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  • In January 2006, a tiny wasp was released onto a Waikato farm to kill an enemy that was devastating pastures. Ten years on, these tiny wasps have saved farmers nearly half a billion dollars.

    An insect pest invades

    In 1996 it was discovered that a little weevil, called the clover root weevil, had arrived in New Zealand, probably from Europe. It may have come in via a shipping container, then climbed into farming equipment that ended up on a Waikato farm. It thrived in our country, gobbling up the clover that helps give Waikato farms some of the richest pasture in New Zealand.

    It wasn’t until 1997 that farmers discovered the clover in their paddocks was disappearing – alarmingly quickly! They were worried, because cows eating clover make the best milk.

    The adult clover root weevils eat the leaves of the clover, while the larvae eat the roots, nodules, and stolons.

    Clover root nodules produce nitrogen which goes back into the soil, fertilising the ground and resulting in rich, green pasture. When the nodules are eaten, the reduced nitrogen content in the soil causes the whole pasture to become more yellowed. Stolons are runners which allow the clover to spread to new areas. If the stolons are eaten, the clover plants can’t spread as easily.

    Wasps to the rescue!

    The paddocks were not looking good. Milk production went down, and the milk was not as good as it had been. The farmers were not happy!

    They turned to scientists to help find a solution to this new problem. The scientists already knew about a tiny wasp used to control a different sort of weevil - the lucerne weevil. This wasp, called Microctonus aethiopoides, originally came from Morocco.

    The wasp is a parasitoid wasp because it lays its eggs inside the weevil. When the wasp eggs hatch, the wasp larvae eat their way out of the weevil. This kills the weevil. The wasp larvae then find a safe place in the dead plant material on the soil surface (the leaf litter) and each larva makes a cocoon. Eventually they emerge as adult female wasps, reproducing asexually and laying their eggs in another unsuspecting weevil.

    Not just any wasp!

    Tests showed that the wasp that was killing the lucerne weevil would not lay its eggs in the clover root weevil. In other words, it would be no good at controlling the clover root weevil.

    Next, scientists tested a different strain of Microctronus aethiopoides wasp, this time from Europe. When they tested this wasp in their laboratories, they found that the wasp could kill the clover root weevil. They were very excited. BUT the tests also showed that the European variety began to mate with the original Moroccan wasp. This mating resulted in hybrid wasps that were ineffective against both the lucerne weevil and the clover root weevil.

    Going on a wasp hunt

    In 2004, a group of scientists from AgResearch went to Europe to see if they could find another strain of the wasp. One of these scientists thought he would extend his trip with a holiday in Ireland. While he was there, he collected some weevils from the Irish clover. He wondered whether any of these weevils would contain wasp eggs that might be of interest in New Zealand. He brought them back with him, making sure that the appropriate people at border-control knew what he was doing and that he had all the correct permits.

    A wasp with an interesting difference

    Later, it was found that wasps did emerge from the Irish weevils. Once again, they were the same species of wasp that had already been tested, Microctonus aethiopoides, but a different strain, or genetic variant. Importantly, all the wasps were female, and they could reproduce asexually. They did not need males!

    This was fantastic news. It meant that the Irish wasp would kill the clover root weevil, but there was no danger of it breeding with wasps of different strains and producing hybrids as the European wasp had done.

    Further testing

    Before being released, any new organism needs to be thoroughly tested to make sure it will not have any negative or unexpected effects on New Zealand’s natural environment. For example, scientists needed to make sure our native weevils and beneficial biocontrol agents would not be wiped out. Research in quarantine showed that while a small number of native weevils are potential targets for attack by the wasp, the clover root weevil is always the preferred host.

    Approval for release is given by the Environmental Risk Management Authority (ERMA), later replaced by the Environmental Protection Authority.


    After several years of testing, the wasp was ready to be released on real farms.

    The first release was in January 2006, on a Waikato farm. Since then there have been other releases in different parts of the country. Scientists continue to monitor the effectiveness of the wasp at controlling clover weevil numbers, that clover growth is increasing, and how the wasp is spreading around New Zealand.


    AgResearch reports the Irish wasp is reducing the clover root weevil by around 90% in monitored sites. The clover root weevil had spread throughout the North Island by 2006 and by 2010, had made its way to Southland. AgResearch scientists responded by organizing 91 wasp releases in the lower South Island. Weevil populations started to drop within a year.

    In December 2016, AgResearch estimated that the benefits to farmers is aboout $158 million per year. This is through reduced production losses and reduced fertiliser usage. The programme has more than paid for itself. Research and development cost $8.2 million - and the savings over ten years are estimated to be $489 million!

    Although the Irish wasp is successful at reducing weevil numbers, AgResearch scientists suggest that farmers adopt a few farm management practices to maximize clover growth and biocontrol benefits. Visit the AgPest website for more information.

    Learning about biocontrol

    To investigate the use of the Irish wasp as a biocontrol agent for the clover root weevil in New Zealand, and to understand its significance in the wider context of dairy farming, see our Unit plan for primary and lower secondary schools: Biocontrol in action.

    Related content

    Find out more about the life cycle of the parasitoid wasp.

    Originally written by Barbara Ryan, NZ Science Mathematics and Technology Teacher Fellow, 2006.

      Published 10 October 2007, Updated 5 December 2016 Referencing Hub articles
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