Temporal population dynamics of stored maize insect pests and efficacy of safer grain protectants in the smallholder sector of Zimbabwe

The overall objective of the study was to establish the efficacies of different inert dusts alone and in combination with safer levels of a synthetic pyrethroid or a bio-pesticide as grain protectants. Parallel on-station ecological studies at the Institute of Agricultural Engineering (IAE), Harare, were conducted to establish sources of primary infestations that are responsible for most grain damage incurred seasonally. Results from both were integrated to advocate for the combination of the use of safe grain protectants with grain handling practices that minimise or eliminate resident infestations. The efficacies of new food grade imported diatomaceous earths (DEs) (A2 and A3), imported new DE (MN51), registered imported DE (Protect-It®) and local raw DE (Chemutsi®) combinations with spinosad (Spintordust® 0.125%) and reduced quantities of deltamethrin were tested against Prostephanus truncatus (Horn), Sitophilus zeamais (Motsch.) and Tribolium castaneum (Herbst) under controlled conditions of 27±1oC and 60±5% relative humidity. Shumba Super® dust (fenitrothion 1% + deltamethrin 0.13%) applied at 0.5 g/kg and untreated grain were used as positive and negative control treatments, respectively. Cumulative mortality data were recorded at 7, 14 and 21 days, while F1 progeny emergence was recorded at 49 (S. zeamais) and 70 (P. truncatus and T. castaneum) days after treatment. Protect-It® gave significantly higher (P < 0.001) insect mortalities than all the other DEs across all test species. Chemutsi was equally efficacious against S. zeamais with significant differences (P < 0.001) from other DE treatments except Protect-It®. In combination treatments, except for spinosad 0.5 mg/kg + Chemutsi 0.05% w/w that gave low mortality, there were no significant differences between all the treatments and the positive control (Shumba Super® dust 0.5 mg/kg) on all the three test insects. There were no significant differences in P. truncatus F1 progeny emergence across all the DE treatments. However, there were significant differences (P < 0.001) between Deltamethrin 0.05 mg/kg + Chemutsi 0.1% (w/w) and other cocktail treatments in S. zeamais progeny emergence. Out of the 31 laboratory-tested materials, Protect-It® 0.1% (w/w), Spintordust® 1 mg/kg, Spintordust® 0.3 mg/kg + Protect-It® 0.1% (w/w), Deltamethrin 0.05 mg/kg + Protect-It® 0.08% (w/w), Deltamethrin 0.05 mg/kg + Chemutsi 0.1% (w/w), Deltamethrin 0.1 mg/kg + Chemutsi 0.08 % (w/w), Deltamethrin 0.1 mg/kg + Protect-It® 0.08 % (w/w), Spintordust® 0.5 mg/kg + Chemutsi® 0.05 % w/w were further tested for 24 weeks at IAE . These treatments were compared with Shumba Super® 0.5 g/kg and untreated control. Grain damage and weight loss in the untreated control were significantly higher (P < 0.001) compared to all other treatments. On-farm trials were conducted in Musami Communal Area, Murehwa district, using five best combination treatments selected from the laboratory bioassay results. The treatments were: Chemutsi 0.08% (w/w) + Deltamethrin 0.1 mg/kg, Chemutsi 0.05% (w/w) + Spinosad 0.5 mg/kg, Chemutsi 0.1%(w/w) + Deltamethrin 0.05 mg/kg, Protect-It® 0.05% (w/w) + Deltamethrin 0.1 mg/kg, Protect-It® 0.1 % (w/w) and were compared with Chikwapuro® 0.4 g/kg (pirimiphos-methyl 2.5% w/w + deltamethrin 0.1% w/w) and untreated control. At 24 weeks after treatment, grain damage (22.5 %) and weight loss (1.96 %) in the untreated control were both significantly higher (P < 0.001) than in all other treatments. Sitophilus zeamais was the most abundant pest species observed at all the on-farm trial sites. In the ecological studies, bulk grain was used to determine the critical source of infestation in stored maize. Two sets of 150 kg of grain were fumigated and placed in clean re-plastered granary compartments; one was completely sealed, while the other was left open. The same was repeated using un-fumigated grain. Resident pest infestations caused significantly higher grain damage (P < 0.001) in all the unfumigated treatments than the fumigated ones. Damage was more pronounced at top and middle levels in unfumigated closed granaries but was higher at the bottom in unfumigated open ones. High S. zeamais populations were observed at top levels of unfumigated open granaries, though it manifested heavily on all unfumigated treatments as well as fumigated open ones. Trash per kg of grain was significantly higher (P < 0.001) in all unfumigated treatments where P. truncatus was recorded in high populations. The results of the study showed that local and imported DEs can be combined with deltamethrin and spinosad to replace the organophosphate component of the synthetic grain protectants currently available commercially. Resident insect infestation causes more damage and at a faster rate than incoming infestation. Opening or closing the granary is of little importance when the grain is initially free of resident infestation. The elimination or avoidance of resident infestation in harvested maize grain retards insect pest build-up, delays damage and loss, and hence saves resources, time and reduces the risk of pesticide poisoning. Further investigations need to lengthen the study period to a whole storage season (about 40 weeks) to establish if the results obtained at 24 weeks will still hold.
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Region Focus: 
Southern Africa
RUFORUM Theses and Dissertations
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Open Access
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Dr. B.M. Mvumi (University of Zimbabwe) and Dr. P. Chinwada
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