Efficacy of the insect growth regulators methoprene, fenoxycarb and diflubenzuron against Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) on maize and paddy rice

The activity of fresh deposits of methoprene, fenoxycarb and diflubenzuron against F₁ progeny of Rhyzopertha dominica on maize and paddy was compared with that on wheat, at two equilibrium relative humidities. There were differences between slopes of log concentration-probit (lc-p) lines for different compounds, and for the same compound on different grains. Judging by values of the IC_99.9, i.e. the concentration which inhibited progeny production by 99.9%, the order of activity against F₁ progeny on different grains was: methoprene, wheat and paddy > maize; fenoxycarb, wheat > paddy > maize; diflubenzuron, wheat and maize > paddy. Equilibrium relative humidity (e.r.h.) had no consistent effect on activity—at 90% e.r.h., the IC₅₀ of fenoxycarb on wheat was reduced and the IC₅₀ of diflubenzuron on maize was increased compared with 70% e.r.h., and other treatments were unaffected.

The efficacy of these compounds on maize and paddy against F₁ and F₂ progeny was evaluated during 48 weeks storage at 30°C, 70% r.h. The resolved S isomer of methoprene was also included. Slopes of lc-p lines were greater against the F₂ than against the F₁, particularly using diflubenzuron on paddy, with corresponding smaller values of the IC_99.9. Equally effective concentrations did not decline systematically over 48 weeks. Minimum effective application rates were judged as the concentrations that prevented living F₂ progeny in at least 2 of 3 replicates. Estimates for 48 weeks protection on maize were: methoprene, 2 mg kg⁻¹; S-methoprene, 1 mg kg⁻¹; fenoxycarb, 10 mg kg⁻¹; and diflubenzuron, 5 mg kg⁻¹. Corresponding estimates on paddy were 0.15 mg kg⁻¹, 0.05 mg kg⁻¹, 5 mg kg⁻¹, and 5 mg kg⁻¹.