AgResearch science and team leader, biocontrol and biosecurity, Alison Popay explains.
- Pasture pests cost farmers more than $600 million each year.
- Populations of three weevil pest species have been suppressed in the short term using introduced braconid wasps.
- Natural pathogens of grass grub and porina can reduce these pest populations, but relying on natural occurrences of such pathogens alone is risky.
- Two bacteria used as biopesticides have been proven to significantly reduce grass grub populations.
Biocontrol of invasive exotic weeds and pests has proved highly-cost effective in New Zealand. The country’s productive sectors are vulnerable to invasion by species that arrive without their natural enemies, hence its strong focus on biosecurity. However, the careful selection and introduction of one or more of these natural enemies has led to the long-term suppression of the target weed or pest throughout the country.
Parasitoids and Predators
Parasitoids (parasitic wasps and flies) and predators are key components of sustainable pest management in pastures, whether they are natural inhabitants or deliberate introductions. Conservation measures that preserve and enhance their populations, such as maintaining host plant diversity and refuges where they can survive through unfavourable conditions, can help pest suppression.
Implementing a classical biocontrol programme involving identification and release of a suitable agent (or agents) requires careful consideration of the risks and benefits of introducing organisms into a new environment to ensure they don’t harm non-target species. There are strict regulatory controls on introducing new agents into New Zealand and extensive testing is undertaken to ensure they will have limited effects on native species.
In New Zealand, biological control programmes have been successfully implemented for three major weevil pests, the lucerne weevil, Argentine stem weevil (ASW) and most recently the clover root weevil (CRW). For each of these weevil pests, different species of parasitic wasps (Microctonus spp.) were introduced (Figure 1). These tiny wasps lay their eggs in adult weevils making the females infertile, and the larvae develop within the weevil, eventually killing it when they emerge as pupae.
- To control Lucerne weevila a Moroccan biotype of the parasitic wasp M. aethiopoides was released in the South Island between 1982 and 1985 with the parasitoid naturally dispersing throughout New Zealand by 1998 and providing a high level of pest suppression that has continued to this day1.
- In the early 1980s it was discovered that fungal endophytes protected ryegrass against attack by ASW, a major pest of ryegrass. To provide another tool for control of this serious pest, the wasp, M. hyperodae, was introduced2. This biocontrol agent initially appeared highly successful, but there is now evidence that parasitism levels may no longer be effectively reducing populations of this pest3 and work is in progress to understand why.
- The most recent parasitoid releases have been against CRW, first found inhabiting pasture in 1995. In this case an Irish biotype of M. aethiopoides has been released with spectacular success4. Beginning in 2006, the parasitoid was widely distributed throughout the North Island using nursery sites and many ‘mini-releases’ of parasitized weevils to farmers4. After CRW reached the South Island in 2006, strategic releases were made based on knowledge of dispersal of both the weevil and the parasitoid5. This project culminated in a mass release program in two regions in the southern South Island after very high populations of CRW caused clover to largely disappear from the area. Over two years an estimated 900,000 parasitized weevils have been distributed to farms across these regions by a small group of scientists and technical staff in conjunction with industry representatives. Already clover is returning to pasture with a cost benefit of $15/ha/year to dairy farmers in this region alone6. Overall, the programme has a cost benefit of $20/ha/year to dairy farmers nationwide (unpublished AgResearch data).
The majority of endemic and native insects, such as grass grub and porina, are likely to be associated with entomopathogens (disease organisms that attack insects), including fungi, bacteria, viruses, microsporidia and protozoa. The natural pathogens of grass grub and porina are able to reduce pest populations without human intervention. To be successful, the disease organisms need to be continually replenished in the soil by deaths of diseased individuals from the previous generation. Climatic and farm management interventions, such as drought and cultivation, can break the cycle of disease through successive generations by reducing both the host population as well as viability of the pathogens. Thus a reliance on natural disease cycles to maintain effective population levels is risky, particularly under intensive farming regimes where there is a low tolerance of yield reductions.
The term biopesticide is applied to the mass production and application of pathogenic microbes such as bacteria, fungi, microsporidia, viruses, protozoa or nematodes for the management of pests. Biopesticides are usually not self-sustaining and repeat applications are required to suppress populations. Factors that limit the mass production of effective organism and their use in the field include shelf-life and stability, consistent efficacy in the environment, rapidity of effects and ease of application.
