Using Fine-Scale Relatedness To Infer Natural Enemy Movement

Natural enemies often move among habitats to track prey and resources. Indeed, biocontrol often depends on natural enemies dispersing into crops after disturbances such as tillage and pesticide applications. However, the small size of many natural enemies makes it difficult to observe such movements. Here we used genetic relatedness among entomopathogenic nematodes, tiny, soil-dwelling, and thus cryptic natural enemies, to infer their movement across an agricultural landscape. We collected strains of two nematode species, Heterorhabditis bacteriophora and Steinernema feltiae, by placing sentinel hosts into eight irrigated Solanum tuberosum fields across arid central Washington State. We then used restriction site associated DNA sequencing to generate single nucleotide polymorphisms and infer relatedness among strains across our study sites. We identified 3,367 and 138,286 polymorphic loci for H. bacteriophora and S. feltiae, respectively. Genetic differentiation for both species increased with greater distance between sites, although there was considerable variation. While strains collected from the same field were generally more closely related than those from different sites, for both species, strains from different fields were sometimes quite closely related. Altogether, our results suggest a surprising amount of genetic similarity among nematodes from distant sites, despite the presumably limited distances that can be traversed by individual worms. This is consistent with the nematodes moving, at least occasionally, between far-apart locations. More generally, we suggest that recent advances in population genomics are providing powerful new tools for mapping natural enemy movement across broad landscapes.