Phylogeography and Historical Demography of the Agricultural Pest Grasshopper Dichroplus elongatus Across South American Biogeographic Provinces.

Understanding the evolutionary history of insect pests is crucial for predicting population outbreaks. We studied the phylogeographic pattern and demographic history of Dichroplus elongatus, an agroeconomically important grasshopper in South America. We sequenced a 560-bp fragment of the mitochondrial COI gene across 16 populations spanning three biogeographic provinces in Argentina: Pampeana, Monte, and Chaco. Network and phylogenetic analyses revealed that most haplotypes are shared among Biogeographic Provinces. Consistent with this, a weak signal of regional genetic structure was revealed by both AMOVA and BAPS analyses, suggesting high historical gene flow or recent range expansions. However, regional-scale demographic analyses revealed contrasting scenarios. The Pampeana province showed robust signs of recent population expansion, characterized by unimodal mismatch distributions and significantly negative Tajima’s D and Fu’s Fs values in individual populations within the region. Conversely, Monte and Chaco provinces exhibited demographic stability with multimodal mismatch patterns. Bayesian Skyline Plots (BSP) further confirmed these asynchronous regional dynamics, tracking distinct effective population size (Ne) fluctuations over time. Bayesian divergence dating placed the TMRCA of the biogeographic provinces in the Late Pleistocene, ranging from ~88,600 years ago in Chaco to ~59,600 years ago in Pampeana. These results indicate that despite high connectivity undermining profound genetic structure, regional environmental variations shaped distinct demographic trajectories. This highlights the complex evolutionary history of D. elongatus and provides vital insights into the regional drivers of its population dynamics.