Detection of irradiated Ceratitis capitata
With the possible withdrawal of methyl bromide, interest has increased in gamma irradiation as a quarantine treatment, in particular against fruit flies. In USA, a minimum dose of 150 Gy was recommended against fruit flies infesting fresh fruits and vegetables. As this treatment does no cause immediate death but prevents completion of development to the adult stage, it is important to develop tests to check that any live fruit fly larva found on a treated commodity has received a sufficient irradiation dose. It has been shown that phenoloxidase activity is reduced by gamma irradiation and that this enzyme has an important role in melanization of insect cuticle. In previous studies carried out on Anastrepha suspensa (EPPO A1 quarantine pest), it was observed that insufficiently irradiated larvae, when killed by freezing, rapidly melanize, and that tests could also be applied to measure the phenoloxidase activity (see EPPO RS 94/234). Similar studies were carried out on Ceratitis capitata (EPPO A2 quarantine pest) and showed that the observation of melanization after freezing or the measurement of the phenoloxidase activity in C. capitata larvae are good, sensitive, and reliable indicators of irradiation treatment. However, both assays have temporal limitation: irradiation must occur before the 3rd instar and sufficient enzyme activity is present only during the 3rd instar. But the authors felt that because only high quality fruit will be treated (no visible infestation at early stage), and because there is a delay between treatment and inspection at the point of entry which allow for the eggs or young stages to proceed to more advanced stages, these assays can be used efficiently for quarantine purposes.
Sources
Mansour, M.; Franz, G. (1996) Effect of gamma irradiation on phenoloxidase activity in Mediterranean fruit fly (Diptera: Tephritidae) larvae.
Entomological Society of America, 89(3), 695-699.