Deltamethrin-induced feeding plasticity in pyrethroid-susceptible and -resistant strains of the maize weevil, Sitophilus zeamais

Phenotypic plasticity contributes to the adaptative evolution of populations exposed to new or altered environments. Feeding plasticity is a component of phenotypic plasticity not usually considered in insect strains adapted to insecticide-altered environments, but which may either accentuate or mitigate insecticide resistance. This is a concern in the pyrethroid-resistant strains of the maize weevil Sitophilus zeamais Motsch. (Col., Curculionidae), and the reason for this study. A pyrethroid-susceptible and two pyrethroid-resistant strains of maize weevil were subjected to free-choice and no-choice tests with maize grains sprayed with increasing doses of the pyrethroid, deltamethrin. The insects from the pyrethroid-resistant strains exhibited higher feeding avoidance with increased deltamethrin doses than insects from the susceptible strain when subjected to free-choice tests. The strains of maize weevil physiologically resistant to pyrethroids were also behaviourally resistant to deltamethrin [long dash] an additional management concern. The resistant strains avoid deltamethrin-sprayed grains and are less nutritionally affected by this compound, with divergent responses from the susceptible strain with increased doses of deltamethrin. Furthermore, the higher relative growth rate and consequently higher efficiency of food conversion observed in the insecticide-resistant strains were significant even without insecticide exposure, indicating that these traits are stimulus-independent and may persist even without further insecticide selection, potentially limiting the options available for their management.