Use of iridium-191 and nitrogen-15 to label winter colza insects

In preparation for an investigation of winter colza insect population dynamics for the flea beetle (Psylliodes chrysocephala L.) and the stem weevil (Ceuthorrhynchus napi Gyll.), a study was made of the possibility of using as labels iridium-191 analysed by gamma spectrometry after activation in a reactor, and nitrogen-15 measured by means of optical spectrometry. The insects are fed on colza leaves previously soaked in a solution of iridium (H2IrCl6) and/or nitrogen-15 ((NH4)2SO4). The iridium has the advantage of remaining in the body of the animal for a long time and enables large populations to be screened. In the stem weevil, the label remains fairly stable with time, but in the flea beetle it is weaker and less persistent. Nitrogen-15 is not as advantageous as iridium because its detection involves, of necessity, the preparation of each insect tested. It should only be used for subjects already labelled with iridium. In the release and recapture experiments for which these tests were intended, the main aim was to explain the disparities in infestation levels often observed from one field to another, and to draw lessons from them as regards crop supervision recommendations. Checks performed on the reared animals sampled after release indicate that, in three cases out of four, the freed insects were satisfactorily labelled. (author). 9 refs, 8 tabs