New projects get underway

Three new research projects aim to help fruit growers navigate the challenges of spotted wing drosophila. By Anna Mouton.

Hortgro, BerriesZA, SATI, and SAWine have joined forces to fund two crop-protection projects led by Prof. Pia Addison of the Department of Conservation Ecology and Entomology at Stellenbosch University.

A third project, funded by Hortgro and led by Dr Renate Smit, investigates postharvest mitigation. This research will be conducted at PHYLA, Hortgro’s bespoke phytosanitary research facility.

All three projects launched in October 2024 and are scheduled for completion at the end of 2027.

A risk-management system

Spotted wing drosophila has become a significant pest of soft-skinned fruit worldwide. Its wide host range and rapid reproduction enable it to adapt and thrive seemingly wherever it goes. How will it fare in South Africa?

A multi-industry project seeks to understand how spotted wing drosophila interacts with the range of hosts and environments in our main fruit-growing regions. The project’s data could be used to create a web-based tool to help growers identify their risk.

To assess the flies’ fruit preferences, the researchers will establish a spotted wing drosophila colony in the Stellenbosch University insectary. Captive flies will serve on taste panels, choosing preferred fruit types and cultivars in which to lay eggs.

The researchers plan to test the flies’ responses to stone fruit, cherries, blueberries, and wine and table grapes. They will also expose flies to fruit of different maturities. The flies will have 24 hours to lay eggs before the researchers remove and incubate the fruit. The number of emerging flies will provide a measure of host preference and suitability.

To learn more about the flies’ population dynamics, traps and temperature loggers will be deployed in the Western, Eastern, and Northern Cape, as well as Limpopo Province, for one growing season. One early and one late plum, cherry, blueberry, and wine- and table-grape cultivar will be included.

Traps will be monitored two-weekly, and fruit damage assessments will be conducted at various intervals.

In the Western Cape, the most efficient trap placement within a block will be studied by placing several traps in different positions. This will also provide information on potential edge effects.

The laboratory and field data will be integrated with published population models and the TerraClim platform to generate maps of egg-to-adult development success. These will then be further refined to account for crop susceptibility, spatial crop distribution, and fruit ripening and provide growers with crop-specific risk maps.

Integrated pest management

The ability of spotted wing drosophila to lay eggs in ripening fruit presents growers with a dilemma. On the one hand, the flies can destroy the fruit, and on the other, pesticide applications can lead to residues and rejections.

Furthermore, excessive or inappropriate pesticide use is highly likely to lead to resistance. There’s a reason geneticists love drosophilids as model organisms — the flies breed quickly and mutate readily. Resistance to spinosad has already been reported from California.

This is why integrated pest management, the focus of the second multi-industry project, will be critical to successfully controlling spotted wing drosophila in South Africa. The research will include evaluating biocontrol and developing an attract-and-kill system.

Previous work conducted by Dr Urban Spitaler, who heads the plant-protection-product testing group in the Institute for Plant Health at Laimburg Research Centre in South Tyrol, identified strains of a yeast, Hanseniaspora uvarum, that are highly attractive to adult spotted wing drosophila.

Spitaler’s research is in the early stages of commercialisation. He hopes his work will lead to an attract-and-kill product for growers. Read an interview with him here.

For the South African work, a student of Spitaler will join the Stellenbosch University team to determine whether we have local strains of spotted wing drosophila-attracting yeasts. If so, this could form the basis of an attract-and-kill product.

In parallel with the yeast hunt, commercially available attractants will be evaluated for use in attract-and-kill products.

To explore the potential biocontrol of spotted wing drosophila, the researchers will survey plums, cherries, blueberries, wine and table grapes, natural vegetation, and alternative hosts for parasitoid wasps. They will sample fruit from sites where trap catches show the presence of spotted wing drosophila. They will also check the traps from the risk-management-system project for parasitoids.

The researchers will also screen local and commercial entomopathogenic fungi and nematodes against final instar spotted wing drosophila larvae.

Postharvest mitigation

Although spotted wing drosophila occurs in most of our export markets, it is an invasive pest in those countries and has a restricted distribution in some of them. Therefore, their plant-protection organisations may impose phytosanitary treatments on our fruit. The third new spotted wing drosophila project aims to ensure we are prepared if this happens.

International research on fumigation, cold treatments, and a combination of these already exists for table grapes, cherries and berries. The Hortgro-funded project will focus on the efficacy of these measures in stone fruit and cherries.

Three cold-sterilisation treatments will be tested: minus 0.55 °C for 22 days (T107e USA false codling moth protocol), 0.5 °C for 12 days (Belgium cherry protocol), and 2.2 °C for 18 days (T107a USA fruit-fly protocol).

Fumigation with nitric oxide under ultra-low oxygen conditions will also be tested. A concentration of 1% nitric oxide has been shown to control first and second instar larvae, and a concentration of 3% nitric oxide to control eggs and larvae. Fumigation will be done at 2 °C for 8 hours.

Once the most effective cold or fumigation treatment has been identified, further trials will be performed to generate supporting statistical data. If necessary, the results will be available to present to any of our markets, thereby protecting access for our stone fruit and cherries.