Cold treatment unlocks new fruit fly control options

In 2017, a systems approach for mitigation of the risk of FCM in citrus exported from SA to Europe was developed and implemented under the name of the FCM risk management system (FMS).

Two years later, the EU amended its regulation regarding non-European fruit fly pests on specific fruit types, including citrus, imported from specified countries. Fruit originating from these countries would either have to come from fruit fly free areas or be subjected to effective fruit fly treatment.

In response, the South African citrus industry applied a systems approach for fruit fly, similar to the FMS.
One of the postharvest measures required by the FMS for citrus (except lemons and limes), is shipping at temperatures in the range of -1°C to 4°C. While there are internationally approved cold treatments for fruit flies in citrus at temperatures of 3°C or below, the efficacy of treatments at temperatures higher than 3.2°C for fruit fly pests on citrus in Southern Africa required evaluation.

One treatment regime that satisfies the phytosanitary requirements for both FCM and fruit flies exported from Southern Africa, presents obvious efficiency and cost-effectiveness advantages to the local citrus industry. Hence a project was designed by Citrus Research International (CRI) and co-funded by the Postharvest Innovation Programme to determine if fruit fly pests can be controlled by temperatures above 3.2°C, as provided for in the FMS.

The two-year study was led by Dr Aruna Manrakhan from CRI.

Study design

Three fruit fly pest species have been recorded on some citrus types in Southern Africa, namely Ceratitis capitata, Ceratitis rosa and Bactrocera dorsalis. Studies to test the efficacy of cold treatment on all three species would be time consuming and costly. Hence, it was decided to determine which of the three was the least sensitive to cold and proceed with further investigations on that species alone.

With this principle established, the following study objectives were formulated:

1. Determine the larval development time of the three fruit fly species in an artificial rearing medium
2. Compare the in-vitro (in the artificial rearing medium) and in-vivo (in fruit) cold tolerance of third instar larvae of the three fruit fly pests at 3.5°C
3. Determine the efficacy of a 3.5°C treatment for the most cold-tolerant fruit fly species at 4, 8, 12, 16, 20, 24 and 27 days (exploratory testing)
4. Confirm the efficacy of a 3.5°C treatment for the most cold-tolerant fruit fly species for 24 consecutive days.

Findings

In terms of the first objective, the researchers found no significant differences in larval development time between the three fruit fly species. However, the results of the second objective clearly showed the Ceratitis capitata was significantly more cold tolerant than B. dorsalis and C. rosa at 3.5°C, in both the in-vitro and the in-vivo tests.

Working with the third instar larvae of C. capitata, the third objective was carried out in a three-replicate study at three different temperatures, ranging from 3.53°C to 3.57°C. In the first replicate, exposure to cold was evaluated for up to 27 days. In the two other replicates, exposure to cold was evaluated only for a period of up to 24 days. No survivors of third instar larvae of C. capitata were recorded beyond 16 days of cold treatment.

Objective 4 was fulfilled with a large-scale test carried out on Valencia oranges in three replicates. At mean temperatures between 3.5°C and 3.8°C for 24 days, there were no survivors among a total estimate of 26 753 third instar larvae that were treated.

Results and recommendations

The study proved that a cold treatment at or below 3.5°C for 24 days is effective for disinfestation of C. capitata in citrus. This treatment, either on its own or within a systems approach, will effectively mitigate the risk of fruit fly pests on export citrus produced in SA.