Abstract

Sitophilus zeamais Motschulsky and Prostephanus truncatus (Horn) are the two most destructive post-harvest insect pests of maize worldwide. Bioassays were carried out to determine the efficacy of commercially available dilute dust grain protectants (pseudonamed protectant A, B, C, D, E and F) in controlling S. zeamais and P. truncatus under laboratory conditions. The products tested were: Hurudza® (fenitrothion 1.7% + deltamethrin 0.05%), Shumba Super Dust® (fenitrothion 1.0% + deltamethrin 0.13%), Actellic Super Chirindamatura Dust® (pirimiphos-methyl 1.6% + permethrin 0.3%), Chikwapuro® (pirimiphos-methyl 2.5% + deltamethrin 0.1%), Ngwena Yedura® (pirimiphos-methyl 2.5% + deltamethrin 0.2%) and Actellic Gold Chirindamatura Dust® (pirimiphos-methyl 1.6% + thiamethoxam 0.36%). These products were evaluated against four and five populations each of P. truncatus and S. zeamais, respectively, collected from Headlands, Murehwa, Bindura, Zvimba and Masvingo. Laboratory cultures for both P. truncatus and S. zeamais, provided courtesy of University of Zimbabwe Biological Sciences Department were also exposed to the dilute dust insecticides. None of the laboratory strains (both P. truncatus and S. zeamais) had been subjected to any regular insecticide selection pressure specifically designed for its control for more than five years on whole maize grain. Although, all the grain protectants were very effective against S. zeamais and were not significantly different among themselves in their effects on adult insect mortality, some differences were noted with respect to P. truncatus. The level of efficacy was also dependent on the population of P. truncatus tested. Actellic Super Chirindamatura Dust® was the least effective (11.6-34.6% mortalities) against P. truncatus while Actellic Gold Chirindamatura Dust® managed to control both P. truncatus and S. zeamais, achieving 100% mortalities across all populations tested. While Hurudza® and Shumba Super Dust® were very effective against the Bindura and “laboratory” LGB; the two products seemed not to be effective against the Headlands and Murehwa LGB populations.

Key words: Prostephanus truncatus, Sitophilus zeamais, grain protectants, susceptibility.