Wednesday, May 22, 2024

PULPIT: Native hunters shun exotic pests

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Planting botanical species to attract the enemies of pests is becoming popular around the world as primary producers search for effective, non-synthetic pest-control strategies.  It’s a pest-management strategy known as conservation biocontrol.
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But while it might work elsewhere it won’t work in New Zealand pastures.

Our native insects have not evolved to live in exotic pasture or to prey on the invasive insects that do. With few exceptions, especially grass grub and porina, native insects don’t migrate into our exotic pasture. Rather, they stay in their native forest or grassland ecosystems and they contribute little if anything to the control of pasture pests.

In a paper just published in the Journal of Insect Science with my co-authors, I outline why NZ has a very high exotic pasture pest burden. 

The main reason is a distinct lack of natural enemies to push back on the exotic pests when they get in, which, in turn, relates to the lack diversity in our pasture species.

NZ’s pastoral landscapes might look similar to pasturelands in Europe and elsewhere but that’s where the similarity ends. 

Our intensively farmed, entirely exotic pastures comprise less than 1% of northern hemisphere grassland species and are dominated by perennial and annual ryegrass. 

This lack of diversity means much more damage from exotic pests than occurs in the species-rich grassland areas in the pests’ native ranges. 

While we inadvertently imported pasture pests into our pastures or they self-introduced, their natural enemies did not come in with them. As a result, the exotic pest species that arrive find themselves in the enviable position of having an extensive, suitable habitat and very few if any enemies.

For example, the Argentine stem weevil only very rarely inflicts damage in native South American pastures – certainly nowhere near the $200 million of damage a year it often causes in NZ pastures. 

The clover root weevil has not been found in densities higher than 30 a square metre in its native range yet here it has been reported to reach densities of 1400 a square metre.

The root weevil causes significant economic and environmental impacts. 

NZ farmers are often unable to maintain clover in pastures so they compensate by increasing their use of nitrogenous fertilisers – costing the sector about $225m a year and also creating downstream environmental impacts.

However, the good news is there are very clear reasons to focus on what’s called classical biocontrol – importing natural enemies, often from where the exotic pests came from. We already have ample evidence this works outstandingly well in NZ pastures. 

Using introduced parasitoids, biocontrol of the NZ populations of the Argentine stem weevil, the clover root weevil and the lucerne weevil has reduced all three to below damaging levels. Based on the international track record of biological control the chances of that happening were one in 1000. 

Again, this shows there is something distinctly different in NZ. We are pretty sure the difference is that the introduced natural enemies also do not have natural enemies and so they are they thriving.

Going back to conservation biological control there are many excellent reasons for planting native bush and restoring native wetlands and other ecosystems on farms but attracting the natural enemies of pasture pests isn’t one of them. 

If you use exotic plants the native natural enemies won’t respond to them. If you establish native plants around the edges of pasture it will attract native natural enemies but experience has shown they will stay around the edges and not move into the pasture. 

None of this should be taken as a dismissal of conservation biological control – it works well in many ecosystems. 

But those ecosystems are usually ones where farming and horticulture are based on species that have evolved together, in that place.

NZ’s farming ecosystems did not evolve here and are very incomplete. 

We simply cannot copy what works elsewhere – we have to understand what does work here and ensure our resources focus on getting that right. 

And what does work extremely well here is bringing in natural enemies as biological control agents. 

Who am I?
Professor Stephen Goldson is deputy director of the Bio-Protection Research Centre at Lincoln University. The centre was set up in 2003 to drive innovation in sustainable approaches to pest, pathogen and weed control. It has seven partners: AgResearch, Lincoln, Massey, Canterbury and Otago Universities, Plant and Food Research and Scion.

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