AgResearch today took a significant step in the quest to control clover root weevil, a white clover pest capable of costing the pastoral sector up to three hundred million dollars annually.

More than eight years research by the AgResearch Biocontrol and Biosecurity Group resulted in a first release of about 3000 clover root weevils infected with the larvae of a biocontrol agent known as Microctonus aethiopoides.  The release was made on a farm at the “epicentre” of the clover root weevil problem in Morrinsville, near Hamilton.  Microctonus aethiopoides is a tiny parasitic wasp. 

The release is the first in a managed series of four releases around the North Island this summer. The next areas targeted will be in Hawkes Bay and Manawatu later this month. These experimental releases will be closely monitored to make informed decisions about the best way to carry out future releases and to optimise the chance of successfully establishing the biocontrol agent in New Zealand.

AgResearch Chief Science Strategist Dr Stephen Goldson, who led the team that discovered the wasp near Galway in Ireland, said today’s release was significant on several fronts.

“This has major importance for the pastoral sector. Clover root weevil is a severe pest of white clover, which is important for nitrogen fixation in pastures and contributes greatly to feed value for livestock.

“It also represents what has been some exemplary cooperation between companies, organisations and governments both domestically and internationally.

“The weevil has already caused severe damage to pasture, resulting in lost productivity.  Estimates of ultimate impact sit at approximately $300 million a year and the release of the wasp should help to alleviate this,” Dr Goldson said.

Dr Goldson said today’s release would not have been possible had it not been for the co-operation and support of several key stakeholders offering both funding and expertise. These included Dairy Insight, Meat & Wool New Zealand, The Agricultural and Marketing Research and Development Trust (AGMARDT), The Foundation for Research Science and Technology (FRST), the Alma Baker Trust, Deer Industry New Zealand and AgResearch. 

A number of European research agencies and the United States also made very valuable contributions in developing this programme.

Special mention was also made of former Biosecurity Minister, the Rt Hon Simon Upton, who played a crucial part in getting the co-operation going between the various New Zealand contributors.

Dairy Insight chairman Doug Leeder said the dairy industry would be holding its breath, waiting for the outcome of this initiative.

“The implications are huge – not only for dairying, but for the New Zealand economy.

“New Zealand’s livestock industries are overwhelmingly pasture-based and white clover is estimated to contribute more than $3 billion to the economy. White clover is a crucial component of our farming systems and the threat posed by the clover weevil cannot be underestimated,” Mr Leeder said.

Meat & Wool New Zealand General Manager for Research and Development, Dr Richard Templer, said his company was also pleased to see the parasitic wasp being released today. 

“It is a practical demonstration of what can be achieved by cooperation between government and the pastoral industry to deliver world class research and development.  It is a significant first step in answering a challenge facing New Zealand’s pastoral farmers,” he said.

FRST Investments Operations Group Manager John Smart said the benefits of this project to the pastoral sector are “significant”.

“It showcases how important investment in research and development is for New Zealand’s key industries.”

AGMARDT chairman Sam Robinson said he was thrilled that his agency had helped to bring together the collaboration that made the project possible.

“Given the extraordinary amounts of money at stake to New Zealand’s economy it is vital that all efforts are made to secure the future of the clover plant and that its numerous benefits are maximised.”

Dr Goldson said the project is well complemented by the work of the agricultural nutrients company, Ballance, which is working to produce a granulated form of a fungus, also discovered overseas by the weevil research group, to act as another biocontrol agent to counter the destructive weevil, especially in intensive systems.

The parasitic wasp, which is totally harmless to humans, kills clover root weevil by injecting it with an egg which makes female weevils sterile. This breaks the weevil life cycle. The wasp larvae go through four stages with the last larval stage killing the weevil as it breaks out of the weevil’s body. The larva then turns into a pupa which in turn becomes the new generation of wasp.

Clover root weevil larvae do the most damage to white clover, by attacking its roots and root nodules.

Dr Goldson led a programme involving fellow AgResearch scientists, Dr Pip Gerard, Mark McNeill, John Proffitt and Dr Craig Phillips in Europe and the USA to search for an appropriate biocontrol agent that would kill the pest. After years of research funded by Dairy Insight, Meat & Wool New Zealand, AGMARDT, FRST, the Alma Baker Trust, Deer Industry New Zealand and AgResearch, a number of candidates to counter the clover killer had been identified.

Mr McNeill found a parthenogenetic strain of Microctonus aethiopoides in Ireland. That meant it would not interbreed with another strain from Morocco that was already in New Zealand controlling the lucerne pest, Sitona discoideus.

Clover root weevil was first discovered in New Zealand in 1996 but, by the time it was detected, it was too widespread in the North Island for eradication to be feasible so the search for management tools, including biocontrols, began.

The clover root weevil biocontrol programme is now led by Dr Gerard.

BACKGROUND
by Dr. Pip Gerard
AgResearch

Why clover root weevil biocontrol is good news for farmers?

The clover root weevil Sitona lepidus is one of the most serious clover pests found in New Zealand. To understand the impact it is having on pastures and how the biocontrol agent will remedy this, it is useful to know a little of its biology.

In the North Island there are generally two generations per year with adult weevils emerging in late spring and again in the autumn. The weevil is entirely dependant on the clover plant. The adults feed on the foliage, particularly favouring newly germinated seedlings. They live 2-7 months depending on the weather, and lay eggs continuously if clover is plentiful and rainfall adequate. The newly hatched larvae seek out root nodules that are fixing nitrogen. As they mature they move onto the side roots and then onto the main nodal roots and stolons. Adults survive longer and larvae establish better in autumn and winter. Therefore larval populations are greatest in late winter/early spring, and with a much smaller generation of larvae over the summer.

