Toxicity of Ecdysone Agonists Correlates with the Induction of CHR3 mRNA in the Spruce Budworm

Four ecdysone agonists, RH-5849, RH-5992, RH-0345, and RH-2485, were tested for toxicity against fifth-instar larvae ofChoristoneura fumiferana.RH-2485 was the most toxic (LD50= 1.05 ng/larva) followed by RH-5992 (LD50= 8.8 ng/larva), RH-0345 (LD50= 41.0 ng/larva), and RH-5849 (LD50= 236.9 ng/larva). The relative potencies of these compounds to induce the transcription factorChoristoneurahormone receptor 3 (CHR3) was studied. Detectable levels of CHR3 mRNA were induced by RH-2485 at a concentration as low as 1.5 ng/larva, but similar inductions required concentrations as high as 7.5 ng of RH-5992/larva, 37.5 of RH-0345/larva, and 187.5 of RH-5849/larva. The order of potency of these four ecdysone agonists for inducing CHR3 mRNA is the same as their larval toxicity.