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1 February 2020
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Crop Protection
Crop Production

Controlling pests in brassicas

Forage brassicas form the cornerstone of forage cropping in New Zealand, because they are fast to establish and relatively cheap to grow. 

The ability to grow a fast establishing abundant feed, high in protein, energy and digestibility for livestock, as well as, in many situations, forming a useful break in the pasture cycle allows the paddock to have a period of time not growing grass or clover to control various weeds and bugs.

Brassicas may sound like the perfect crop, but there are a few issues to contend with to maximise their success in delivering valuable livestock feed. Whilst seedbed preparation, plant nutrition and weed control are all important, the one area that can have a devastating effect is pests. This is due to the yield value diluting the growing costs which can have a massive economic impact on the crop.

As farmers, we see a growing crop as feed for our livestock. If we plant and grow fresh, succulent leaves for our animals, there are millions of other animals that are also grateful for having a feed source that, in many cases, is essential for them to complete their lifecycle. A crop can cope with some amount of feed being lost to these other species, but when they start to have an economic impact on the crop, they become a pest and have to be dealt with.

The growth stage of the crop and weather patterns have an influence on what pests are likely to attack your crop. This allows you to be more focussed on what to look out for and formulate a control plan. At establishment, we need to be observant of pests, such as slugs, snails, nysius and springtail. At this time of year, though, our crops are more likely to be infested by caterpillars (diamondback moth and white butterfly), aphids and fly larvae (leaf miner). 

There are also a large number of other species that make their home in the crop, and some of these actually feed on the pests and do the control for us. These are called beneficial species and they can negate the requirement for spraying with an insecticide. Whilst it is tempting and easy to say ‘just spray with a broad spectrum insecticide like an organophosphate (OP)’, this is not environmentally responsible nor is it the safest option. It also increases the risk of the insect pest you are wanting to control becoming resistant to the spray and, with few new products being registered year-on-year, we may lose the ability to control them in the future.

This is why monitoring of pest and beneficial species is important so that an informed decision can be made on whether that pest is having an economic effect on the crop and whether an insecticide is needed. 

The correct identification of the pest is critical if a spray is required because it enables us to use an insecticide with more selective activity thus leaving the beneficials safely behind. This, in turn, reduces the likelihood of the pest numbers increasing again because the beneficial numbers are higher and control them for you. This process is called Integrated Pest Management (IPM for short).

To learn more about IPM and the beneficials in your brassica crop, contact your local PGG Wrightson Technical Field Representative, or visit the PGG Wrightson YouTube channel.

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