If pest researchers had a most-wanted hit list identifying the worst pests in New Zealand pastures the latest research puts grass grub as number one offender.
Based on a new economic impact study grass grub makes the dubious claim of inflicting almost $600 million of damage a year on pastures, about a quarter of the $2.3 billion of losses a year caused by an assortment of pests along the length of the country.
In a research first, the study went beyond simply calculating the amount of foliage eaten or reductions in pasture production.
“This time we went a step further and worked to calculate what the losses were in terms of actual production, the value of the milk or meat that could not be produced as a result of that pasture loss,” AgResearch lead scientist Colin Ferguson said.
Essentially, measuring the opportunity cost of the pests’ impact has delivered some eye-watering figures that will now provide researchers with solid grounds to base economic decisions on when seeking control agents for any of the pests.
For grass grub, dairy farms wear about $380m a year and drystock about $205m a year.
The research is based on values earlier determined for pasture and clover with pasture estimated in 2016 to be worth $14.6b to NZ’s GDP and clovers $2.4b. But this figure for clover has also been estimated to be worth more, with one unpublished work putting it at $4.1b.
Other significant pests identified by the report, and with a price on their heads, include black beetle, Argentine stem weevil, clover root weevil and porina.
The research is even more seminal in light of the lack of work done in the past 25 years on pest impacts.
The report reveals between 1992 and 2016 only 3% of the research done on pests looked at estimating their economic impact. That compares to 30% of the research done when government-funded agency research was almost at its peak between 1972 and 1981.
The concerning thing for modern researchers is that farm systems and pasture species have changed significantly since much of that work was done but there has been no updated research to take account of that.
Ferguson said the findings don’t hold any major surprises for the researchers in terms of what the main suspects are but he was enlightened at the significance of the black beetle in the overall suite of pests.
“I think if you are from up north you will understand full well the impact of the black beetle. For us down south it was quite an eye-opener in terms of national importance.”
The beetle is now distributed throughout the North Island, including much of Waikato and Bay of Plenty, even extending along coastal margins to Foxton and Hawke’s Bay.
Sporadic outbreaks can cause widespread and severe damage across these regions and they have few natural enemies.
The need for pasture renewal has doubled under black beetle outbreaks over the past 24 years, with only endophytes appearing to help deter it.
Having some numbers around the pests’ costs is likely to help researchers in coming years as they work through possible control options.
Ferguson said AgResearch is examining a number of options for pest management including biopesticide controls.
“Usually for biopesticides we are talking about things like fungi or bacteria. There is one in the market now for grass grub, and others in the pipeline including one which has given promising results against Black Beetle and porina.”
Some earlier research found Black Beetle populations, both larvae and adults, can be affected by a small RNA virus that reduces their fecundity and length of life.
He likens the endophyte used to control Argentine stem weevil more as a biological control agent.
“In the case of endophytes, if you could put an economic value on that, it would be really significant but it is difficult to do.
“The natural state for perennial ryegrasses and some other species is a symbiotic relationship with endophytes, the value of that is probably immense in terms of pest management. On top of that the value of improved endophytes is between $336m and $525m per year.”
The development of endophytes used in modern ryegrasses for Argentine stem weevil control have also had a halo effect with activity identified against other pests like black beetle and porina.
Biological control organisms have had mixed success in NZ and the report has also proved important in putting a value on one of the country’s success stories, the wasp M. aethioppoides introduced in 2006 to control clover root weevil, discovered in 1996.
A very conservative estimate for clover root weevil’s effect if left untreated was a massive $391m a year.
However, the beneficial effect of the wasp has been similarly very conservatively estimated at $156.5m a year from this year onwards and the continuing $234m a year damage still leaves a strong case for further control development.
Ferguson said while the wasp has been an outstanding success the research data and other work also highlights risks biological controls can face.
A wasp introduced to control Argentine stem weevil is having declining success after a good start because it is parthenogenetic – only female larvae are produced, with no male DNA to improve genetic variability.
“The Argentine stem weevil appears to be withstanding the wasp more than it did and we are working to try and determine why that is the case.
“We have a very similar situation with clover root weevil and are very aware a similar situation could arise with that pest. Unlike for Argentine stem weevil there is no back-up for clover root weevil control and that should have alarm bells ringing.
“Knowing the cost of pests and the contribution their control agents make should provide an easier pathway in future to funding new controls and methods,” Ferguson said.
