Two peas in a pod: Efficacy of S-methoprene in combination with piperonyl butoxide for the control of different strains of the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae)

The lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), is one of the most destructive pests of stored cereals worldwide, and resistance to insect growth regulators such as S-methoprene has been documented in several populations. In the present study, we evaluated the effects of S-methoprene alone and in combination with the synergist piperonyl butoxide (PBO) against susceptible (QRD14) and resistant (QRD551) strains of R. dominica . Bioassays were conducted on wheat treated with two concentrations of S-methoprene (0.1 and 0.3 mg/kg for QRD14; 10 and 30 mg/kg for QRD551 and two concentrations of PBO (0.27 and 1.35 g/kg). Treatments were applied under three exposure scenarios: (i) sequential application of S-methoprene followed by PBO with a 5-h interval, (ii) sequential application of PBO followed by S-methoprene with a 5-h interval, and (iii) simultaneous application of both compounds. Adult mortality was generally low in both strains, not exceeding 30 % even after 21 d, although the combination. In contrast, progeny production was almost completely suppressed in QRD14 across all treatments, while in the QRD551 strain, progeny suppression was dose-dependent and strongest when S-methoprene at 30 mg/kg was combined with PBO at 1.35 g/kg. These results demonstrate that progeny suppression, rather than adult mortality, is the key effect of S-methoprene against R. dominica , and that PBO can enhance its efficacy, particularly in resistant populations. • S-methoprene and PBO were evaluated against susceptible and resistant strains of Rhyzopertha dominica . • Adult mortality was low, confirming progeny suppression as the main effect. • PBO enhanced S-methoprene efficacy in the resistant strain. • Higher doses and simultaneous application maximized progeny suppression. • Synergist–IGR combinations show promise for resistance management.