM, contact your local PGG Wrightson Technical Field Representative. 

 
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