Genetic diversity and its geographic structure in Sitophilus oryzae (Coleoptera; Curculionidae) across India – implications for managing phosphine resistance

The rice weevil, Sitophilus oryzae, is a serious global pest of stored grains. Fumigation with phosphine gas is the primary control method for S. oryzae, but the indiscriminate and prolonged use of phosphine gas has led to the development of heritable resistance. Developing and implementing an effective phosphine resistance management strategy for S. oryzae relies on an understanding of its genetic diversity and any structuring of that diversity geographically. We therefore sequenced the mitochondrial cytochrome c oxidase subunit I gene from 143 S. oryzae specimens collected from 37 locations across India, and from that assessed the genetic diversity of the species and its phylogeographic structuring. In addition, we compared the genetic diversity in Indian S. oryzae populations (the hypothesised origin of this beetle) to global populations. Genetic diversity was low in Indian S. oryzae, with only eight haplotypes (including two very common haplotypes) identified. The low level of mitochondrial diversity observed in this species appears typical of stored product pests, perhaps suggesting that low mitochondrial diversity is associated with repeated phosphine fumigations, which may eliminate low frequency haplotypes. The genetic diversity of S. oryzae in India is, however, higher than in many other countries, though comparable levels were identified in China. There was no evidence of population genetic structure across India, with most haplotypes found in three of the broad biogeographic regions. This lack of phylogeographic structuring indicates significant gene-flow across India, most likely through the incidental anthropogenic transport of this relatively poor (or reluctant) flyer. The major practical implication is that phosphine resistance management for S. oryzae needs to be dealt with country wide, as populations are not isolated.