Toxic effect of LC20 of phosphine on carbohydrate metabolism and macromolecular concentrations in two larval instars (4th and 6th) of the khapra beetle, Trogoderma granarium, at different exposure periods (24-120 h) was determined. Two populations of khapra beetle used in this research possessed different levels of susceptibility to phosphine. Based on LC50 one population was termed as a susceptible population (never exposed to phosphine previously) while the other was a tolerant population (having a history of phosphine fumigation for 13 years). The LC50 of 4th larval instar of susceptible and tolerant populations was 6 and 9 ppm while for 6th larval instar it was 5.4 and 8.8 ppm, respectively. The phosphine concentration at LC50 value caused about 100 percent mortality when exposure period was prolonged to 120 h. The trehalose, glycogen, lipids and nucleic acid contents of both larval instars of these populations were significantly reduced, while trehalase and amylase activities were significantly increased in larvae of both populations throughout the phosphine exposure period with reference to their respective unexposed group values. In case of soluble proteins, an elevation was observed which was followed by a decline after 72 h. Similarly, free amino acid contents showed increasing trend in both larval instars of tested populations throughout the exposure period with the exception of the 4th larval instars of susceptible population which decreased after 72 h. The concentration of glucose showed significant increase in 6th larval instars of both populations at each exposure period but in 4th larval instars it started to decrease after 72 h exposure. These phosphine induced metabolic alterations over varying exposure intervals may be helpful in controlling this insect pest by monitoring period of fumigant exposure.
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