Weevil fight in wasp’s favour
An Irish wasp smaller than a sandfly has got the costly clover root weevil on the run, and saved the pastoral industry $490 million.
AgResearch scientists say the benefit will last even longer, estimating that the value to the primary sector of controlling the weevil is $158m a year ‘‘into perpetuity’’. The cost of the programme’s research and development is a one-off $8.2m.
AgResearch science team leader Dr Alison Popay said the programme was ‘‘a real success story in the continuing battle against pests on New Zealand’s farms’’.
‘‘The wasp was so successful the team found that it reduced weevil populations by around 90 per cent in monitored areas where the wasp is well established,’’ Popay said.
A mainstay of agriculture, clover is not just a high quality feed, it also fixes nitrogen. While nitrogen losses can be offset by applying nitrogen fertiliser, that costs money and risks higher nutrient run-off and higher greenhouse gas emissions.
Scientists first became aware of the clover weevil Sitona lepidus in 1996, by when it had spread to 200,000 hectares of the North Island.
A 2005 study estimated that without control, the weevil could cut farm margins by 10 per cent to 15 per cent. It spread readily in New Zealand because of a lack of natural enemies, abundant clover, minimal competition from other species, a favourable climate, and because it is a highly mobile species.
In 2006 the Irish parasite wasp Microtonus aethiopoides was released in an experimental phase after testing to make sure it would not have an impact on other organisms.
All of the wasps are female and propagate by parthenogenesis injecting eggs into the adult weevil. The eggs grow into grubs which feed inside the still-living weevil, rendering it infertile. Once fully grown, the parasite grubs eat their way out of the weevil to pupate in the soil, and the weevil dies.
Once the pupae have developed into adult wasps the cycle begins again. One Irish wasp can kill about 85 clover root weevils.
Early results from the 2006 release were promising. The same year scientists discovered the weevil had crossed Cook Strait, most likely hitching a ride on cars or freight.
The Irish wasp spread from its release locations at about 20 kilometres per year, mainly as eggs and grubs within adult clover root weevils, which can fly well.
AgResearch tracked the spread of the weevil and the wasp and made additional releases to ensure all affected farmers would benefit from the biological control programme as quickly as possible. It developed a computer model to prioritise releases.
Scientist Colin Ferguson said the clover weevil would probably never be completely eradicated but the wasp should keep it in check.
‘‘It would be unusual for it to be eradicated - the wasp would be signing its own death warrant if that happened.’’
It was possible the weevil would be able to evolve in such a way it could combat the wasp, as has happened with the Argentine stem locations for wasp weevil, which appears to have developed resistance to a South American parasite wasp introduced in the 1990s.
If that happened scientists would have to investigate a new control method.
The control programme has been supported by DairyNZ, Beef + Lamb New Zealand, Federated Farmers, the Clover Root Weevil Action group, New Zealand Landcare Trust and fertiliser companies.