Phosphine resistance in saw-toothed grain beetle, Oryzaephilus surinamensis (Coleoptera: Silvanidae) in the United States

Sub-lethal dose application of phosphine (PH3) that is mostly caused by leakage during fumigation has resulted in resistance in Tribolium castaneum, Rhyzopertha dominica, Cryptolestes ferrugineus, Sitophilus oryzae, and other stored-product insect pest species worldwide. However, PH3 resistance in the saw-toothed grain beetle, Oryzaephilus surinamensis, has not been reported in any country. Additionally, the discriminating dose of PH3 for eggs of O. surinamensis has not been estimated. In this study, the discriminating dose for eggs of the susceptible strain of O. surinamensis was estimated as 28.4 ppm applied for 3 d. Adults from 4 out of 14 field-collected populations showed detectable resistance to PH3 whereas eggs in 9 out of 14 populations had detectable resistance. Resistance frequencies in both adults and eggs in Box BF, Box BR and OKWat populations were > 90%. Levels of resistance (LC99) in these three populations were estimated using probit analysis. LC99 values for adults of Box BF, Box BR, and OKWat populations were 320.5, 290.7 and 263 ppm, respectively, whereas those in eggs from the same populations were 1055.9, 1030.7, and 564.5 ppm, respectively, over 3-d fumigation. Resistance levels of adults and eggs of the most resistant population, Box BF, were 24.3- and 43.6-fold, respectively, higher than those of the lab-susceptible strain. The resistance levels in eggs from these three populations were > 3-fold higher than that in adults and this shows eggs of O. surinamensis are more tolerant to PH3 than adults. These results indicate that it may not be practical to use PH3 to control Box BF and Box BR populations. Therefore, it is important to develop alternative pest management strategies for controlling highly PH3-resistant populations of stored-product insect pests.