ARC Infruitec-Nietvoorbij’s gamma radiation facility is ideally suited for sterilizing the fruit flies. All fruit flies destined for release are irradiated in the pupal stage at a dose of 90 Gy. Each batch has a radiation dose indicator to verify that a minimum of 90 Gy was given.
Sterility in the context of sterile insect technique (SIT)
If males receive too much radiation, it damages them to such an extent that they cannot perform (mate with wild females) properly. The radiation treatment of fruit flies for an SIT programme is therefore a compromise between limiting the likelihood of irradiated males giving rise to fertile eggs, and leaving them in such a condition that they can compete successfully with wild males in the field.
The males that are released are not, and cannot be, 100% sterile for the reasons given below. Male flies released by Fruit Fly Africa are on average 99% sterile; however, these males will not compromise the success of the SIT programme.
- To be effective in an SIT programme, male Medflies treated with radiation need to be able to compete effectively in the field with wild, unirradiated males; i.e. the irradiated males need to be able to fly properly, find females successfully, perform a complex courting procedure, mate with the female, and ultimately transfer sufficient sterile sperm to the female that she does not lay fertile eggs. Depending on the dose, radiation to a greater or lesser extent affects the ability of the males to be competitive.
- To fully sterilize male medflies so that 100% of them would be 100% sterile would require a radiation dose so high that they would be incapable of performing any of the above actions, if they survived at all.
- The radiation dose chosen in an SIT programme is one that is high enough to reduce to an absolute minimum the likelihood of males transferring fertile sperm, while still enabling the males to successfully find and mate with females. This is the principle adopted by all SIT programmes throughout the world.
- The radiation doses around the world for Medfly SIT vary from programme to programme, and are usually between 90 Gy and 120 Gy. The higher dose is usually used in eradication programmes. The South African SIT programme is currently a suppression programme.
- The minimum sterility of male Medflies in SIT programmes is prescribed in an international SIT Quality Control Manual as 95%. Male flies that are between 95 and 99% sterile will not automatically give rise to wild progeny.
- Should any wild female mated with a male irradiated between these sterility levels lay any fertile eggs, they will be exceedingly few, and even fewer eggs will hatch. If any eggs do hatch, the great majority of the larvae, if not all, will die before reaching maturity. Any adult flies that may result from these eggs will mostly be 100% sterile or nearly so, or incapable of mating successfully.
- The SIT Africa facility uses 90 Gy in accordance with the advice of an overseas SIT expert, with a view to greater male competitiveness in the field. Trials recently carried out by Fruit Fly Africa comparing 90 Gy with 110 Gy showed that the 90 Gy gives rise to males that are more than 95% sterile; the average sterility in 2012 was 99%. Any fertility from matings by these males should be cancelled out by their increased competitiveness in the field.
- The amount of fruit damage that could possibly arise from Fruit Fly Africa males is thus minimal and of no consequence to the SIT programme or to cost-effective fruit production.