Higher dosage of phosphine is required to control resistant strains of pests in outdoor grain storage systems: Evidence from Zimbabwe

Abstract

Pest control in stored grain crops in southern Africa is largely dependent on the use of phosphine generating fumigant, aluminium phosphide (AlP), albeit using blanket dosages, regardless of storage system type. An assessment and a field experiment were conducted to determine AlP (phosphine) fumigation practices, insect pest occurrence and effect of AlP dosage (11, 15 and 22 pellets t−1) on fumigation success using two outdoor storage platforms (compacted gravel and concrete floor) at the Grain Marketing Board (GMB)'s storage facilities. Fumigation failures were 320% higher in compacted gravel than concrete floor platforms. Cryptolestes ferrugineus (Stephens), Prostephanus truncates (Horn), Rhyzopertha dominica (Fab.), Sitophilus zeamais (Motschulsky) and Tribolium castaneum (Herbst) were common insect pest species across GMB depots. Aluminium phosphide dosage had differential effects on pest species with 95% mortality of C. ferrugineus at 11 pellets t−1 and 99.9% at 15 and 22 pellets t−1 for R. dominica. The compacted gravel floor platform accumulated 38.8% lower phosphine (phosphine) than the concrete floor platform. It can be concluded that C. ferrugineus and R. dominica are resistant to phosphine and that concrete floor platforms are more effective for phosphine fumigation than compacted gravel floor platforms. Grain handlers should consider avoiding phosphine on compacted gravel floor platforms and only fumigate grain stored on concrete floor platforms using minimum dosage of 22 pellets t−1 in order to kill all life forms of all pests.