Noureddine Soltani

Effects of diflubenzuron on the pupal-adult development of Tenebrio molitor L. (Coleoptera, Tenebrionidae): Growth and development, cuticle secretion, epidermal cell density, and DNA synthesis

Diflubenzuron (DFB), applied by dipping on newly emerged pupae of Tenebrio molitor L., disturbs the pupal-adult development. Four main types of treated insects were obtained according to external morphology: blocked pupae, adults unable to ecdyse, adults partially ecdysed, and adults completely ecdysed. The proportion of these four types varied with the time of treatment during the pupal life. When DFB was administered by dipping at a 10 g/liter concentration to newly emerged pupae, HPLC measurements showed that the different responses were due to a difference in the levels actually incorporated. Histological and ultrastructural observations of sternal cuticles revealed that DFB disturbs the cuticle deposition: reduction of its thickness and modifications of its architecture. Tritiated thymidine incorporation and cell density measurements in sternal epidermis show that DFB affects both mitoses and DNA synthesis in blocked pupae. These findings suggest that DFB interferes with other biochemical processes besides the chitin biosynthesis; whether these effects are primary or secondary remain to be determined.

Laboratory evaluation of Alsystin, a chitin synthesis inhibitor, against Culex pipiens pipiens L. (Dip., Culicidae): effects on development and cuticle secretion

Alsystin, a benzoylphenylurea derivative, was evaluated on Culex pipiens pipiens L. Treatment was made on newly third‐ and fourth‐instar larvae for 24 h. Mortality occurred at various developmental stages following treatment. However, death as larvae was relatively important compared with the mortality recorded for the other later developmental stages. Treatment resulted in a dose‐dependent reduction in adult emergence. Moreover, treatment caused morphological aberrations depending upon the importance of larval–adult transformation and significantly increased the duration of both third and fourth larval instars. A histological study conducted on fourth instar larval integument, revealed that Alsystin delayed the ecdysis and significantly reduced the thickness of both larval and pupal cuticles secreted compared with controls. Moreover, the apolysis that occurred at day 4 in controls was not affected by treatment.

Biological activity of flucycloxuron, a novel benzoylphenylurea derivative, onTenebrio molitor: comparison with diflubenzuron and triflumuron

Flucycloxuron, a novel benzoylphenylurea (BPU) derivative, exhibited insecticidal activity when injected into newly ecdysed pupae ofTenebrio molitor. Mortality occurs because of defective adult ecdysis. Treatment caused a reduction in both cuticle thickness and incorporation of14C-labelled precursor into chitin, although it had no significant effect on the protein synthesis. The potencies of other BPU compounds as inhibitors of chitin biosynthesis have been examined and results showed that diflubenzuron was less effective than either flucycloxuron or triflumuron.

Stage-specific effects of diflubenzuron on ecdysteroid titers during the development of Tenebrio molitor: Evidence for a change in hormonal source

The effects of the insect growth regulator diflubenzuron (DFB) were observed on the larval-larval and larval-pupal moulting cycles of Tenebrio molitor, after treatment at ecdysis. In both cases, the first parts of the cycles, from ecdysis to apolysis, were apparently not affected, but the pharate periods were lengthened; treated animals were generally unable to perform ecdysis and died at this step. The ecdysteroid titers in the hemolymph of treated animals were measured with a radioimmunoassay and compared to controls. During larval-larval cycles, the single ecdysteroid increase was not affected by DFB treatment. On the contrary, during larval-pupal development, a significant modification was observed; whereas two ecdysteroid peaks occurred in controls, the second peak of treated animals was significantly reduced and slightly delayed; however, the first peak was not modified. Taking into account that previous observations demonstrated a complete inhibition of the ecdysteroid peak in Tenebrio pupae, these stage-specific differences could reveal either a change in the DFB sensitivity of a sole endocrine source (i.e., prothoracic gland) or a change in hormone origin during metamorphosis. Ligation experiments during the last larval stage, in combination or not with DFB applications, clearly demonstrated the change in the moulting hormone source at the end of larval development in Tenebrio.

Inhibition of ecdysteroid increase by diflubenzuron in Tenebrio molitor pupae and compensation of diflubenzuron effect on cuticle secretion by 20-hydroxyecdysone

Summary Dipping of newly ecdysed Tenebrio molitor pupae in an acetone solution of diflubenzuron prevents most of them (73%) from carrying on further development. Such blocked animals cannot secrete adult cuticle and remain apolysed. Their ecdysteroid levels analysed by radioimmunoassay do not present any increase. However, injection of 20-hydroxyecdysone (2–10 μg) several days after diflubenzuron application allows the secretion of a new cuticle with an abnormal architecture but with a high content in N-acetyl-aminosugars as revealed by fluorescent wheat germ agglutinin.

