Codling moth biocontrol with Mastrus ridens
Adult parasitoidwasp Mastrus ridens seeking codling moth larvae in corrugated cardboard collars
Orchardists require economically viable production techniques that allow them to meet market requirements so that they can stay in business. Apple and pear growers in Australia contend with a range of insect pests that affect productivity, fruit quality, and economic viability.
The key pest in all Australian apple and pear growing regions except Western Australia is the codling moth. The Australian industry has for many years been investing in research to reduce reliance on synthetic pesticides by adopting biorational (ecologically benign, highly selective or behaviour modifying) techniques integrated with biological control and agronomic practices to provide economically and environmentally sustainable pest and disease management.
One of the desired outcomes identified in the Apple and Pear Strategic Investment Plan 2017-2021 is to improve industry profitability and global competitiveness. The plan includes the strategy of continuing to build the body of knowledge around pest and disease management and prevention, considering both biosecurity risk mitigation and cost reduction. The national PIPS (Productivity Irrigation Pests and Soils) program conducted from 2009-2014 developed new tools and knowledge for use by fruit growers. The Integrated Pest and Disease Management (IPDM) subproject team:
- assessed options for biocontrol of codling moth;
- investigated the biotypes of woolly apple aphid present in Australia in preparation for introduction of a predator to supplement control exerted by the parasitoid wasp Aphelinus mali;
- developed an improved scab management program; and
- reviewed the interactions between various control measures, pests and diseases, and agronomic practices in the orchard.
One of the results from this five-year national research was approval to import and release a biocontrol agent, the parasitoid wasp Mastrus ridens, against codling moth to supplement existing biorational techniques such as pheromone-mediated mating disruption. As a prerequisite to gaining this approval the research team conducted extensive host-specificity testing in specialized quarantine facilities, using native and endemic introduced insects (including species introduced as biocontrol agents of weeds). A second five-year phase, PIPS2, commenced in 2015 and included a four-year IPDM component tasked to complete field releases of Mastrus ridens in pome fruit orchards in eastern Australian states and to evaluate its effectiveness as a biocontrol agent against codling moth.
Mastrus release
Mastrus has now been released into sites in Qld, NSW, SA, Tas, and Victoria. The first releases were at Merrigum and St Germaines in Victoria in 2014. Releases were conducted at two orchards in the Stanthorpe area of Queensland in late April 2016. Releases were also made: during April 2017 in five NSW orchards (two in Batlow, two in Orange, and one near Young); one orchard in Tasmania (Grove) in May 2017; two orchards in South Australia and one in NW Victoria in May 2018; one orchard at Ardmona Victoria in spring 2018; and another in Batlow NSW in autumn 2019. Adult parasitoids were evident at all sites for a week after release. In the season following release, codling moth populations crashed at all release sites bar one (2019 Batlow site which has not yet completed a follow up season), resulting in much-reduced levels of damage.
Rearing Mastrus and codling moth
A large component of the project involved rearing the parasitoid (Mastrus ridens) and the host (codling moth) in separate cultures. The cultures were maintained at separate locations (Tatura for codling moth and Bundoora for Mastrus) so that the parasitoid could not inadvertently contaminate the stock culture of codling moth. The stock culture of codling moth consumed about 920kg of artificial diet, produced 296,000 larvae/year and an additional 78,000 larvae/year were reared under diapause (a form of hibernation) inducing conditions for storage and ultimately parasitism by Mastrus. The diapausing codling moth larvae spun cocoons inside corrugated cardboard bands that simulate cocooning sites under the bark of fruit trees. Mastrus adults were exposed to the bands in special cages that allowed the adult wasps to search for and parasitise the codling moth larvae. The parasitised larvae can then be stored for a couple of months before release in large numbers in orchards.
Summary of direct toxicity and sub-lethal effects (fertility and fertility of offspring) resulting from exposure of Mastrus ridens adults to dried residue of pesticides registered for use on pome fruit. Key to ratings (with IOBC accepted scale (Hassan et al. 1985) enclosed in brackets): L= Low (harmless); M = Moderate (slight-moderately harmful); H= High (high-very high); NS=no survivors to test.
