Sterile Medfly production

Mass-rearing facility

Depending on the time of year, our Medfly mass-rearing facility in Stellenbosch produces between 30 and 65 million Medfly males on a weekly basis. Since continuous fruit fly production is needed, the facility works 7 days a week throughout the year. Most staff members have attended intensive 3-month training programmes in the mass-rearing of fruit flies at the International Atomic Energy Association’s (IAEA) research laboratories at Seibersdorf in Austria.

The IAEA has developed a special genetic sexing strain (GSS) for Medfly (called the “VIENNA 8^D53+”) for use in SIT programmes. This strain is characterised by the female eggs having a sensitivity to heat (temperature sensitive lethal (tsl)), which enables the production of male-only fruit flies by eliminating female eggs with a heat-treatment. The GSS also results in pupae colour differences (female pupae are white & male pupae are brown) to enable the identification of males and females in the pupal stage. These scientific advances greatly saves on rearing costs and improves production efficiency.

The rearing process starts with the collection of eggs in the adult room. Females lay eggs through the gauze sides of the specially designed “cages” and eggs are collected in water troughs below. Collected eggs are heat treated to remove female eggs. Once “harvested”, the eggs are placed on an artificial diet (which includes water, bran, yeast, sugar and anti-microbial agents) on which the larvae develop.

After about 10 days the mature larvae leave the diet and are collected in water troughs. The larvae are then placed on vermiculite (which simulates soil) to pupate. Pupae are sterilised with gamma radiation 1 day before emergence. All sterile pupae are dyed with a fluorescent dye and placed in paper bags with an agar/sugar mixture for food and water. The sterile pupae are transported to regional eclosions to hatch under controlled conditions (temperature and relative humidity). Once the sterile male flies become sexually mature, they are cooled down, loaded into a specially designed chiller box before being released with a helicopter over the designated areas.

In any SIT programme, the production of high quality sterile males is the primary goal – poor quality sterile males will not compete successfully with wild males for the females. A number of parameters are used to measure quality, including egg hatch, the ratio of pupae obtained from a given number of eggs, pupal weight, adult emergence, flight ability of the sterile males, and degree of sterility. Each batch of flies produced is subject to the Quality Control tests, and remedial action taken if necessary.

A Quality Management System (QMS) has been implemented in the rearing facility, the purpose of which is to be able to provide clients with an assurance of quality for the product. All of the many rearing and sterilization procedures are defined and described step-by-step. Instructions are specified for every procedure. Non-conformance reporting, auditing of procedures, and training are an integral part of the QMS. Daily Planning Meetings and Weekly Review Meetings are held.

The genetic filter process

Purifying the strain

When fruit fly strains selected for certain genetic characteristics are mass-reared for a long time, they tend to lose these characteristics due to genetic recombination to the ‘normal’ state. In the case of VIENNA 8 this would means unless the ‘bad genes’ are removed (filtered) from the rearing process, the production of males only for releases, and the white pupae/brown pupae characteristic would be steadily lost.

To prevent this, the colony is constantly purified by removing ‘wrong sex’ pupae (white males and brown females) in a separate filter unit. This process also automatically removes any females that become resistant to the heat treatment. The colony is thus constantly regenerated using the purified material, ensuring that the colony maintains its genetic integrity, and preventing the release of females into the field, even though they are sterile.

Following additional funding from government, the Fruit Fly Africa Medfly Facility in Stellenbosch was extended in 2009. The additional building will increase the production capacity to approximately 40 million sterile males per week.