Into the weeds: matching importation history to genetic consequences and pathways in two widely used biological control agents

Authors: HOPPER, JULIE - University Of Southern California; McCue, Kent; Pratt, Paul; DUCHESNE, PIERRE - University Of Laval; GROSHOLZ, EDWIN - University Of California, Davis; HUFBAUER, RUTH - Colorado State University

Submitted to: Evolutionary Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2019
Publication Date: 1/5/2019
Citation: Hopper, J.V., McCue, K.F., Pratt, P.D., Duchesne, P., Grosholz, E.D., Hufbauer, R. 2019. Into the weeds: matching importation history to genetic consequences and pathways in two widely used biological control agents. Evolutionary Applications. 12(4):773-790. https://doi.org/10.1111/eva.12755.
DOI: https://doi.org/10.1111/eva.12755

Interpretive Summary: Non-native species of plants or animals can spread rapidly in their exotic range, which can result in displacement of native species, increased competition for nutrient resources and encroachment on physical spaces. The invasion of the non-native water hyacinth in the waterways of Northern California is both a biological and physical problem causing competition for resources and physical obstruction of canals and water transport infrastructure. As an alternative to herbicides, biological control agents (weevils) from the plant’s native range are used to feed on water hyacinth and reduce the health and spread of the plant. These weevils have come to California through different routes from their original South American origin and are observed to have varying effects at controlling the spread of water hyacinth. To understand the basis for effective weed control and relate this to source and existing populations, we are using molecular markers to monitor the diversity and spread of the weevils in relation to their ability to control the water hyacinth. We discovered that although two species of weevils have been introduced, and created mixed populations, the diversity of the population still reflects the original source populations. This suggests the need to optimize the original source population from an environment most similar to the environment being treated for maximal efficiency of biological control.

Technical Abstract: The intentional introduction of exotic species through classical biological control programs provides unique opportunities to examine the consequences of population movement and ecological processes on the genetic diversity and structure of introduced species. The weevils, Neochetina bruchi and N. eichhorniae (Coleoptera: Curculionidae) have been introduced globally to control the invasive floating aquatic weed, Eichhornia crassipes, with variable outcomes. Here, we use the importation history and polymorphic microsatellite markers on populations of these weevils from their native and introduced ranges to examine the effects of introduction processes on population genetic diversity and structure. We report the first confirmation of hybridization between these two widely used biological control agents and suggest for future work to investigate the causes and effects of hybridization. For both species, we found the highest number of rare alleles in populations in the native range compared to the introduced populations. N. eichhorniae also had higher allelic richness in the native population compared to the introduced populations. Neither the initial propagule size nor the number of introduction steps appeared to consistently affect genetic diversity. We found evidence of genetic drift, inbreeding and admixture in several populations as well as significant population structure. Analyses estimated two populations and eleven sub-clusters for N. bruchi and four populations and seven sub-clusters for N. eichhorniae, indicating divergence of populations during and after introduction. Genetic differentiation and allocation of introduced populations to source populations generally supported the documented importation pathways, and clarified the importation history in cases where multiple introductions occurred.