Genetic diversity and population structure of wheat stem sawfly and its Bracon spp. parasitoids in the Great Plains

The wheat stem sawfly, Cephus cinctus Norton (WSS, Hymenoptera: Cephidae), is a major pest of wheat in the U.S. Great Plains. While populations of the larval parasitoids Bracon cephi and B. lissogaster (Hymenoptera: Braconidae) are abundant in wheat-growing regions of some states, they remain low in Colorado. To determine whether reduced parasitism reflects local parasitoid population differentiation or ecological constraints, we combined greenhouse parasitism assays with mitochondrial cytochrome oxidase I (COI)-based population genetic analyses of WSS and Bracon cephi collected from Colorado, Nebraska, and North Dakota. Greenhouse assays showed no differences in parasitism rates of Colorado WSS among Bracon populations from different states, indicating compatibility across regions. However, COI analyses revealed contrasting genetic patterns between host and parasitoid. WSS populations exhibited strong regional genetic differentiation, with Colorado samples showing evidence of recent demographic expansion, including a star-like haplotype pattern centered on a dominant haplotype and a significantly negative Tajima's D. Nebraska displayed the highest WSS haplotype diversity, while North Dakota samples clustered within the northern haplogroup. In contrast, B. cephi populations showed little geographic structure with haplotypes broadly shared among regions. AMOVA analyses revealed substantial genetic differentiation among WSS populations (56.52%), whereas B. cephi showed no detectable population structure, with most of the variation within populations. Low parasitism in Colorado appears consistent with ecological or demographic factors associated with recent WSS expansion rather than genetic incompatibility. These findings highlight the evolutionary history of the genetic structure of WSS and Bracon populations, providing insights for biological control strategies in WSS management.