Detection of irradiated Anastrepha suspensa larvae
The loss of ethylene dibromide as a fumigant and the possible loss of methyl bromide has increased interest in irradiation as a quarantine treatment. For quarantine purposes, a minimum dose of 150 Gy has previously been recommended. However, it is important to develop tests to verify that any live insect intercepted on an imported commodity has been irradiated with the required dose. Studies have been carried out in USA on first instars of Anastrepha suspensa (EPPO A1 quarantine pest) which were irradiated at the following doses: 0, 5, 10, 20, 50, 75, 100, 150 Gy and then killed in the freezer. Tested larvae were observed for whole body melanization until they reach late third instar. Control larvae rapidly melanize, whereas larvae irradiated at doses ³ 20 Gy failed to show typical melanization after freezing and thawing. However, this melanization test after death is not applicable to 1st and 2nd larval instar. The authors point out that this is a very simple test that requires no special training or equipment, except a freezer.
In addition, a simple qualitative spot test for phenoloxidase activity (which is much reduced by irradiation) have been developed. It can be performed with only a portion of the crushed body of a single third instar, a red colour developed in 15 min or less on a transparency film containing 100 µg dried 2-methyl DOPA. The red colour is due to the phenoloxidase action on the 2-methyl DOPA substrate. Irradiation of first instars at doses ;³ 20 Gy caused a failure in third instars to develop normal phenoloxidase activity and therefore no red colour appear. However, this spot test can only be used on third larval instars.
Sources
Nation, J.L.; Smittle, B.J.; Milne, K. (1995) Radiation-induced changes in melanization and phenoloxidase in Caribbean fruit fly larvae (Diptera:Tephritidae) as the basis for a simple test of irradiation.
Annals of the Entomological Society of America, 88(2), 201-205.