Area-wide fruit fly control helps South African fruit retain global market share. The Mediterranean fruit fly or Medfly is a global menace to fruit production. Both Medfly and its close relative, the Cape fruit fly, are phytosanitary pests. South African fruit growers must control these infernal flies to protect their harvests and markets.
FruitFly Africa, in partnership with DALRRD, supports the South African fruit industry through area-wide control of Mediterranean and Cape fruit flies, as well as surveillance for yet another menace to our industry, the Oriental fruit fly.
“We focus on providing cost-effective user-pay services,” says Ghian du Toit, manager of FruitFly Africa. He lists their core functions as monitoring, aerial baiting, and sterile insect technique (SIT). DALRRD partially funds monitoring and SIT. All other costs are recovered from producers.
Medfly birth control
“Sterile insect technique is a strong component of our services,” says Du Toit. “It has wonderful advantages — it’s internationally proven and environmentally friendly, which counts for a lot in our environmentally aware day and age.”
FruitFly Africa released approximately 65 million sterile male Medflies per week between October 2021 and May 2022. These sterile males are just as eager as wild males to mate with females — SIT works by flooding an area with sterile males that outnumber and outcompete fertile wild males.
Medfly SIT uses the Vienna strain of fly specially developed in Austria by the United Nations Food and Agriculture Organization in collaboration with the International Atomic Energy Agency (IAEA). Like computer operating systems, the Vienna flies are continually updated — FruitFly Africa is currently on Vienna 8.
Female Vienna Medflies carry a temperature-activated lethal gene — exposing fly eggs to high temperatures kills the females. The male survivors are raised until they pupate, and the pupae are then sterilised by radiation.
FruitFly Africa multiplies and sterilises flies at their headquarters in Stellenbosch. Irradiated pupae are packed in paper bags and sent to eclosion facilities in different fruit-production regions. Eclosion is the emergence of adult flies from pupae.
Most sterile male adults are chilled to 2–3 °C before taking a helicopter trip for their aerial release. During the most recent growing season, aerial releases covered nearly 42 000 hectares per week in Elgin-Grabouw-Vyeboom-Villiersdorp, the Hex River Valley, the Warm Bokkeveld, and Wolseley-Tulbagh.
During the off-season, approximately 30 million sterile males per week are released on the ground. A new eclosion facility opened in 2022 to provide flies for year-round ground release in the Kakamas area, focusing on Blouputs.
All this sounds complicated because it is. FruitFly Africa has the distinction of being the only facility in Africa producing sterile Medflies for SIT. But Du Toit is quick to praise its international partners such as the IAEA. “The free support we get is amazing. We can access their technical people and manuals, which is a great help.”
Area-wide aerial baiting
Between November and April, FruitFly Africa coordinates bait sprays from aircraft. During the 2022 season, the total treated area was about 208 000 hectares and included Elgin-Grabouw-Vyeboom-Villiersdorp, the Hex River Valley, the Langkloof, the Warm Bokkeveld, Wolseley-Tulbagh, and the Orange and Olifants River Valleys.
FruitFly Africa receives statutory funding for a minimum number of bait sprays per region, but growers can request additional aerial baiting.
Baits are called baits because they attract fruit flies that are killed when feeding on insecticide-laced droplets.
FruitFly Africa baits with a spinosad formulation. Spinosad is an insecticide naturally produced by bacteria originally discovered in a disused rum still in the Virgin Islands. Spinosad has low toxicity for mammals — it is used to treat fleas in pets and head lice in children — and is considered to have a low environmental impact.
An eye on the numbers
As every fruit grower knows, monitoring is a key part of pest control. FruitFly Africa monitors for Mediterranean, Cape, and Oriental fruit flies in Elgin-Grabouw-Vyeboom-Villiersdorp, the Hex River Valley, the Langkloof, the Warm Bokkeveld, Wolseley-Tulbagh, and the Olifants and Orange River Valleys.
In addition, monitoring by FruitFly Africa is an early-warning system under their agreement with DALRRD. When necessary, FruitFly Africa can respond to threats by rapid implementation of control measures.
Growers can also request monitoring for false codling moths and mealy bugs on a user-pay basis. Unlike fruit-fly monitoring, these services are not partially funded by DALRRD.
FruitFly Africa’s monitors deploy and service traps in each of their areas. They provide weekly catch data to the area coordinator, who communicates the results to growers.
Collecting fruit-fly catch data is a finicky business. First, monitors must discriminate between the target fruit flies and anything else that may have strayed into the trap. Bycatch is reduced by lures that specifically attract either the closely related Medflies and Cape fruit flies or Oriental fruit flies.
Next, monitors distinguish between sterile and wild Medflies in areas with SIT. To facilitate identification, a fluorescent dye is applied to sterile male pupae before release so that adult flies glow under ultraviolet light. Wild flies are sorted into males and females.
FruitFly Africa needs these detailed data to tell whether they are releasing sufficient sterile flies and whether baiting is working. They also identify and address hotspots through localised ground-based measures such as deploying attract-and-kill systems and releasing additional sterile males.
Suspected catches of Oriental fruit flies are confirmed by an entomologist and reported to DALRRD, which has a committee that decides on appropriate action, such as instructing FruitFly Africa to survey the extent of the infestation or conduct an eradication programme.
In January 2023, FruitFly Africa also began monitoring for a new threat to South African fruit producers: the polyphagous shot-hole borer. They have deployed traps in Elgin-Grabouw-Vyeboom-Villiersdorp, the Langkloof, Stellenbosch, and the Warm Bokkeveld. The project is funded by Hortgro.
Sustained effort brings success
The first pilot study of SIT in South Africa dates from 1999 when relatively small numbers of sterile male Medflies were released in the Hex River Valley. Although this provided proof of concept, SIT only gained traction after the National Department of Agriculture — now DALRRD — signed a Memorandum of Understanding (MoU) with the Deciduous Fruit Producers’ Trust — now Hortgro — in 2008.
Under this agreement, which is renewable every three years, the Department contributes 50% of the cost of fruit-fly monitoring and SIT. It boosted effective SIT implementation, culminating in the establishment of FruitFly Africa in 2013.
Data from the Hex River Valley — the cradle of South African fruit-fly SIT — show that trap catches decreased by 73% in the decade following the first MoU. Trap catches in the other SIT areas have also dropped significantly.
However, Du Toit stresses that area-wide control is just one component of an integrated-pest-management system. “Producers also need to do their part in terms of proper orchard sanitation,” he says. “And although the aerial baiting does wonders, they need to keep in mind that, in weeks without aerial baiting, producers need to do ground-based baiting.”
He also highlights the role of home gardens. “We know that fruit flies migrate to home gardens after the commercial fruit has been harvested. If we can clean up or replace fruit trees and other hosts in home gardens, we can bring down fruit-fly numbers.”
Ultimately, Du Toit sees FruitFly Africa as an integral part of the South African fruit industry. “FruitFly Africa is here to serve the industry — our people are committed to helping create a healthy rural environment and a pest-free fruit industry.”