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Sterile flies, sexual development and the surprising role of caffeine in Qfly control

Biosecurity

Early results from pilot sterile Queensland fruit fly releases in NSW and Victoria are promising, but scientists are aiming to drive even greater effectiveness by accelerating the sexual maturity of the sterile males using, among other things – caffeine.

The trial release of millions of sterile Queensland fruit flies weekly this season across Cobram (VIC) and Hillston (NSW) aimed at curbing the major horticultural pest is showing early promise with plenty of sterile male flies in the area and low wild fly counts. The releases between September last year and April this year are the culmination of hundreds of meticulous hours of work sterilising and rearing out populations of male Qfly using sterile insect technology (SIT), a key tool in the fight to eradicate Qfly outbreaks. Once released the sterile males outnumber the wild male population, limit the opportunity for wild females to mate with wild males and thereby disrupt mating and reduce subsequent generations.

Two million sterile flies mass reared in the purpose-built SIT fly factory at Port Augusta in SA are being released across Hillston and Cobram each week in the ‘post factory pilot’. Each site has unique characteristics, with the Cobram site focused on an urban area with surrounding production and the Hillston site centred on a spatially isolated production area. It follows releases early last year in South Australia, and it is planned that further seasonal trial releases will continue in the 20/21 and 21/22 seasons coinciding with Qfly activity. Whilst only early in the release program, the monitoring data gained to date is promising. Macquarie University Research Fellow Dr Bishwo Mainali notes that for the Cobram area “Most sterile fly captures have been within a 3km radius of the centre of the drop zones which indicates a good level of fly dispersion and prevalence across the sites with wild fly capture rates low”. This means that the sterile flies are persisting in the environment and can adequately engage with the wild fly population.

In Australia, SIT has been used as part of a suite of tools to successfully eradicate outbreaks, with examples including Qfly in Western Australia (1989-91) and Oriental fruit fly in Cairns QLD (1990s). Recent use of SIT saw its application to mosquitoes (Aedes aegypti) in Far North Queensland in 2018 achieving an 80 per cent reduction in population numbers. There has been some incredible success with SIT, but there remains significant opportunity to refine the process, reduce cost and improve the impact of SIT programs, especially for Qfly.

This is where the SITPlus program comes into play. Funded through the Hort Frontiers Fruit Fly Fund, SITPlus is a collaboration between some of the world’s leading fruit fly researchers and organisations working together to deliver an integrated solution to Qfly management.

What is Sterile Insect Technique (SIT)?

Sterile Insect Technique or SIT is a process where insects are mass reared, deliberately sterilised and released into the environment. The goal of this release is for sterile male insects to mate with wild females resulting in eggs that do not hatch, reducing the ability of the population to reproduce. Over time this suppresses the wild population and may eventually lead to eradication. Like any pest control method, there are advantages and disadvantages of using SIT. A key advantage of SIT is that it is socially and ecologically sustainable. SIT has the benefit of being pest specific i.e. it does not impact upon other insects and so is compatible with IPM programs. Likewise, its use leaves behind no residues unlike traditional pesticides, and so is more appealing to markets domestically and internationally. SIT is also a proven technology; it has demonstrated significant pest reduction achievements previously across a range of locations and pests.

The disadvantages are that the technique requires a level of investment to develop the necessary infrastructure and knowledge to support the production, sterilisation, rear out and application of the flies. SIT is also most effective when pest numbers are low, and sterile insects can overflood the wild population. Because of this SIT is best used as part of a broader Area Wide Management (AWM) initiative. AWM is a coordinated approach which is taken to manage a pest over a defined geographic area by a community, not just growers. This is quite important when dealing with a highly mobile pest with an extremely numerous and diverse host range like Qfly, as the potential for reinfestation from unmanaged areas such as urban areas, abandoned orchards and feral trees is quite high. Area wide applications of sterile insects can however effectively treat these unmanaged areas. The website area-wide-management.com.au has a range of resources which explore AWM in more depth.

Getting sterile flies on the job faster

The actual release of sterile flies is not the only work being conducted; a diverse range of enabling research is being undertaken to improve the quality and performance of the sterile flies and update our application of SIT to world standards. An example of this enabling research is the work recently conducted by Saleh Adnan at Macquarie University. Saleh’s work has broadly focused on the use of methoprene (a growth regulator similar to juvenile insect hormone) as a supplement to reducing the time needed to achieve sexual maturation of sterile male insects.

The issue

The goal of a SIT program is for sterile males to successfully mate with wild females, ultimately suppressing wild population numbers. To maximise the impact, you really need every sterile male released able to mate with their wild female counterparts. After pupa emergence, adult Qflies have a lag period of 10-14 days before they reach sexual maturity. Additionally, Qflies are anautogenous meaning that they require access to a protein source in addition to other nutritional needs before they can complete sexual development. Qflies released as two–threeday-old immature adults to reduce damage to the flies and costs still have around a week before they are sexually mature and ready to mate. During this time, they need to successfully navigate environmental pressures, e.g. predation, pathogens and abiotic stresses, all of which increase mortality.

Figure 1: Relationship between age (post-emergence) and proportion of male Q-flies mating after being fed one of two diets combined with methoprene supplementation (0, 0.05, 0.1, 0.5 per cent).

A solution?

Adnan’s work began by looking at the use of methoprene and how it was applied. Previous research suggested that methoprene administered topically would increase sexual maturation of Qflies, but this has the disadvantage of requiring an acetone solution carrier which can cause defects in the flies. A novel solution was to incorporate methoprene into the diet. This was achieved by using a commercially available product NOMOZ; a water soluble pesticide for mosquito larvae control. This proved suitable and more cost effective than analytical grade methoprene. By using a combined methoprene, sugar and yeast diet administered for two days post emergence, the time required to reach sexual maturity was significantly decreased. This was evidenced through an increased propensity for mating by males and a longer mating duration for both male and female flies (Figures 1 and 2). Later experiments conducted showed that these increases in sexual performance held true for irradiated sterile Qflies as well. Further field trials again proved that the sexual performance of the flies was improved in a simulated crop setting and that these improvements were the result of sex organ development and not just behavioural responses.

Whilst initially Adnan’s work did not reveal issues around the use of methoprene and longevity, further exposure trials to stresses such as extreme dryness, a lack of water, and food did indicate an issue. Methoprene supplements could increase the susceptibility of Qflies to these stresses and reduce their lifespan, most likely due to elevated metabolism levels.

A caffeine hit?
Working from the premise of a stimulant improving sexual development Adnan has also recently conducted research into the effects of caffeine on Qfly. In a yet to be published study, Adnan experimented with supplemental dietary doses of coffee, guarana and analytical caffeine. Adnan found that whilst guarana and coffee had no effect, caffeine was able to significantly increase the mating propensity and reproductive organ development in male Qflies. The effects were similar to those seen with methoprene and raspberry ketone dietary supplements.

What does this mean for Qfly SIT?
The use of pre-release dietary supplements holds promise to accelerate sexual development for Qflies. However, the use of these supplements should be considered and based upon the prevailing environmental conditions to ensure longevity in the field or a willingness to accept a trade-off between earlier mating and potentially decreased longevity in the field. There also remains the opportunity to further refine the use of the supplements identified and work to discover others which may be of practical use in reducing the delay between release and maturity of Qfly in SIT programs.

This article appeared in the Autumn edition of Australian Fruitgrowers Magazine, and was written by Chris O’Connor, Industry Liaison and Outreach Coordinator, SITplus Post-production Pilot Project, Macquarie University.

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