Abstract
The success rate of weed biological control programs is difficult to evaluate and the factors affecting it remain poorly understood. One aspect which is still unclear is whether releases of multiple, genetically distinct populations of a biological control agent increase the likelihood of success, either by independent colonization of different environmental niches or by hybridization that may increase the agent's fitness and adaptive ability. Since hybridization is often invoked to explain the success of unintentionally introduced exotic species, hybridization among biocontrol agents may be similarly important in shaping the effectiveness of biological control programs. In this study, we first evaluated intraspecific hybridization among populations of a weed biological control agent, the ragwort flea beetle, Longitarsus jacobaeae. These insects were introduced as part of a classical biological control program from Italy and Switzerland. We genotyped 204 individuals from 15 field sites collected in northwest Montana, and an additional 52 individuals that served as references for Italian and Swiss populations. Bayesian analysis of population structure assigned seven populations as pure Swiss and one population as pure Italian, while intraspecific hybrid individuals were detected in seven populations at frequencies of 5%–69%. Subsequently, we conducted a 2‐year exclusion experiment using six sites with Swiss beetles and three with hybrid beetles to evaluate the impact of biological control. We found that biological control by Swiss beetles and by hybrid beetles is effective, increasing mortality of the target plant, Jacobaea vulgaris, by 42% and 45%, and reducing fecundity of surviving plants by 44% and 72%, respectively. Beetle densities were higher and mortality of larger plants was higher at sites with hybrids present. These results suggest that hybridization of ragwort flea beetles at high‐elevation sites may improve biological control of tansy ragwort and that intraspecific hybridization of agents could benefit biological control programs.
Highlights
Classical biological control, the use of specialized natural enemies from the native range of an invasive species to exert top‐down con‐ trol, is a crucial component of sound long‐term management of many invasive species
Tansy ragwort is native to Eurasia and introduced to North America, where it is prevalent along the Pacific coast from northern California to British Columbia, and has spread to inland sites in Oregon, Idaho, and Montana (Bain, 1991; Littlefield, Markin, Puliafico, & de‐ Meij, 2008; Szűcs, Eigenbrode, et al, 2012)
We found that natural hybridization between Swiss and Italian bee‐ tles is widespread in northwest Montana with hybrids being present at most sites where originally both parental populations had been released
Summary
The use of specialized natural enemies from the native range of an invasive species to exert top‐down con‐ trol, is a crucial component of sound long‐term management of many invasive species. Cryptic species are inadver‐ tently released, and in others, genetically distinct populations from the native range are released (Bean et al, 2013; Phillips et al, 2008; Winston et al, 2014) The consequences of such diverse releases are generally unknown, including whether distinct biotypes are more likely to establish, whether intra‐ or interspecific hybridization be‐ tween those occurs in situ (Szűcs, Schwarzländer, & Gaskin, 2011), and whether hybridization may lead to different levels of control in the field. We predicted that (a) hybrids would produce more offspring than parental lineages, (b) that hy‐ brids would build up higher densities per plant, and (c) they would reduce survival and/or reproductive output of the weed more significantly than beetles from parental lineages
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