First report of strong phosphine resistance in stored grain insects in a far northern tropical region of Australia, combining conventional and genetic diagnostics

Although phosphine resistance monitoring and management programs are well advanced in Australia, the tropical northern part of the continent has received little attention. To address this, grain storages were sampled in the Townsville region in the Burdekin river catchment area of northern Queensland, and insect populations were subjected to resistance testing. Sampling of storages including silos, food processing facilities and feedlots was carried out during July to August 2019 and February to March 2020. Of the several species collected, populations of two major pest species, Rhyzopertha dominica and Tribolium castaneum were subjected to phenotypic resistance and molecular screening using discriminating doses and gene specific DNA marker, respectively. Utilising phenotypic assay, of the 17 populations of R. dominica, 12 were diagnosed as weakly resistant and five as strongly resistant; whereas of the 34 populations of T. castaneum, two were found to be susceptible to phosphine, 15 were determined as weakly resistant and 17 as strongly resistant. The molecular diagnostics, however, determined rph2 alleles for strong resistance in some populations each of R. dominica and T. castaneum that were diagnosed as weakly resistant in the phenotypic assay. The most prevalent rph2 alleles were found to be P49S and K142E in R. dominica populations and P45S and G131S in T. castaneum populations. Our results highlight the need of more comprehensive study towards determining the level of pests and resistance risks in this region.