OPINION: Most of us are under the impression that all of New Zealand has fertile soil that’s great for growing food. It’s more or less a fairy tale.
If such conditions recur this autumn, slug populations will quickly bounce back from the hot, dry summer and pose a risk to autumn-sown crops and grass.
We all know that slugs can be devastating to newly sown crops and pastures, so it makes sense to check paddocks before sowing to see how bad the risk of slug damage is.
Paddocks should be checked for slugs before and after drilling. Walking paddocks and checking under objects like sticks, stones, cowpats or thick layers of trash can give an initial idea of slug presence.
However, a slug mat can give a much more accurate assessment of slug populations, which can be crucial in ensuring the solid establishment of your crop.
When conditions favour slug activity, slug mats should be left out overnight and checked for slugs the next morning. A common ‘rule of thumb’ is that three to four slugs per mat means there are enough numbers to cause damage. The mats can then be moved to new locations to monitor what is happening with slug numbers over time.
Even after acting to limit slug numbers, it is imperative to monitor the crop until the risk of slug damage has passed. This is particularly important where slug populations are high or where conditions favour slug development.
There is a range of cultural techniques that can help manage the risk of slug damage.
Some crops, such as brassicas, provide a better environment for slugs. So, managing your crop rotation and considering the sequence of crops can limit the rise of slug populations.
Minimising residue from the previous crop, e.g. by grazing and/or spraying out the residual vegetation, will reduce feed available to slugs and limit their options for shelter.
Cultivation will also reduce slug numbers through habitat disruption and direct physical impact, whereas direct drilling poses a higher risk of slug survival. Rolling after drilling helps because slugs are not good at burrowing into the soil, so fine, firm seedbeds are better than loose cloddy ones.
Depending on the circumstance, there are different baiting strategies that can be employed.
Where there is a risk of slugs migrating into the drill row, applying slug bait with the seed can be beneficial. This may help where you have shallow sowing depths or if soil conditions mean that it is not easy to close over the drill row.
Even if some slugs do migrate into the drill row, a broadcast bait may still be required to control those that do not. Broadcasting bait with fertiliser after the spray-out and before or after drilling can be very effective because the bait is applied when the feed source for the slugs is disappearing and before slugs get a chance to cause damage.
Remember that even if slugs are hard to find in the paddock, they can migrate in from the fence lines – making it worthwhile to broadcast bait on the outside round. With continued monitoring, you will know if a repeat application of bait treatment is required three to four weeks later.
Pasta-based Endure slug baits have been developed for the best combination of attractiveness, palatability, spreadability and durability for slug control in wet conditions, when slugs are more active.
The active ingredient metaldehyde will not harm beneficial insects, such as carabid beetle and earthworms. So, it is possible to use repeat applications when necessary.
• George Kerse, is product manager agrochemicals at Ravensdown.