Role of uptake and in vitro binding on toxicity of nonsteroidal ecdysone agonists in the cotton leafworm and the Colorado potato beetle

Abstract

Tebufenozide, methoxyfenozide, and halofenozide were administered through diet to newly moulted last-instar larvae (0-4 h) of cotton leafworm Spodoptera littoralis and Colorado potato beetle Leptinotarsa decemlineata. Treated larvae underwent a premature and lethal larval moult that was observed 24-96 h after treatment. In S. littoralis, methoxyfenozide was the most active compound (LC50 = 44.6 (38.0-51.8 mg/litre)) followed by tebufenozide (LC50 = 7.47 (6.52-8.41 mg/litre)) and halofenozide (LC50 = 1.41 (0.79-1.50 mg/litre)). In L. decemlineata, halofenozide was most toxic (LC50 = 54.0 (42.5-66.3 mg/litre)) followed by methoxyfenozide (LC50 = 6.23 (5.15-7.22 mg/litre)) and tebufenozide (LC50 = 1.41 (1.20-1.65 mg/litre)). Halofenozide was slightly more retained in S. littoralis than methoxyfenozide and tebufenozide with half-life values of 4.5, 3.1 and 3.9 h, respectively, but were not significantly different (P = 0.05). The patterns of excretion followed a similar inverse curve. In L. decemlineata, halofenozide was retained at significantly higher levels than methoxyfenozide and tebufenozide. The latter 2 yielded similar levels of radioactivity in the larval body. The half-life values were 4.9, 0.2 and 0.6 h for halofenozide, methoxyfenozide and tebufenozide, respectively. Concentration response curves for induction of cultured imaginal wing discs of S. littoralis by the different compounds were parallel and indicated that the relative order of activity was: methoxyfenozide»tebufenozide»halofenozide»20E _ RH-5849.