A citrus perspective on Oriental fruit fly

What can the Western Cape learn? By Grethe Bestbier.

Up to now the Western Cape has dodged the Oriental fruit fly bullet, but other parts of South Africa have not been so fortunate. According to Dr Aruna Manrakhan, research entomologist and Fruit Fly Programme Coordinator at Citrus Research International, Oriental fruit flies are becoming increasingly established in the northern and northeastern parts of the country.

What does a single fly tell us?

The Oriental fruit fly first invaded South Africa in 2013. Since then it has established in Limpopo, Mpumalanga, North-West and Gauteng Provinces as well as parts of KwaZulu-Natal.

Manrakhan has been studying the pest for several years. The Oriental fruit fly population is growing in the north and a higher population poses a threat to other areas of the country. “There is a lot of pressure on other areas, ” says Manrakhan.

It is important to distinguish between incursions versus establishment of flies. Manrakhan explains that a single detection of a male only indicates that it is necessary to determine whether a population is present. “One fly is considered ‘no control action’, except for placing more traps around,” she says. “When we talk about incursions per se, it is more than one fly in an area and this triggers ‘eradication actions’.”

However, Manrakhan warns, if Oriental fruit fly incursions are not eradicated in the Western Cape and if conditions are suitable, the Western Cape could have the same experience as the north: flies all around, constantly being found in traps, indicating an established pest.

How fond of citrus are Oriental fruit flies? Compared to, for example, mangoes grown in the north or deciduous fruit grown in the Western Cape, citrus is not their preferred host. It presents certain barriers to infestation, such as the peel’s toxicity to eggs and developing larvae.

A study done in 2016–2017 looked at the infestation of sweet oranges in the northern Limpopo and Mpumalanga Provinces. Rates of Oriental fruit fly infestation were low. The researchers also found that damaged or vulnerable fruit, such as those already infested or dropped from trees, were more susceptible to infestation.

Two top-notch techniques for control

“The pest is actually fairly easy to control, because of two techniques that are available, with one using a specific male lure,” says Manrakhan. “With these two methods, we target males and females to bring populations down.”

Where the pest is present, growers are advised to apply bait sprays that target predominantly females, in combination with male annihilation technique. The latter uses blocks of wood impregnated with a lure and a pesticide that attracts and kills males. These two techniques, together with good orchard sanitation, can reduce fly catches to as low as two flies per trap per week.

“We’ve tested these techniques in some citrus plantations for two years, and we consistently found that you can keep to the threshold that has been established by DALRRD [Department of Agriculture, Land Reform and Rural Development],” says Manrakhan. “Below three flies per trap per week is acceptable, and it is possible to do that with the combination of these techniques plus orchard sanitation.”

Citrus grows abundantly in the Western Cape and international markets are increasingly intolerant of Oriental fruit fly. Hence the pressure to keep these pests out. According to Manrakhan, alertness has never been so important. “Now is the right time to take action. There are high chances for incursions in the Western Cape, so we need to increase vigilance. Traps need to be continuously checked. If a fly is detected, it should be reported immediately, and actions should be rolled out fast. We need to make sure our fruit is pest-free.”