Insecticide susceptibility status of Prostephanus truncatus (horn) and Sitophilus zeamais (Motschulsky) in Zimbabwe and assessment of enhancement of grain protection through incorporation of eucalyptus leaves
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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 in controlling S. zeamais and P. truncatus under laboratory conditions. Efficacy parameters tested included adult insect mortality, population growth rate and grain damage caused in Actellic Super Chirindamatura Dust®-treated grain. 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. The reproductive performance of P. truncatus and S. zeamais as well as resultant grain damage in maize treated with half recommended rate of Actellic Super Chirindamatura Dust® were assessed. Damage due to S. zeamais averaged 39-93% in untreated grain; however, the weevil could not successfully establish in grain treated with half recommended rate of Actellic Super Chirindamatura Dust®. Prostephanus truncatus successfully established in both treated and untreated grain and caused damage of 49.4-83.8% and 56.3–87.8% in treated and untreated grain, respectively over a storage period of 10 weeks. For both P. vi truncatus and S. zeamais, progeny numbers were positively correlated with grain damage. The toxic and repellent effects of grain admixed with gum tree (Eucalyptus citriodora) leaf powder (10% w/w) on P. truncatus and S. zeamais were evaluated under laboratory conditions. Untreated control and the conventional insecticides were used as negative and positive controls, respectively. The efficacy of the synthetic insecticides was better when applied singly than when combined with E. citriodora leaf powder. With the exception of Actellic Super Chirindamatura Dust®, all conventional insecticides significantly reduced the number of adult insect progeny. The protective effects of gum tree leaf powder alone against P. truncatus and S. zeamais was low since it achieved mortalities below 11%. However, its high repellent properties (up to 68.9%) against P. truncatus and S. zeamais show that it can be used in combination with conventional dilute dust insecticides so as to enhance pest management through repellency.