Hilde Eelen

Differential effects of nonsteroidal ecdysteroid agonists in coleoptera and lepidoptera: Analysis of evagination and receptor binding in imaginal discs

Abstract The effects of 20-hydroxyecdysone (20E) and the first nonsteroidal ecdysteroid agonists, RH-5849 and tebufenozide (=RH-5992), were tested on in vitro cultured mesothoracical imaginal wing discs of last-instar larvae of the Colorado potato beetle, Leptinotarsa decemlineata Say, and the greater wax moth, Galleria mellonella L. In general, the response of the nonsteroidal agonists was qualitatively similar to that induced by 20E, although the concentrations required to induce evagination varied widely. The EC 50 of 20E, RH-5849 and tebufenozide, that indicates the compound concentration which elicits evagination in 50% of the isolated imaginal discs, was 60.7, 461 and 757 nM, respectively, in L. decemlineata . In wing discs of G. mellonella the order of biological activity (expressed as EC 50 ) resulted in tebufenozide (8.9 nM)>20E (321 nM)>RH-5849 (865 nM). In a second series of experiments, a competition receptor binding assay was developed with whole imaginal discs, and showed that the I 50 value of 20E, RH-5849 and tebufenozide to compete 50% of the specifically bound amount of 3 H-ponasterone A in imaginal discs of L. decemlineata was 425, 740 and 1316 nM, respectively. In wing discs of G. mellonella that respective value was 106, 911 and 22 nM. The current results showed that the binding affinity (I 50 ) of 20E reached a similar level as the concentration to elicit a biological response (EC 50 ) in imaginal wing discs of the two insect species; a similar conclusion could be drawn for the two nonsteroidal ecdysteroid agonists. In this regard, it is suggested that the imaginal disc assay allows us to measure the potency of an ecdysteroid active compound on the one hand and its affinity to bind on the ecdysteroid receptors (EcRs) in competition with 3 H-ponasterone A on the other. In addition, the current data suggest that the specific activity and insect selectivity of the nonsteroidal agonists, especially the high toxicity of tebufenozide for Lepidoptera and the nonsusceptibility of Coleoptera, is related to selective binding on the EcRs.

Persistence of the sulfonylurea herbicide iodosulfuron-methyl in the soil of winter wheat crops

On four winter wheat fields grown on soils of different textures in Belgium, 10 g a.i. ha−1 of the sulfonylurea herbicide iodosulfuron-methyl-sodium was applied post-emergence in the spring. A procedure was developed for the analysis in field soils of iodosulfuron-methyl 1 and of its metabolites iodosulfonamide 2 and iodosaccharin 3 with a sensitivity limit of 0.3 µg of equivalents of iodosulfuron-methyl 1 kg−1 dry soil. GC and GC-MS was used after purification of the soil extracts by repeated TLC, and methylation. The results of the chemical analyses were confirmed by means of bioassays using sugar beet as test plants. On a winter wheat crop grown on sandy loam soil of pH 6.2 at Melle, iodosulfuron-methyl-sodium 1 was applied at the beginning of April. The iodosulfuron-methyl 1 soil half-life in the 0–10 cm surface soil layer was 60 days. At the end of June, the sum of the concentrations of the metabolites 2 and 3 in the 0–10 cm surface soil layer attained a maximum corresponding to 27% of the applied dose. Green manures were sown after the harvest of the wheat at the end of August. No phytotoxicity at all was observed during the growth of the green manures, in spite of the very low residues of iodosulfuron-methyl 1 remaining in soil in September and October. At the mid of November, iodosulfuron-methyl 1 and its metabolites 2 and 3 were no more detected in soil. On three other winter wheat crops grown on clay soils of pH of about 8 at Leke, Gistel and Zevekote, iodosulfuron-methyl-sodium 1 was applied at the beginning of May. The soil half-life of iodosulfuron-methyl 1 in the 0–10 cm surface soil layer was between 30 and 44 days. Since the application and until the mid of November, in all the trials made on sandy loam or clay soils, iodosulfuron-methyl 1 (and its metabolites 2 and 3 in the trial made on sandy loam soil) were never detected in the 10–15 and 15–20 cm surface soil layers, indicating their low mobilities in the field soils.

Persistence and Movement of the Herbicide Propoxycarbazone-Sodium in Winter Wheat Crops

The new herbicide propoxycarbazone (sodium methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1H-1,2,4-triazole-1-yl)carbonyl]amino]sulfonyl]benzoate, 1 ) has been measured in the soil of winter wheat crops. In the soil extract, propoxycarbazone was separated from its potential soil metabolites by repeated TLC. Propoxycarbazone was methylated with diazomethane. In the GC and GC-MS apparatus, the N -methylpropoxycarbazone 2 (methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1H-1,2,4-triazole-1-yl)carbonyl]methylamino]sulfonyl]benzoate) generated N -methylsaccharin 4 (1-dioxy-2- N -methyl-3-keto-1,2-benzisothiazol) which was measured. Propoxycarbazone has been applied in the spring at the rate of 70 g ha m 1 post-emergence on winter wheat crops grown in several sites different as to their soil texture and composition. In the 0-10 cm surface soil layer of the winter wheat crops grown on sandy loam (Melle) or on clay loam (Zevekote) soils, the half-life of propoxycarbazone was 54 days. In the winter wheat crop grown on loam soil (Cortil-Noirmont), the propoxycarbazone soil half-life was 31 days. The difference between the propoxycarbazone soil half-lives at the different sites was related to the organic fertilizer treatments applied in the past. After the winter wheat harvest at the end of August, the concentration of propoxycarbazone in soil was very low at Cortil-Noirmont; at the end of September, propoxycarbazone was no more detected. At Melle and Zevekote, the concentration of propoxycarbazone in soil was very low in September, and disappeared completely at the end of October. Since the treatment and until the end of October, propoxycarbazone was not detected in the 10-15 and 15-20 cm surface soil layers of the three trials.