Response of phosphine-resistant mixed-age cultures of lesser grain borer, Rhyzopertha dominica (F.) to different phosphine-carbon dioxide mixtures.

Mixed-age cultures of a phosphine-resistant strain of lesser grain borer, Rhyzopertha dominica (F.) were
exposed to various concentrations of phosphine and phosphine plus 10, 20 and 30% carbon dioxide
combinations (CO2) for different time intervals at 25 �C to evaluate the comparative efficacy of admixtures
of phosphine-CO2. The mortality data of the mixed age cultures were recorded taking the adult
count at weekly intervals for 8 weeks. Lethal concentrations were determined by probit analysis.
Phosphine with carbon dioxide exerted significant synergistic effect on the mortality of mixed-age
cultures at 4, 6 and 7 days of exposure, while individually phosphine or carbon dioxide did not show
a raised increase in their response. The addition of 30% carbon dioxide to lower concentrations of
phosphine over shorter exposures of 4 days showed better synergistic effect among the treatments. The
present study revealed that CO2 augmented the effect of phosphine thereby reducing the concentration
and exposure time required to bring about significant mortality in the mixed-age cultures of R. dominica.