NZFarmer : February 17th 2009
12 Straight Furrow • February 17, 2009 Pasture Management It has been more than ten years since clover root weevil munched a devastating swathe through the Waikato, Bay of Plenty and Auckland regions, earning its reputation as the ‘evil weevil’. Since then, it has spread throughout the North Island and in 2006, established footholds in Nelson and Canterbury. In 2006 AgResearch entomologists released a biocontrol agent – a tiny Irish wasp – to help strike back at the weevil. Three years on, who is winning the war: the weevil or the clover? This update is from AgResearch Entomologist, Dr Pip Gerard. UR research shows that clover root weevil is still a problem pest even though populations in some districts are now lower than when they first arrived. This is typical of the boom and bust cycle for an inva- sive pest arriving in a new area. For example, our study near Morrinsville in the Waikato shows that larvae reached a massive 1800/m2 (see graph) shortly after the arrival of clover root weevil in the mid 1990s. With this number of weevils in the system, white clover virtually disappeared and without food, the weevil population collapsed. Later, both the clover and weevil numbers recovered. O The graph shows that the level of clover root weevil larvae varies throughout the year. There is a small summer population and a large population over winter and spring, reaching economically damaging levels in most years. Prior to the Irish wasp release, the main thing preventing weevils reaching the whopping 1997 lev- els was the availability of the nitrogen fixing nodules found on the roots of clover. Female weevils can lay hun- dreds of eggs over several months, resulting in strong com- petition for available nodules: without them newly hatched lar- vae won’t thrive. The clover plant must put a lot of its resources into repairing roots and growing new nodules, resulting in less production above ground. Our research has shown that typical Waikato winter popula- tions of around 300 larvae/m2 reduce clover production by 35 per cent, mainly in spring. Economic assessments at the farm system level using farm deci- sion support models (StockPol and UDDER) show this level of damage decreases farm gross margins by 10 to 15 per cent if no intervention is taken. Pasture management Using nitrogen fertiliser is the quickest way to ensure feed sup- ply and farm profitability in wee- vil-infested regions. With clover root weevil present in the system, nitrogen fertiliser enhances clover and grass production. The New Zealand Clover Root Weevil Action group obtained MAF Sustainable Farming funding to investigate how best to use fer- tiliser and look into pasture man- agement strategies that may help improve clover vigour, enabling it to better withstand weevil attack. The key outcomes were: • Clover-friendly grazing man- agement (including preventing shading of stolons in spring but providing cover in summer) pro- duced 60 per cent more summer clover than the ‘conventional’ grazing and between 6 to 13 per cent more summer pasture overall. • Even when using 200kg N/ha/annum or more of fertiliser, clover content can be lifted by aiming for low post-grazing residues in spring and autumn and higher residues in summer. • High rates of nitrogen fertilis- er (over 200 kg N/ha/annum) appear to increase pasture vulner- ability to drought. • Nitrogen application at low rates aids nodule production in clover root weevil-infested pas- tures. • At the main site in the Morrinsville area, an application of 200kg N/ha/annum resulted in good pasture production, nitro- gen fixation, and weed control. An overall lower annual rate (e.g. 150kg N/ha) applied in split dress- ings after grazing from mid- autumn to late spring is likely to give the same benefits. • Two-year-old pasture follow- ing a maize or turnip crop had more clover, fewer weeds, and higher autumn production com- pared to pasture following white clover/grass. This is because going through a crop helps clean up weed, pest and disease prob- lems and provides better condi- tions for pasture establishment. For a typical Waikato dairy farm, this equated to gains of $490 to $541/ha for pasture estab- lished after crops compared to $336/ha for white clover/grass to www.straightfurrow.co.nz Biological control of ‘evil weevil’ working Clover root weevil biocontrol The use of nitrogen fertiliser is not an option for all farm systems. Consequently, a long-term eco- nomically and environmentally sustainable solution for clover root weevil was required to ensure farmers could benefit from the animal production and nitro- gen-fixing advantages of clovers. AgResearch entomologists white clover/grass renewal. • Overall, two-year-old diploid perennial ryegrass/white clover pastures had more grass and less clover than tetraploid perennial ryegrass pastures. There were no differences in total pasture production in spring, but diploid pastures had higher production than tetraploid in summer and autumn. The farm decision programmes (StockPol and UDDER) indicated it was most economically beneficial to use diploid ryegrass in the North Island sheep and beef farm and tetraploid ryegrass in the Waikato dairy farm. • With the support of the Meat & Wool NZ FITT fund, the knowl- edge gathered in this project has been collated and is available as a pamphlet (contact industry con- sultants or tina.eden@agre- search.co.nz for a copy). • These two sources summarise the life cycle of clover root weevil and suggest strategies to manage clover in the presence of this pest. They also contain an identi- fication panel which illustrates other pasture pests and how to distinguish them from clover root weevil. released the Irish parasitoid wasp Microctonus aethiopoides at experimental sites in the Waikato, Hawke’s Bay and Manawatu in early 2006. This tiny wasp attacks adult weevils, rendering females sterile almost immediately and breaking the life cycle. The host dies when the wasp larva emerges to pupate. The wasp has done extremely well at all these sites, even with the drought earlier this year. It has spread at least 15km from the Hawke’s Bay release site and 11km from the Bulls release site. Two strategies were then imple- mented to spread the Irish wasp throughout the North Island. One approach was the establishment of regional nursery sites as point sources for natural and assisted dispersal. With the assistance of Regional Councils and Landcorp, these have been set up at sites favourable to the wasp, and locals are being given training to under- take collections and distribution of parasitised weevils. Establishment has been con- firmed at all sites, except in Northland where it is suspected the wasp is not suited to the sum- mer twilights and climate. Last year the programme also received a boost when AGMARDT agreed to invest $30,000 into the Irish Wasp Dissemination pro- gramme in conjunction with Dairy NZ and Meat & Wool New Zealand. The second approach was to give out small samples of para- sitised weevils to farmers. Hundreds of these samples have been given out through pastoral industry networks and events. Farmers should watch for rural events where the samples will be available over the summer, or contact their local DairyNZ con- sulting officer or their Meat & Wool NZ regional manager. In districts not covered by events or the relevant personnel, the AgResearch biocontrol team is enlisting the support of Ravensdown account managers to get the samples to farmers. In the South Island, the Irish wasp has been released and established in Nelson. It is hoped that the wasp will spread with the weevil and control their popula- tions, so the pest doesn’t build up or spread as fast as it did in the North Island. No additional release activities are planned in the Nelson region. The recently-discovered Canterbury infestation is being monitored by AgResearch ento- mologists at Lincoln, who will decide on Irish wasp release options this year. DairyNZ, Meat & Wool NZ and the Foundation for Research Science and Technology have sup- ported this programme since its inception in 1997.
February 25th 2009