So what does this mean in terms of pasture production?  While leaf notching is an excellent indication of the presence of the weevil, the adult feeding damage is only of importance when renovating pasture. Sowing clover into a clover root weevil-infested pasture is a waste of money unless the adult weevils are controlled by insecticide or driven away by spraying out residual clover several weeks beforehand.

It is the larval feeding damage hidden below ground that is of greatest significance. Clovers may already be under stress from other factors, such as nematodes, poor grazing management or drought. The loss of roots can cause plant death. But if clover plants in pasture are not under stress, they can produce replacement nodules and new roots. This diverts nutrients to the roots and thus away from herbage production. Research by Dr Pip Gerard has shown that typical annual populations levels found on infested farms of around 300 larvae/m² caused yield reductions between 1600-2600 kg DM/ha in annual clover dry matter production in two year old pasture. Most of this production loss was in spring, when rapidly growing young animals and lactating cows and ewes have high nutrient demand.

Dairy farmers in the Waikato who have experienced chronic weevil infestations since 1996 have found the use of fertiliser nitrogen essential to maintain pasture production and productivity. The fertiliser nitrogen is partially used by clover to cover its needs and allow it to continue to make contributions as a quality feed component within the pasture even if it is not fixing nitrogen. A new research programme supported by the Sustainable Farming Fund and AGMARDT is now underway investigating optimal nitrogen fertiliser application levels in association with pasture management strategies. However, nitrogen fertiliser application is not a suitable option for all farms and current usage levels may not be economically or environmentally sustainable in the future.

So how will the Irish wasp help? The wasp injects an egg into the clover root weevil adults. This does not kill the weevil immediately but it makes the females sterile so no more eggs are laid. After several weeks the wasp larvae breaks out of the weevil, killing it. The larva then pupates in the pasture litter, after which a new parasitoid adult emerges to attack any clover root weevil that are nearby. The wasp will multiply through several generations over summer and will be most abundant when the clover root weevil adults from the small summer larval generation emerge in autumn. This should prevent the winter generation of clover root weevil building up to economically damaging levels, thus allowing farmers the option of maintaining quality ryegrass/white clover pastures without the addition of high levels of artificial fertilisers.

Countdown starts for release of Irish wasp to control clover root weevil.

With the Environmental Risk Management Authority (ERMA) granting approval on 8 November, AgResearch entomologists, headed by Dr Pip Gerard at Ruakura, are keen to make the first experimental release of a tiny Irish wasp Microctonus aethiopoides on 5 January. This will be a major milestone in the research effort to provide farmers effective long term control of clover root weevil.

The clover root weevil was first found in the Waikato in 1996 and is now throughout the North Island. Without effective means of control, pest numbers are determined by the amount of rainfall in early summer and the availability of well-nodulated clover. Entomologist Mark McNeill found that the Irish wasp performed well in quarantine trials and if it does as well in the wild as it did in the laboratory, expects it should reduce clover root weevil numbers in most regions of New Zealand.

One reason for the scientists’ optimism is that the Moroccan strain of the same wasp species has already proved an excellent biocontrol agent for the very closely-related lucerne weevil in New Zealand. However, the presence of this Moroccan strain, which does not attack clover root weevil, has been a two-edged sword. Early research also showed different strains of Microctonus aethiopoides could interbreed.  As the resulting hybrids were much less effective against the target weevils, the introduction of the wrong strain could put existing control of the lucerne weevil by Moroccan M. aethiopoides at risk. In addition, Dr Barbara Barratt at Invermay had previously discovered that the Moroccan strain, introduced in 1982 before ERMA came into being, also attacked a number of native weevils. Therefore, AgResearch has taken extra care with this introduction. All strains of the wasp were declared ‘risk species’ and it became ERMA’s responsibility to weigh up the risks, costs and benefits to New Zealand regarding the proposed release of the Irish strain.

The problem of hybridisation was overcome by the serendipitous discovery of an asexual strain of M. aethiopoides in Ireland. This all-female strain has been collected from sites across Ireland from Belfast to Galway and does not interbreed with the Moroccan strain. In addition, it is much less aggressive against native weevils. With clover such a vital component of our pastoral economy, ERMA has granted approval with the condition that only this Irish strain is released.

It will take several years before the impact on the clover root weevil will become apparent. Four sites have been selected for the initial experimental releases, one near Morrinsville in the Waikato where the Ruakura team have been monitoring the weevil since 1996, and the others on farms near Fielding and inland from Napier which have been infested much more recently. A further site near Bulls will be focussed on detecting if any other weevil species are attacked in the field. Once the team knows how fast the wasp builds up in the weevil population and at what rate it spreads into neighbouring paddocks, farms or districts, a strategy can be developed with farming and regional organisations to get the wasp to all affected farmers as quickly as possible.

The mass-rearing of thousands of wasps is a very labour-intensive operation and could not commence until a good supply of host adult weevils became available in early December. The clover root weevil biocontrol team is working over the holiday period to ensure the releases commence as soon as possible to ensure good establishment and rapid increase of the wasp populations during summer and autumn. This will be the culmination of seven years of research supported throughout by FRST, Dairy Insight and Meat and Wool New Zealand.