Under AgResearch Invermay pest specialist Colin Ferguson’s direction almost a million parasitised weevils were vacuumed up from Canterbury farmland and transferred to Southland where they were released to farmers in packs of 100.
AgResearch scientist Dr Scott Hardwick leads the Lincoln University-based CRW team. He said the establishment of the Irish wasp parasite into the Southland CRW population had been very successful. The parasitised wasps had caused reductions of greater than 90% of the CRW population in monitored North Island farms where the wasp had been released so the measure in Southland was to speed-up the control in the region where a massive infestation in the previous mild winter had exacerbated the spread.
“The mild conditions had allowed a greater number of clover root weevil eggs to hatch and more of the larvae survived right through the winter. As a result the population of the weevils exploded,” Hardwick said.
“Tremendous pressure was being placed on pasture from the increased numbers of larvae that fed on the clover roots and their nodules last winter. That damage was followed by the large population of adult clover root weevil emerging in early summer and feeding on the clover leaf. Normal spring and summer grazing of clover had added to the stress on already struggling plants.
“Affected farmers either saw clover disappearing very quickly once grazed or a complete absence of clover plants in their pasture. Unfortunately there is little that farmers can do by changing their management practices to minimise the impact of clover root weevil and there are no current effective pesticides.
“If the farm budget allows farmers can use nitrogen fertiliser to boost pasture production and cover for the lack of clover. However, there are no quick fixes and they shouldn’t adopt any unproven miracle cures,” Hardwick said.
Sampling has shown that on some sites 95% of CRW have been successfully parasitised but the success of the biocontrol measures will not mean CRW will disappear completely.
“You are always going to see some damage and a small amount of leaf notching – you will never completely wipe out the clover root weevil,” Hardwick said.
The clover root weevil.
Counting the cost
A cost-benefit analysis completed on the Southland CRW project has positive news for farmers and accountants alike. The paper, to be presented at the upcoming NZ Plant Protection Society conference, tracks the cost at $600,000 spent on spreading the parasitoid and finds the benefits paid for costs in only one year because of dropping weevil numbers.
Simulations were used to estimate the returns (dollars per hectare a year) using a wide range of values including the cost farmers would face if they had to replace the nitrogen-fixing ability of clover with synthetic nitrogen (urea). However, it didn’t include costs such as growing additional non-susceptible forage plants and-or an increase in supplementary feed. Returns were positive in almost all the simulations.
The project found that on dairy farms biological control returned $14.78/ha/year on an average Southland farm or $2.3 million over the 158,017ha estimated to be in dairy production. On sheep and beef farms the estimated return to individual farmers was $6.86/ha/year or $4.7m over 719,854 ha, AgResearch scientist Dr Scott Hardwick said.
“The results show in 98% of cases the money we spent of releasing the parasitic wasp in Southland was recovered in a year.
“This parasitic wasp is proving to be quite a firecracker. It’s establishing everywhere we put it – it’s proving very effective, and very quickly.”
Hardwick said the results of the research highlighted the benefits of biocontrol which offers a cost-effective, environmentally-friendly and permanent solution to pest control. Carefully selected biocontrol agents target only the pest, don’t harm desirable species and don’t pollute the environment. Once established they travel wherever the pest spreads and can return again and again to kill off new pest populations, all without human input.
