Further demonstration of widespread phosphine resistance in Cryptolestes ferrugineus (Coleoptera: Laemophloeidae) in Oklahoma

Abstract: In 2013, we established a discriminating dose for Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) to be 56.2 ppm of phosphine gas (PH3), based on a 20-h fumigation. During 2015 and 2016, eight populations of C. ferrugineus from Oklahoma grain storage facilities that were evenly distributed geographically were collected, and subjected to the discriminating dose to detect resistance to PH3. Results showed three of the populations had resistance frequencies > 90%, and these were populations from one terminal and two cooperative elevator storage facilities. Three populations had resistance frequencies in the range of 60.7-86%. The remaining two populations were from cooperative elevator facilities and had no detectable resistance (100% mortality); similarly and expectedly, mortality of individuals in samples of the laboratory susceptible strain (Lab-S) was also 100%. Dose response tests were conducted over a 3-day period on the two populations of C. ferrugineus from Stillwater (Still) and a terminal elevator in Enid (Enid Term. 1) which had resistance frequencies ≥ 98%. LC99 values determined by probit analyses for the Lab-S, Still, and Enid Term. 1 were 7.3, 699.5, and 1,275 ppm, respectively, over a 3-day exposure period. The level of resistance in populations from Stillwater and Enid were 96.5 and 225 times that of the susceptible laboratory strain, respectively. Data from this study show that C. ferrugineus resistance to PH3 is widely distributed geographically within the state of Oklahoma with some populations highly resistant to PH3. This study demonstrates the need for a wider survey of PH3 resistance in stored grain insect pests in grain growing parts of Oklahoma and USA.