Unmasking phosphine: Assessing its continued effectiveness on stored product insects in India across populations and life stages

Phosphine is the primary fumigant to disinfest majority of world's grain against insect pests. However, the toxicity of phosphine against stored grain insect populations from different locations in India has been compared rarely. Under the present study, comparative toxicity of phosphine was determined for the storage insect pests of wheat and rice at different locations. It also evaluated phosphine’s efficacy against Callosobruchus maculatus (Fabricius) and Hypothenemus hampei (Ferrari) for which information is limited. Developmental stages including adults, larvae and eggs of red flour beetle, Tribolium castaneum (Herbst), adults of rice weevil, Sitophilus oryzae (L.) and lesser grain borer, Rhyzopertha dominica (F.), larvae of khapra beetle, Trogoderma granarium Everts, eggs of C. maculatus and larvae, pupae and adults of H. hampei , were exposed to phosphine concentrations ranging from 0.001 to 2.25 mg/L for 24, 48, and 72 hours. Sitophilus oryzae and R. dominica strains of Almora were observed to be more resistant to phosphine when compared with the most susceptible population from Cuttack. Tribolium castaneum strains of Delhi and Almora were more susceptible than strains of Cuttack. All the tested developmental stages of H. hampei were found to be highly susceptible; while, eggs of C. maculatus and larvae of T. granarium were highly resistant. Results showed significant variations in susceptibility among insect populations and life stages. The study also revealed that median lethal concentrations decrease with an increase in exposure levels irrespective of the insect species and life stages tested, suggesting the need for location-specific dosage and exposure periods for phosphine fumigation.