Field efficacy and persistence of synthetic pesticidal dusts on stored maize grain under contrasting agro-climatic conditions

Grain storage trials were conducted in two districts of Zimbabwe with contrasting agro-climatic conditions (mean annual temperature of 18–30 °C and 28–42 °C; total rainfall of 750–1000 mm per annum and <450 mm per annum; respectively) to determine the comparative efficacy of commercially-available grain storage synthetic pesticides under contrasting climatic conditions. The five grain protectants, namely Shumba super dust® (fenitrothion 1% + deltamethrin 0.13%), Actellic gold dust® (pirimiphos-methyl 1.6% + thiamethoxam 0.36%), Super guard® (pirimiphos-methyl 1.6% + permethrin 0.4%), Chikwapuro® (pirimiphos-methyl 2.5% + deltamethrin 0.1%) and Ngwena yedura® (pirimiphos-methyl 2.5% + deltamethrin 0.2%) were evaluated at manufacturer's rates on stored shelled maize. The trials were conducted for a 40 week-long storage season in 2014/15 and again in 2015/16. Samples were analysed for insect grain damage, total insects per kilogram, grain weight loss, insect feeding dust and chaff as well as grain moisture content. Temperature and relative humidity within stores were recorded using data loggers. The results highlighted the generally poor efficacy of the synthetic pesticides under both cooler and hotter climatic test conditions. The pesticides failed to prevent insect grain damage or suppress insect pest numbers. Only Actellic gold dust®, introduced in the 2015/16 storage season was effective under both the agro-climatic conditions. The current study suggests that only Actellic gold dust® can be recommended for smallholder farm grain protection under both cooler and hotter climatic conditions. The findings confirm the frequent claims of smallholder farmers in east and southern Africa regarding poor storage pesticide performance, and emphasize the need to develop alternative effective storage insect pest control options.