Dr Aruna Manrakhan of Citrus Research International reviews its use in South African citrus and deciduous fruit. By Engela Duvenage.
“Prior to 1980, fruit industries relied on single-point disinfestation treatments to mitigate insect pest risks,” said Manrakhan. “The ban or restricted use of halogenated fumigants like methyl bromide and ethylene bromide in the 1980s gave birth to the development of a systems approach.”
Manrakhan explained that a systems approach relies on the implementation of measures which work independently to cumulatively reduce risk.
Various systems approaches to manage phytosanitary risks in fruit have been developed and implemented with increasing success since the early 2000s. These combine at least two independent risk-reduction measures.
The international standard for the systems approach — ISPM 14: The use of integrated measures in a systems approach for pest risk management — was adopted in 2002 and has become the basis for at least 60 international protocols.
Greater than the sum of its parts
Systems approaches typically include pre- and post-harvest measures, for example, cultural practices, crop-protection products, resistant or tolerant plants, and harvesting during a period which is less risky for infestation.
ISPM 14 provides guidelines for the implementation of a critical-point system to monitor and reduce risks.
“This can include thresholds for actions such as the number of pests allowed,” clarified Manrakhan, “for instance sample-trap thresholds in orchards or pest limits at fruit-sampling points.”
Manrakhan highlighted the importance of establishing thresholds that indicate when responses need to be adjusted. “Thresholds can be set based on practical experience and refined as data become available.”
The international standard for the systems approach for pest risk management of fruit flies adopted in 2012 is largely based on ISPM 14. Its risk-reduction measures focus on:
* Minimising exposure to pests by creating areas that are free of pests or have low pest prevalence. This includes pest management, pest exclusion and growing produce in tunnels to exclude pests.
* Minimising host vulnerability by using a less susceptible fruit type or maturity stage.
* Reducing infestation rates through post-harvest treatments and inspection.
* Reducing the risk of pests establishing by using models to determine if a destination climate is a good or poor pest habitat.
The systems approach protocol to import stone fruit such as apricots from Spain to the United States for instance includes mitigation measures against the Mediterranean fruit fly. This includes trapping to demonstrate a low Medfly threshold of not more than 0.5 flies per trap per day.
Fruit flies in the South African context
Manrakhan shared observations from her time working in the Western Cape deciduous-fruit sector in the mid-2000s. “We saw very low fruit-fly prevalence early in the season in many orchards, and in many areas in which different techniques such as baiting and sterilised insects were used.
“Over the course of two years, fruit-fly infestation rates in various deciduous crops were 0.6% at most at harvest. Most infested fruit were infested by Medfly.”
She also discussed the use of a systems approach to control fruit flies in South African citrus destined for the European market. It was approved in 2019. “The industry had varied success with interceptions on citrus in 2019. There were none in 2020, but unfortunately, 2021 had two interceptions.”
Three species of fruit fly are a worry to South African citrus exporters: Mediterranean fruit fly, Natal fruit fly, and Oriental fruit fly.
Two different systems approaches — one for lemons and limes, and another for oranges, mandarins, and grapefruit — are used, based on the susceptibility of specific citrus types.
“Our two-year survey of 43 000 export-grade lemons showed that lemons are not fruit-fly hosts,” reported Manrakhan. “We did not find any infestation, despite the presence of fruit flies in those orchards.”
The official inspection of samples by the Perishable Products Export Control Board acts as a control point for packed lemons and limes. If there are any interceptions, the fruit is rejected for export.
Three measures are used for oranges, mandarins, and grapefruit. The first control point is good agricultural practices, including orchard-trap monitoring and control measures at specific times of the year, and pack house inspections. Symptomatic fruit is rejected on the pack-line, and samples of packed fruit are officially inspected. Cold shipping further mitigates any remaining risk from fruit flies.
Studies between 2013 to 2019 determined the effectiveness of various fruit-fly baits. “Mostly there was less than one female per trap per week,” said Manrakhan. “We found no infestation in different citrus types. If baits are applied correctly, these can therefore already reduce fruit-fly risk at orchard level.”
More post-harvest practices to reduce risk
Manrakhan and her colleagues are currently quantifying the efficacy of pack house sorting and grading to remove symptomatic fruit. Since 2019, the local citrus industry has distributed identification guides to pack houses to help sorters identify internal and external damage and refine their sorting process.
A cold-shipping regime further mitigates the risk of fruit flies. Cold-treatment measures were determined based on international protocols against Medfly, as experiments showed it to be the most cold-tolerant of the three fruit flies of economic importance to the local citrus industry.
Pulp temperatures are 0–3 °C for 16–22 days. “In recent post-harvest treatments, we exposed 50 000 larvae to 3.5 °C for 24 days, and 70 000 larvae to 5 °C for 27 days. There were no survivors,” said Manrakhan.
Her team is currently in their second year of quantifying the efficacy of the systems approach for South African citrus. This work is necessary to generate the evidence that will maintain market access for South African fruit. Because, as Manrakhan reminded the audience, the onus is on South Africa to prove our case to importing countries.
Image: Dr Aruna Manrakhan of Citrus Research International. Supplied by Echo Media.