Yellow-legged hornet: NZ has a narrow window to stop it, here’s how everyone can help – The Conversation

The yellow-legged hornet (far left) compared with species established in New Zealand: (from left to right) German wasp (Vespula germanica), Asian paper wasp (Polistes chinensis) and Australian paper wasp (Polistes humilis). Photo / Biosecurity New Zealand

The first Asian yellow-legged hornets observed in Auckland in winter were two old and slow males. Many people were concerned and worried.

Now, at the end of spring, what we’ve seen is a potential nightmare.

Over recent weeks, the Ministry for Primary Industries (MPI) response team has discovered around two dozen spring queens and small nests across the suburb of Glenfield.

Unless New Zealand rapidly scales up its search effort, we could soon be confronting an incursion far tougher and costlier to stamp out.

Or worse, we could end up with this pest as a permanent resident.

Overseas, nest densities can exceed 12 per square kilometre.

These nests can produce as many as 500 new queens in autumn.

If our current incursion stemmed from a single nest that produced hundreds of queens, the two dozen queens and small nests detected this spring may be only a small fraction of what is actually out there.

An incursion of such a scale would spell particular trouble for our honey bees, which can make up as much as 70% of this hornet’s diet.

In parts of Europe where the species has established itself, they have wiped out 30% to 80% of hives.

People are at risk, too.

Those who approach nests too closely risk severe stings known to cause intense pain, rapid swelling and, in some cases, life-threatening allergic reactions.

People have died after being stung by these hornets.

In dollar terms, the cost to countries – in control and lost productivity – can be in the tens of millions.

Is New Zealand doing enough while it still can?

We need more boots on the ground

New queens are still being found in Auckland, and the capture rate is increasing.

That could be due to better monitoring, bigger nests or more eyes on the ground.

Whatever the reason, the increasing captures are a major concern.

The more we search, the more hornets we find.

That trend must reverse.

The ideal scenario is for teams to spend days – and weeks – searching without finding anything new.

Only then could we be confident that hornet populations are being effectively controlled.

Clearly, we are not there yet.

A queen yellow-legged hornet develops an embryo or primary nest (pictured). Worker hornets raised in the primary nest build a larger, secondary nest during the summer. Photo / Ministry for Primary Industries

MPI teams are spending long hours searching on the ground and have increased staff numbers.

They are doing an excellent job and deserve real credit.

But it is clear more searchers are needed.

This would allow coverage of a much wider area, and there is debate about how far the search zone should extend.

MPI has focused on detection areas of 200 metres around each nest, supported by traps out to five kilometres, in line with international experience and guidance.

But other evidence and opinion from Europe suggest this may not be enough.

Year-to-year observations show new nests can appear kilometres from previous sites, while worst-case early invasion spread rates have been estimated at around 30 kilometres a year, accelerating to 75 to 112 kilometres each year once populations become established.

If these European patterns apply even partially to New Zealand, the search radius must expand dramatically, requiring many more people in the field and a careful watch from the public in the wider region around Auckland.

Eradication is the goal

The only successful eradication of the yellow-legged hornet occurred on Majorca, off the coast of Spain.

The programme ran for six years across an area of about 35 by 25 kilometres.

Success depended on a mix of citizen reports, active nest searching by volunteers and government staff, and mechanical nest removal.

After three years of finding and destroying nests, Majorca recorded three consecutive years with no detections.

This shows eradication is achievable over a substantial area.

New Zealand also has an advantage with toxic bait such as Vespex, which is locally designed for invasive wasp control and has been shown to be safe for use near beehives.

We’ve seen Vespex knock down wasps by more than 97% in thousands of hectares of New Zealand forest.

While its effectiveness against this hornet is not yet fully known, it has the potential to be a powerful tool.

For it to work, foraging worker hornets must collect the bait.

January and February are likely to be the best months in which to deploy it – when nests are large enough to produce active foragers but before they begin generating new queens and males.

Later in the summer, we’ll need to be prepared to find and control any remaining nests.

Those can be high and hidden in tree tops, requiring equipment to lift people high into the canopy, and long poles to spray pesticides into nests.

With a ramped-up search effort and using all available tools, including Vespex, the Majorca experience suggests eradication here is realistic.

The priority now is to maximise the search effort, map the true boundary of the infestation and destroy nests before they produce new queens and males.

• Everyone can play their part by staying alert and reporting online any suspected sightings, or by phoning 0800 809 966.
• Beekeepers can find MPI’s surveillance guide and an easy-to-build trap design on its website.

Disclosure statement

Phil Lester serves on the Technical Advisory Group for the Ministry for Primary Industries’ hornet response.

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Te Herenga Waka – Victoria University of Wellington provides funding as a member of The Conversation NZ.

Te Herenga Waka – Victoria University of Wellington provides funding as a member of The Conversation AU.