In New Zealand, two species of bacteria with specific application to grassland pests are Serratia entomophila and Yersinia entomophaga. Pathogenic strains of Serratia entomophila cause ‘amber disease’ in New Zealand grass grub, quickly stopping them feeding although infected individuals take some time to die10. Natural disease outbreaks are known to occur but the bacteria can be mass produced and have been formulated for commercial application as a bioinsecticide11. Farmers are encouraged to apply S. entomophila before grass grub reach damaging levels to establish a source of inoculum in the soil that will continue to naturally infect grass grub larvae in the next generation7.
More recently, a toxin-producing bacterium, Yersinia entomophaga, was discovered naturally infecting grass grub. Although not common in populations, this bacteria has been found to affect a range of major pasture pests in New Zealand, including grass grub and porina larvae (Wiseana spp.)8, 9 and black beetle (Figure 2). Its use as a biopesticide is still in the experimental stage but it can be applied as a spray, in granules or incorporated into bait. The activity of Y. entomophaga is specific to insects and, although it may affect other plant feeders, beneficial species such as predatory staphylinids and earthworms are unharmed 9.
New Zealand pasture ecosystems remain highly vulnerable to outbreaks and chronic infestations of pests, in spite of outstanding achievements in biocontrol and plant resistance. However, armed with better knowledge of what drives these pest populations, new pest control tools can be added, ranging from increased pasture resilience through increased biodiversity, to new endophytes, biopesticides, and classical biocontrol introductions. This will ensure farmers can meet market expectations of sustainable, environmentally-friendly food production systems and still remain internationally competitive.
1. Goldson, S. L., and P. J. Gerard. 2008. Using biocontrol against root-feeding pests, with particular reference to Sitona root weevils. Root Feeders: An Ecosystem Perspective. pp. 115-133.
2. McNeill, M..R., S. L. Goldson, J. R. Proffitt, C. B. Phillips, and P. J. Addison. 2002. Biological Control 24: 167-175.
3. Goldson, S. L, S. D. Wratten, C. M. Ferguson, P. J. Gerard, B. I. P. Barratt, S. Hardwick, M. R. McNeill, C. B. Phillips, A. J. Popay, J. M. Tylianakis, and F. Tomasetto. 2014. If and when successful classical biological control fails. Biological Control 72: 76-79.
4. Gerard, P. J., D. J. Wilson, and T. M. Eden. 2011. Field release, establishment and initial dispersal of Irish Microctonus aethiopoides in Sitona lepidus populations in northern New Zealand pastures. BioControl 56: 861-870.
5. Phillips, C. B., M. R. McNeill, S. Hardwick, C. J. Vink, J. M. Kean, D. Bewsell, C. M. Ferguson, L. M. Winder, I. I.. Iline, M. C. Barron, and B. Stuart. 2007. Clover root weevil in the South Island: Detection, response and current distribution. New Zealand Plant Protection 60: 209-216.
6. Basse, B., C. B. Phillips, S. Hardwick, and J. M. Kean. 2015. Economic benefits of biological control of Sitona obsoletus (clover root weevil) in Southland pasture. New Zealand Plant Protection 68: 218-226.
7. Jackson, T.A. 1999. Factors in the success and failure of microbial control agents for soil dwelling pests. Pest Management Reviews 4: 281-285.
8. Hurst, M. R. H., S. A. Jones, T. Binglin, L. A. Harper, T. A. Jackson, and T. R. Glare. 2011. The main virulence determinant of Yersinia entomophaga MH96 is a broad-host-range toxin complex active against insects. Journal of Bacteriology 193: 1966-1980.
9. Ferguson, C. M., D. M. Barton, L. A. Harper, J. Swaminathan, C van Koten, and M. R. Hurst. 2012. Survival of Yersinia entomophaga MH96 in a pasture ecosystem and effects on pest and non-target invertebrate populations. New Zealand Plant Protection 65: 166-173.
10. Jackson, T. A., D. G. Boucias, and J. O Thaler. 2001. Pathobiology of amber disease, caused by Serratia spp., in the New Zealand grass grub, Costelytra zealandica. Journal of Invertebrate Pathology 78: 232-243.
11. O’Callaghan, M., Jackson, T.A. 1992 Selection, Development and Testing of Phage-resistant Strains of Serratia entomophila for Grass Grub Control. Biocontrol Science and Technology 2: 297-305
This article was originally published in Technical Series September 2016