Effects of carbon dioxide and phosphine mixtures on resistant populations of stored-grain insects

Populations of Rhyzopertha dominica (F.) and Sitophilus oryzae (L.) with high frequencies and high levels of resistance to phosphine (PH3), as detected by the Food and Agriculture Organization (FAO) discriminating dose test, were submitted to various concentrations of phosphine in air and to various concentrations of phosphine plus 10% and 20% carbon dioxide (CO2) for 20 h. Three resistant populations and one susceptible population of each species were tested. Concentration-mortality regression lines to obtain the LC50 and LC99.9, values were determined for each population, The resistance levels for these mortality rates were also obtained. The mixtures of phosphine plus 10% and 20% CO2 reduced the resistance levels of the S. oryzae populations. The more susceptible portion of the R. dominica population showed a decrease in the required phosphine concentration with an increase in CO2, but phosphine alone seemed to be more effective than the mixtures with CO2 for the control of the more resistant portion. The mixtures of phosphine plus CO2 reduced the resistance levels to phosphine in populations of R. dominica and S. oryzae, although a clear pattern could not be determined. Resistant populations of R. dominica and S. oryzae to phosphine showed different mortality responses to treatments with mixtures of phosphine plus 10% and 20% CO2. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.