Gamma irradiation as a mitigation treatment for false codling moth

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DUARION: One year
Lead institution: Citrus Research International
Beneficiary: The citrus industry

South Africa exports citrus to various phytosanitary markets like the USA, Japan and China. These markets require the cold sterilisation of citrus fruit to ensure that pests are eliminated. One such pest is the false codling moth. It is indigenous to southern Africa and particularly prevalent in the Western Cape. The larvae of the moths infest fruit in orchards and cause post-harvest problems, usually in packed fruit. During cold disinfestation, fruit is subjected to -0.6°C for about 22 days. The treatment is not ideal for citrus, since the cold can damage the fruit by causing chilling injury.

Gamma irradiation studies

A project was conducted to test an alternative but safe mitigation treatment for the elimination of false codling moth in South African export citrus. Gamma irradiation, a proven technology for pest sterilisation, was explored. Hendrik Hofmeyr of Citrus Research International led the project at the XSIT insectary in Citrusdal. Preliminary research, entailing gamma dosage rate studies, were conducted at iThemba LABS in 2009.

Experiments on false codling moth eggs and larvae

  • In the first experiment, eggs were aged on wax paper (two A4 sized sheets) and on fruit (six units) for 72-96 hours – just before hatching. The eggs were irradiated with 100 Gray (Gy). After the treatment, the eggs were incubated at 26°C on the fruit and the wax paper. The eggs were inspected, categorised as dead or hatched and counted.
  • In the second experiment, eggs (72-96 hours old) were used on wax paper (two A4 sized sheets). The eggs were irradiated with 100 Gy, disinfected with formaldehyde and 500-800 eggs were inoculated in each of ten rearing jars. The jars were incubated at 26°C until the larvae pupated. The pupae were counted and incubated again until moth eclosion. A similar but untreated control was prepared for comparison. A newly emerged female and male were paired in each of ten 30 ml plastic containers (for test and control purposes). After laying eggs, the females died and were dissected and inspected for the presence of spermatophores in the bursa copulatrix, to confirm successful mating. Seven days later, hatched eggs were inspected to confirm the fertility or sterility of the parents.
  • The third experiment was conducted on mature larvae, contained in three rearing jars per treatment. Fifty additional larvae were retained to confirm larval age via head capsule measurements. After treatment with 75-150 Gy (in 25 Gy increments), the jars were incubated as before. Pupae were counted, incubated and moth eclosion recorded.
  • Experiments were conducted on mature larvae in rearing jars and on fruit, to establish whether irradiation (at 60 Gy) would affect larvae produced in rearing jars and in fruit similarly. Test results demonstrated an equal effect and the production of viable eggs were completely prevented.
  • Test results demonstrated that eggs (50-100 Gy) and larvae (50-400 Gy) become more resistant to gamma irradiation with age.

Outcome of study

  • The project established that a gamma irradiation dosage as low as 60 Gy could prove to be a viable option for the disinfestation of citrus fruit. This was considered improbable until fairly recently, but even lower dosages should now be evaluated.
  • The effect of gamma irradiation on artificially reared larvae in synthetic diet and naturally developing larvae in oranges must also be compared to ensure that the substrate containing the most radio tolerant larvae is used in a final Probit-9 study.