The quality and number of the codling moth larvae exposed to Mastrus is important because poor quality larvae and/or overcrowding of the Mastrus may generate poor quality parasitoids. Influence of host larval size on yield and sex ratio of Mastrus was investigated by allowing Mastrus to parasitise either large or medium sized 5th instar diapausing codling moth larvae, then incubating the parasitised larvae until adult Mastrus emerged. Cold storage of parasitised codling moth rolls for more than 80 days is detrimental to Mastrus emergence and quality. Older Mastrus larvae are less sensitive to prolonged cold storage. Since Mastrus females lay most of their eggs, and higher quality eggs, within a week of emergence it may be more productive to expose codling moth larvae to Mastrus females for a shorter period and then incubate the parasitised larvae to allow further development of a more uniform population of the less cold sensitive larval stage before storing the rolls in the cool room. Further study is required to confirm the benefits, such as condensed emergence and greater fitness during field releases, of such an approach. The literature suggests that supplying good quality hosts (larger codling moth larvae) might improve yields of Mastrus. An experiment that investigated this produced a weak trend that supports that suggestion. The average yield of 2.06 Mastrus/codling moth larva from large sized larvae was at the lower end of the range (2-5 Mastrus/ codling moth larva) obtained by New Zealand workers and suggests that some other factor in addition to size may have contributed to lower yields.
The Mastrus colony comprised about 29,000 wasps/year as breeding stock, 4,000 wasps for pesticide testing, and batches of about 20,000 wasps ready for release. Guidelines for production of the parasitoid and its host codling moth have been developed so that commercial producers of biocontrol agents can assess feasibility of incorporating Mastrus into their suite of biocontrol agents for sale.
Effects of pesticides on the parasitoid
To ensure growers and their advisors have good information to help them support Mastrus populations within orchards to their advantage, the research team investigated the direct and indirect effects of common pesticides on the parasitoid (Table 1). Some pesticides were highly toxic to the adult wasps, whilst others had no direct toxic effect on adults but influenced fertility and subsequent performance of the next generation of wasps. Others were moderately toxic to adult Mastrus at field rates but did not have any effects on fertility or performance of subsequent generations.
Monitoring Mastrus release
Establishment of Mastrus in the release sites was initially monitored using sentinel corrugated cardboard rolls containing non-parasitised diapausing codling moth larvae from the culture at Tatura. This technique had been used at the original release site during the pilot PIPS project and had confirmed movement of Mastrus more than 40m from release trees.
Later monitoring attempts included use of corrugated cardboard trunk bands deployed in late summer-autumn to capture wild codling moth larvae seeking cocooning sites, which would then attract Mastrus, other parasitoids, or hyper-parasites if present. One female and one male Mastrus emerged from a total of 24 bands at the original release site, indicating that Mastrus had survived two seasons in the orchard. As most bands had earwigs present and ants were also present in bands from several locations, it is likely that these insects preyed on the codling moth larvae, as evidenced by few signs of larvae or spent pupal cases present.
Corrugated cardboard collars placed on trees to monitor establishment of Mastrus by presence of parasitised codling moth larvae.
In autumn 2019, before codling moth larvae were leaving infested fruits, fruit damage levels were assessed, and empty bands were deployed to provide cocoon sites for codling moth larvae (that would hopefully become parasitised) in all orchards that had received Mastrus. The bands were retrieved in winter 2019 and examined for presence of Mastrus or signs that Mastrus had been present. In most cases there were very few codling moth larvae recovered, probably because this reflected the low populations in the orchards after the initial populations crashed when Mastrus was released. Almost all bands contained earwigs, spiders and sometimes ants.
Other techniques investigated included use of various trap designs baited with codling moth larvae and/or larval aggregation pheromone. Further work is required to develop a cost-effective method of assessing establishment of the parasitoid, and there is an ongoing need to test new pesticides for their direct and indirect effects on survival of Mastrus.
If Mastrus has established and performs as well as it has in other countries then Australian pome fruit growers will have access to biorational tools targeting all stages of their most damaging pest, the codling moth.
Acknowledgements:
The introduction of Mastrus ridens was funded through the Hort Innovation Apple and Pear Fund, using the apple and pear R&D levy, contributions from Plant and Food Research Limited New Zealand and funding from the Australian Government. Funding for the distribution and establishment of Mastrus ridens was provided by the Hort Innovation Apple and Pear Fund using the apple and pear industry levy and Australian Government matching contributions. In both cases the Victorian Government provided additional funds, not managed by Hort Innovation, through the Agriculture Victoria Research branch of its Department of Jobs Precincts and Regions.
About the author: David Williams is a Principal Research Scientist – Invertebrate and Weed Sciences, Tatura Campus, Agriculture Victoria, Department of Jobs, Precincts and Regions (DJPR).
Email: [email protected]
