Effect of nonsteroidal ecdysteroid agonists on ecdysteroid titer in Spodoptera exigua and Leptinotarsa decemlineata

Abstract

In control last-instar larvae of Spodoptera exigua, and Leptinotarsa decemlineata, major hemolymph ecdysteroid peaks appeared around day 3–4, and 8–9, respectively, which is correlated with normal apolysis. Later on, hormone titers declined to a basal level. In Colorado potato beetle last-instars, a small rise was noted around day 4, which represents the time of digging. In last-instar larvae of S. exigua treated with either RH-5849 or tebufenozide, the ecdysteroid titer remained at a basal level without any elevation. Such larvae showed signs of untimely apolysis within 12–24 h after treatment followed by cessation of fresh weight gain. Likewise, hemolymph ecdysteroid levels and larval weight gain were reduced in last-instar larvae of L. decemlineata treated with RH-5849 as compared with controls. Otherwise, treatment of Colorado potato beetle larvae with tebufenozide caused no hormonal and physiological abnormalities as compared with controls. It is suggested that both nonsteroidal ecdysteroid agonists act via ecdysteroid receptor binding without an increase in ecdysteroid titer.