Abstract:
Maize ( Zea mays L.) is a key staple crop across sub-Saharan Africa, yet it experiences significant
postharvest losses caused by insect pests, including the larger grain borer (Prostephanus
truncatus), the maize weevil (Sitophilus zeamais), and the red flour beetle (Tribolium castaneum).
Heavy dependence on a few insecticides has led to resistance, necessitating evaluation of new
grain protectants. This study assessed the efficacy of Grain-D dust and examined the synergistic
role of pirimiphos-methyl in Actellic Super against resistant strains of P. truncatus. Grain-D dust
(0.2% deltamethrin + 0.36% thiamethoxam) was applied at 25, 50, and 75 g/90 kg maize, with
four replications. Actellic Gold® dust (1.6% pirimiphos-methyl + 0.36% thiamethoxam) served
as a positive control, while untreated maize provided a negative control. For laboratory assays,
100 g treated maize samples were infested with 40 unsexed adults of each pest species, and
mortality was recorded after 7 days. For field simulation, 10 kg portions of treated maize were
stored in hessian baglets under randomized layout, with samples collected every 60 days for 180
days to assess insect damage. Additional bioassays evaluated Actellic Super (1.6% pirimiphos
methyl + 0.3% permethrin) at reduced pirimiphos-methyl strength against P. truncatus strains
from Kiboko, Kitui, Machakos, Makueni, and Mwingi, and S. zeamais from Kiboko and Makueni.
The findings indicated that Grain-D dust achieved ≥95% control of P.truncatus,S.zeamais and
T.castenium with complete protection at 50–75 g/90 kg maize for 180 days, comparable to Actellic
Gold®. Untreated controls recorded 381 insects and 78.8% damage. In contrast, Actellic Super
formulations with pirimiphos-methyl levels below 1.5% were less effective, especially against
Machakos and Mwingi P. truncatus strains (≤91.3% mortality). Grain-D dust provided effective,
long-lasting protection against major maize storage pests and is recommended for use at 50 g/90
kg grain. These findings highlight the importance of continuous monitoring of insecticide efficacy
and optimizing active ingredient combinations to mitigate resistance and safeguard food security.
