The Oriental fruit fly (Bactrocera dorsalis) is a mango and citrus pest of great economic concern which has spread like wildfire through the Lowveld of South Africa. It was first detected in East Africa in 2003, reaching the northern parts of South Africa approximately seven years later. By 2012 it was confirmed in Mpumalanga where much of South Africa’s subtropical fruit industry is based.
Researchers and government are working together to combat fruit fly in the Bushbuckridge and Nkomazi municipalities of Mpumalanga because overripe fruit on mango trees in home gardens are providing an ideal habitat for the fruit fly.
Researchers have found that early Tommy Atkins mangoes as well as cashew nuts are particularly enabling the survival of the fruit fly and they warn that it is vital to deter the Oriental fruit fly from spreading to new hosts, which could include other commercial fruit crops.
There are many subsistence and small-scale farmers in the Bushbuckridge area and also commercial production in the Nkomazi area and due to its proximity to both Mozambique and Swaziland, it is a buffer zone for the intensive mango and other subtropical fruit cultivation areas like Hoedspruit and Tzaneen.
“Overripe mangoes are its main host and we’ve basically provided the fruit flies with a highway. By last year it was everywhere in the Lowveld,” says Andries Bester, horticulturalist at Subtrop, the subtropical fruit industry’s umbrella organisation. The polyphagous Oriental fruit fly can use many fruit for its host, like guavas, but fruit with harder skins like avocado and citrus are, if undamaged, less prone to fruit fly attack.
Commercial mango farmers are taking a three-pronged approach to this new fruit fly: the male annihilation technique, poisoned protein-rich bait and an Efekto Eco Fruit Fly GF120 bait splattered in the orchard rows and on tree stems. Also, mangoes destined for export are picked when mature but still green and a high premium is placed on orchard sanitation. However, the value of their pest management is diminished if there is a continuous population pipeline of Oriental fruit flies only a few kilometres eastwards.
Some commercial mango enterprises have started placing fruit fly traps near where street hawkers are selling mangoes, often from informal orchards where pest management programmes are not in place.
In the Bushbuckridge and Nkomazi municipalities, crop advisers have been trained by the Tropical and Subtropical Crops division of the Agricultural Research Council and the Department of Agriculture, Rural Development, Land and Environmental Affairs (DARDLEA) of Mpumalanga to educate farmer associations, representing mostly subsistence farmers, on the presence of the Oriental fruit fly and the means of combating it, of which the most important is orchard sanitation through the disposal of infested fruit.
Dr Tertia Grové of the ARC, who led the programme, says: “Farmers were also encouraged to harvest mango fruit at a mature green stage and not when fully coloured and ripe. Mango fruit are usually not infested during the early fruiting period but are markedly more prone to attack closer to harvest.” For this reason, growers cultivating mangoes for use in achar, a spicy Indian condiment that is made with unripe mangoes, don’t experience fruit fly problems.
A large part of the programme included setting out traps in home gardens to monitor the incidence and species of fruit fly species, as well as to gauge their seasonality. It was found that the Oriental fruit fly was already well established in this area, as evidenced by large fruit losses to farmers and gardeners. She says that economically accessible monitoring and suppression techniques are needed, specifically aimed at small-scale farmers. Dr Grové emphasises that street hawkers should be included in a programme aimed at creating awareness of Oriental fruit fly.
Research done by the University of Pretoria’s Flies of Economic Significance Research Group and Citrus Research International has found that irrigation in commercial orchards, providing year-round moisture, enables the survival of the fruit fly. “Rainfall had no effect on the adult population in commercial sites, but there was an effect on the population in natural and interface sites, with populations showing an increase 4 months after rainfall. A lack of an effect of rainfall in commercial sites may suggest a role for routine irrigation in the survival of adults. This is due to B. dorsalis' survival being susceptible to dry conditions,” write Drs Weldon, Manrakhan and Theron in the Journal of Applied Entomology.
“This implicates fruit present in December–January, such as early season Mangifera indica (L.) cv. (Tommy Atkins) and Anacardium occidentale (L.) [cashew nut] as the main hosts contributing to increase in population levels in Limpopo and Mpumalanga provinces.”
They warn: “The suppression of B. dorsalis populations is also critical in preventing the spread of this species to new hosts. Allowing population size to reach levels similar to those in East and West Africa may lead to the exploitation of a wider array of commercially grown fruit and other hosts. An increase in population levels could also allow B. dorsalis to outcompete Ceratitis species [other fruit fly species], which has been found to occur in Kenya.”
Dr Grové concludes that natural enemies for the Oriental fruit fly should be sought out and imported.
