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.