Party time for clover root weevil

biocontrol 5 November 2014

Spring is party time for the clover root weevil and farmers in the south are queuing up to get one step ahead of the pest, as it creeps on to their farms.

Pictured: A parasitic wasp (right) release in New Zealand nine years ago is playing a significant role in reducing the impact of the destructive and costly clover root weevil. Image courtesy PestWeb.

We talk to B+LNZ research manager Dr Geoff Ridley and Southern South Island extension manager Paul McCauley about the free wasp biocontrol, as this season's farmer vs weevil battle lines are drawn.

How does the wasp biocontrol work?

The wasp's life cycle revolves around seeking out the adult weevil and laying eggs inside it. They hatch and the wasp larva feed on the weevil's internal organs, before bursting out as fully-formed adult wasps (like a scene out of the sci-fi movie, Alien). It then gets on with finding a weevil to lay eggs in itself. The wasp won't eliminate all CRW, but it does significantly reduce its destructive impact on clovers in pasture.

Is it fully established in parts of New Zealand already?

Yes. The wasp was released nine years ago and has been heading down the country, following the weevil's journey southward. Southland is the last bastion and farmers are trying to get the jump on the weevil, by facilitating additional wasp releases.

Are farmers cautious about its use?

No, farmers recognise the weevil's financial impact. A "back of a cigarette packet" calculation done by the South Otago monitor farm team estimated the cost of lost production – as a result of reduced clover – was about $30,000 per farm. It is worth noting that the weevil is sterilised from the moment it is parasitised, so it won't be laying anymore eggs from then until it dies.

Is every farm suitable for release of the wasp?

Every farm with clover and the CRW is suitable. Remember that the wasp is a treatment – not prevention – so it needs the weevil as its host. If the weevil isn't present, the wasp has nothing to lay its eggs into and feed on. If you can see the weevil, then you have it – and you'll only be seeing the tip of the iceberg.

When is the right time to release the wasp?

Now – after the weevil has been dormant for winter.

Where do you release it on farm?

We talk about establishing a "nursery" for the wasps. That is, a clover paddock that you're not planning to spray out or work up in the next couple of years. A warm spot is good and, ideally, upwind of your property, which helps spread the wasp. It moves at 15km per year, so it will cover your farm in no time.

How do you get hold of the wasp biocontrol?

B+LNZ and AgResearch are hosting a series of workshops in the south (see events calendar for details). Vials of parasitised weevils will be available, but numbers are limited, so registration is essential. Each weevil in the vial has been "infected" with a wasp.

How do I register for a workshop?

See the B+LNZ events calendar for workshop details. Register early, as the places fill quickly.

For more information on clover root weevil and control options, see the AgPest website or download the B+LNZ fact sheet: Combating clover root weevil (PDF, 150KB)

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