EXPERT REACTION: Genetically modified fruit fly to be trialled in WA

First, the amount of tetracycline (tet) antidote waste is not ‘several tonnes per week’ and is in fact insignificant compared to usage for pets, farming and human therapeutic use. For context, looking at our large-scale production of up to 400,000 mosquitoes per week for dengue mosquito control projects (much higher than medfly production for this trial), the amount of tet waste per day is less than what one person excretes when they're on a standard tetracycline dose (2x 500 mg).

Second, there are numerous ways medfly feed can be treated to remove tet before disposal as it rapidly degrades in the presence of UV light and heat. The medfly feed waste will be treated in accordance with the approved protocols for this trial.

The RIDL (self-limiting) technology used in this strain has been around for a while and certainly has potential, but it has not yet been used in any large scale releases, in any insect, including in mosquitoes where experiments have been carried out for a number of years. In the medfly, releases have only been undertaken so far in cage environments and it can be difficult to extrapolate results from cage studies to the field situation where the environment is very different.

The technology depends on males carrying the genetic construct having a high fitness (high mating success) compared to males in natural populations, and this can be a challenge to achieve because insects raised in the laboratory for a number of generations inevitably become adapted to the artificial environment where they are reared.

The approach also depends on expression of the genetic construct in offspring of the released males and this will need to be monitored carefully.  And of course the new technology will need to compete with the traditional sterile insect technique (SIT) where irradiated males (rather than ones carrying an engineered genetic construct) are released in large numbers in the environment. This represents an approach that has proved successful in field situations in other parts of the world particularly when combined with other approaches

The Oxitec Med-flies offer a new form of pest control, very closely related to the well-established Sterile Insect Technique (SIT). In the SIT, flies are mass-reared in factories, sterilised by irradiation, and then released into the environment where they mate with wild females. This causes the wild female flies to lay infertile eggs, and eventually the repeated release of sterile male flies can drive the field population to extinction.

With the Oxitec flies, the general approach is similar, but in this case the mass-released male flies carry a genetic modification which means that all their offspring die. The released flies are environmentally safe and there is no known risk of genetic contamination from the released flies.

The Oxitec approach is not, however, without issues. The flies are genetically modified, and this greatly limits their use under current legislation. Importantly, the lethal gene must be ‘switched off’ in the factory, otherwise the flies could not be reared. This requires the addition of tetracycline hydrochloride to the fly diet. Tetracycline is an antibiotic and how the spent, tetracycline-dosed larval diet from a fruit fly factory (which may run to several tonnes per week) will be properly disposed of will need to be addressed