Contrasting patterns of phylogeographic structuring in two key beetle pests of stored grain in India and Australia

Rhyzopertha dominica and Tribolium castaneum are key pests of stored grains globally. Fumigation with phosphine gas is the primary control method for these beetles, so the development of phosphine resistant populations threatens the continued use of this fumigant. Developing effective strategies to manage the spread of phosphine resistance requires an understanding of the genetic diversity and structuring in these pests. We therefore assessed the phylogeographic structuring of these beetles in Australia and India, two significant grain producing countries. Genetic diversity was high in T. castaneum, with 40 discrete haplotypes identified from the 268 specimens sequenced. Four haplotypes, including the three most common haplotypes, were found in both India and Australia. Genetic diversity was notably lower in India, potentially driven by the higher frequency of phosphine fumigations in India. No genetic structuring was identified in Australia T. castaneum, but a small amount was detected in India. Genetic diversity was much lower in R. dominica, with only three discrete COI haplotypes identified from 215 specimens. The low level of genetic diversity in R. dominica suggests selection may be occurring on the mitochondrial genome, potentially driven by an endosymbiont such as Wolbachia. The contrasting phylogeographic patterns, across species and countries, emphasise the importance of developing insecticide resistance management practices that are both species specific (even if pests share a similar environment), as well as region specific. For instance, the limited genetic structuring identified in T. castaneum suggests phosphine resistance could potentially spread rapidly across Australia, meaning broad-scale resistance management strategies are essential