Effect of two insect growth regulators on the ecdysteroid contents in eggs of the mealworm

RH-0345 (halofenozide), a bisacylhydrazine derivative, is a nonsteroidal ecdysteroid agonist that mimics the action of the moulting hormones, while KK-42, an imidazole compound, is a potent inhibitor of ecdysteroid biosynthesis. Previous results suggested that the observed reduction of ecdysteroid titre, leading to a reduction of reproductive capacity in the mealworm Tenebrio molitor, is due to the direct and rapid action of KK-42 on ecdysteroid biosynthesis. Moreover, RH-0345 was found to increase ecdysteroid production and partially reverse the effects on reproductive events induced by KK-42. Therefore, the present study evaluates the effects of these two insect growth regulators (IGRs) on egg ecdysteroids in mealworms. The IGRs were applied topically (10μg/insect) on newly-emerged adult females. A qualitative and quantitative analysis of ecdysteroids from pooled freshly laid eggs was made by an enzyme-immunoassay (EIA) using two specific antibodies, the rat monoclonal EC 19 antibody to measure for 20-hydroxyecdysone (20E) and the rabbit polyclonal B antibody for ecdysteroids. EIA measurements confirmed the presence of free ecdysteroid and 20E in control and treated series. In addition, the conjugated ecdysteroids were predominant in eggs of untreated mealworms. RH-0345 increased the amounts of 20E relative to the free ecdysteroids. In contrast, KK-42 reduced the amounts of total ecdysteroids, but had no significant effect on the relative hormonal composition of free and conjugated ecdysteroids.