Uptake and distribution of three insect growth regulators — diflubenzuron, flucycloxuron and halofenozide — in pupae and adults ofTenebrio molitor

In a topical bioassay with pupae and adults of the mealwormTenebrio molitor (Coleoptera: Tenebrionidae), an important pest in stored products worldwide, three insect growth inhibitors (IGRs) were tested: the two chitin synthesis inhibitors diflubenzuron and flucycloxuron, and the ecdysone agonist halofenozide (RH-0345). To address differences in toxicity, we determined with the use of a14C-labeled isotope, the pattern of absorption through the cuticle and then the distribution in the different body tissues of gut, hemolymph, integument and reproductive organs (ovaries, testis). The rate of absorption through the cuticle was highest for flucycloxuron, and this concurred with its high toxicity. In general, the patterns of absorption followed a similar trend in young and old pupae and in the adult stage. Accumulation in the reproductive system of females and males was relatively high, which may explain the strong reproductive effects of the IGRs tested. In addition, we recorded large differences in the clearance of insecticide from the insect bodyvia the excrements, with the chitin synthesis inhibitors exhibiting higher rates than halofenozide.

Evaluation of Triflumuron, a benzoylphenylurea derivative, on Tenebrio molitor pupae (Col., Tenebrionidae) : effects on cuticle

Abstract: Triflumuron (TFM), a benzoylphenylurea (BPU) derivative, exhibited insecticidal activity when injected to newly ecdysed pupae of Tenebrio molitor. Mortality occurs by defect in adult ecdysis. Treatment resulted in a significant increase in the duration of pupal stage and caused a reduction in both cuticle thickness and incorporation of 14C‐labelled precursor into chitin compared to controls. Moreover, TFM reduced the amount of cuticle chitin without any significant effect on protein level. Thus, TFM acts by preventing the chitin biosynthesis confirming the primary mode of action of the BPU insecticides.

Insecticidal Activity of Azadirachtin on Drosophila melanogaster and Recovery of Normal Status by Exogenous 20-Hydroxyecdysone

Azadirachtin, an insect growth disruptor, is known to be an antagonist of the juvenile hormone and 20-hydroxyecdysone (20E). However, its mechanism of action remains to be understood. Furthermore, the effects of the interaction of azadirachtin and 20E have not been investigated. The current study examined the effect of azadirachtin topically applied alone or in combination with 20E on Drosophila melanogaster Meigen, 1830 (Diptera: Drosophilidae). In initial bioassays, various doses (0.5, 1, 1.5, 2.5 and 4.5 µg) of azadirachtin was tested alone on newly ecdysed pupae and the inhibition doses (ID) of adult emergence determined. In a follow-up experiment, azadirachtin applied alone at its ID25 (0.59 µg) and ID50 (1.10 µg) was evaluated on catalase (CAT) and glutathion S-transferase (GST) activities, and yolk protein content in fat body and ovaries. Results showed that azadirachtin at the two tested doses increased significantly the activity of both GST at 48 and 72 hours, and CAT at 24, 48 and 72 hours following treatment. Moreover, azadirachtin treatment at these doses reduced significantly the yolk protein content in fat body and ovaries as compared to the control series. Finally, the exogenous 20E (0.25 and 0.50 µg), applied 24 or 48 h after azadirachtin treatment, relatively restored the normal values of CAT, GST and yolk protein content in fat body and ovaries. All these results indicate that exogenous 20E can compensate the depressive effects induced by azadirachtin on D. melanogaster.

Protein synthesis in developing ovaries of mealworm under in vivo and in vitro conditions : Effects of diflubenzuron

Abstract The effect of diflubenzuron (DFB) on protein synthesis in the ovaries of Tenebrio molitor was evaluated during oocyte maturation using in vivo and in vitro assays. When incorporated into the diet (5 and 10 mg g −1 ), DFB was found to affect both the weight, the protein levels and the incorporation of tritiated leucine into proteins of ovaries. In addition, electrophoretic separation of ovarian proteins by sodium dodecyl sulfate-polyacrylamide slab gels (SDS-PAGE) showed that DFB applied in vivo did not have a significant effect on the number of protein bands. When added to the culture medium (5 and 10 μg ml −1 ), DFB resulted in a slight but significant decrease in the rate of incorporation of tritiated leucine into proteins of ovaries cultured for 5 d compared to controls. These in vitro effects of DFB on protein synthesis were less marked than those observed in orally treated females.

Laboratory evaluation of azadirachtin against the oriental cockroach, Blatta orientalis L. (Dictyoptera, Blattellidae): Insecticidal activity and reproductive effects

In this work, the toxicity of azadirachtin applied topically at various doses (300, 600, 1200, 2400, 4800 and 9000 ng/ insect) was evaluated against newly emerged adults of the oriental cockroach, Blatta orientalis L. (Dictyoptera, Blattellidae) under laboratory conditions. Results show that treated insects exhibited toxic symptoms with a dose-dependent mortality. In a second series of experiments, the compound applied topically at its LD 50 and LD 90 on newly emerged female adults was investigated on ovarian growth during the first gonadotrophic cycle (0, 2, 4 and 6 days). Azadirachtin was found to reduce the number of oocytes per paired ovaries and the size of basal oocytes. Furthermore, the compound was examined on ovarian biochemical components. Biochemical data revealed a significant reduction of ovarian contents of proteins, lipids and carbohydrates with the two tested doses. The overall results suggest an interference of the compounds with the vitellogenesis.