Effects of some foliar fungicides on the chrysomelid beetle Gastrophysa polygoni (L.).

Abstract

The effects of the fungicides benomyl, thiophanate-methyl and triadimefon on the chrysomelid beetle Gastrophysa polygoni were investigated in the laboratory. Contact with a suspension of benomyl (1.5 g a. i. litre⁻¹ did not affect the hatchability of the eggs. Larvae were reared on shoots of knotgrass (Polygonum aviculare) that had been sprayed with suspensions of benomyl, ranging in concentration from 0.1 to 5.0 g a.i. litre⁻¹. The mortality to the adult stage, of larvae reared on shoots treated with concentrations of benomyl of 0.5 g a.i. litre⁻¹ and above, was significantly higher than that of control larvae. At concentrations of 2.0 g a.i. litre⁻¹ and above, no larvae survived to the adult stage. The LD₅₀ was 0.78 g a.i. benomyl litre⁻¹. The LT₅₀ values at concentrations of 1.0, 2.0 and 5.0 g a.i. benomyl litre⁻¹ were 22.6, 12.6 and 5.3 days, respectively. The mean weights of adults bred from larvae that had been reared on shoots treated with benomyl (0.5 and 1.0 g a.i. litre⁻¹) were significantly less than those of adults bred from control larvae. The mortality of larvae, reared on shoots of P. aviculare treated with triadimefon (0.5 g a. i. litre⁻¹) or thiophanate-methyl (1.0 g a.i. litre⁻¹), was also significantly higher than that of control larvae. Females kept on plants of P. aviculare treated with benomyl (1.5 g a.i. litre⁻¹) laid similar numbers of eggs to those kept on untreated plants, and the hatchability of the eggs was not affected.