Beyond initial mortality: residual-insecticide effects on feeding, progeny production, and commodity protection in Tribolium confusum and Sitophilus granarius

Residual insecticide performance in storage structures is shaped by pest identity, treated surface, food availability, and residue age. Four commercial formulations; Kafrothrin (deltamethrin 2.5% EC), Fastac (alpha-cypermethrin 10% SC), Katron (lambda-cyhalothrin 5% EC), and Tracer (spinosad 24% SC) were evaluated at manufacturer-recommended label rates against Tribolium confusum and Sitophilus granarius . Adults were exposed on paper, concrete, or wood at 1, 7, 14, or 21 d after treatment, with or without food. Mortality was recorded at 24 and 72 h. Any survivors were maintained for 45 d for progeny production and commodity loss, and separate cohorts provided 5-d nutritional indices. Grouped-binomial models showed strong effects of species, treatment, surface, residue age, and food condition on mortality. Across all treated combinations, pooled 24-h mortality decreased with residue aging, from 44.38% at day 1 to 31.17% at day 21. Mortality was also lower on wood than on paper across the tested conditions. Kafrothrin and Tracer formulations produced the highest pooled 24-h mortality, whereas Tracer produced the highest 72-h mortality. Tracer yielded the lowest progeny production and greatest mean progeny suppression. Multivariate analyses identified a dominant protection-performance gradient with partial overlap among formulations. Residual insecticides should therefore be evaluated according to target pest species and under the surface materials, residue ages, and food availability conditions that influence their practical performance in storage environments.