Sterile Medfly production

Mass-rearing facility

Depending on the time of the season, up to 14 million Medfly males per week are currently produced at the Fruit Fly Africa Medfly mass rearing facility. As 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 mass-rearing fruit flies at the IAEA’s research laboratories at Seibersdorf in Austria. The others have received on-site training by IAEA experts in mass-rearing fruit flies.

A specially developed ‘genetic sexing strain’ of Medfly (“VIENNA 8”) has been developed by the IAEA in Vienna for use in SIT programmes. This strain is characterized by the females having sensitivity to heat (the temperature-sensitive-lethal – tsl – gene), and the white pupae (wp) gene whereby female pupae are white and male pupae are brown. The tsl characteristic enables the production
of only male fruit flies for the releases by eliminating females with a heat-treatment, and the wp characteristic enables the identification of males and females in the pupal stage. This technique saves rearing costs, as sterile female flies play no part in the SIT process, and furthermore, sterile females, if released, can still sting the fruit and cause infection by pathogens.

The rearing process starts with the collection of eggs in the adult room. Females lay eggs through the gauze sides of the cages and are collected in water troughs below. The eggs are placed on an artificial diet on which the larvae develop; this comprises bran, yeast, sugar and water, and anti-microbial agents.

After about 10 days the mature larvae leave the diet and are collected in water troughs below. The larvae are then placed on vermiculite to pupate. Ten days later the cycle is completed when the adult flies emerge.

Cages of flies in the adult room

Eggs on an egg raft on larval medium

In order to produce only males for release, about 80% of the collected eggs are heat-treated, which kills the female eggs. The male eggs are reared in a ‘male-only’ production stream, and all resulting pupae are sterilized 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. After 4 days the sterile males are sexually mature, and the paper bags are taken in a refrigerated truck to the release areas.

A tray of male Medfly pupae

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.

Quality Control Laboratory

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.

Fruit Fly Africa Medfly rearing facility