Effect of grain excavation damages by Sitophilus granarius on the efficacy of grain protectant insecticides against Cryptolestes ferrugineus and Tribolium castaneum

Internally feeding primary pests produce dusty frass that is known to support the development of externally developing beetles. Apart from frass, primary pests create semi-opened cavities in kernels that provide an opportunity for hiding and feeding of secondary pests, increase grain surface area and decrease weight of excavated kernels. It is unknown whether the excavated kernels may affect the efficacy of grain protectants and the development of secondary pests. Therefore, this study explored insecticide droplet distribution on the surface of excavated kernels by Sitophilus granarius. Further, it evaluated the effect of grain mixtures - differing in various proportions of sound and excavated kernels without frass - on the development and efficacy of two different doses of grain protectant insecticides (pirimiphos-methyl and deltamethrin) in sensitive strains of Cryptolestes ferrugineus and Tribolium castaneum. It was found - using dyed insecticide spray - that half of kernels did not receive droplets inside the excavations. Bioassays revealed that (i) the adult survival and progeny production of C. ferrugineus and T. castaneum were recorded only on mixtures with a high proportion of excavated kernels; (ii) the higher dose of pirimiphos-methyl and deltamethrin led to 100% mortality in both pests under all conditions; and (iii) the lower dose enabled survival on grain mixtures with an increased ratio of excavated to sound kernels in both chemicals and secondary pest species. The work first showed that a high proportion of damaged kernels in a grain batch/sample can support the development of C. ferrugineus and T. castaneum even without the presence of frass and that excavated kernels may cause pest survival in the case of a lower concentration of grain protectants.