Hybrid watermilfoil lineages are more invasive and less sensitive to a commonly used herbicide than their exotic parent (Eurasian watermilfoil)

Abstract

Hybridization may stimulate the evolution of invasiveness in human-impacted habitats if unique hybrid genotypes have higher fitness than parental genotypes. Human efforts to control invasive taxa frequently involve the intentional alteration of habitats, but few studies have considered whether hybridization can result in decreased sensitivity to control measures. Here, we investigate whether interspecific hybrids between introduced Eurasian watermilfoil (Myriophyllum spicatum) and native northern watermilfoil (M. sibiricum) are more invasive than parental Eurasian watermilfoil, especially in regard to their relative responses to an herbicide commonly applied for their control (2,4-dichlorophenoxyacetic acid; 2,4-D). In two separate laboratory experiments, hybrids on average grew faster and were less sensitive to 2,4-D compared with parental Eurasian watermilfoil. These two invasive traits appear to be common in hybrid watermilfoils, as opposed to being restricted to a few unique lineages, because they were found in a diversity of hybrid genotypes from several independent hybridization events. In addition, we found that hybrids occurred more frequently than parental species in natural lakes previously treated with 2,4-D. Our results provide compelling empirical evidence that hybridization is associated with the evolution of increased invasiveness in watermilfoils, and have important implications for their management.

Influence of niche similarity on hybridization betweenMyriophyllum sibiricumandM. spicatum

“…Hybrid fitness is often related to transgressive characters (Schwarzbach et al, 2001). Two transgressive traits, faster growth and higher resistance to herbicides, were observed in hybrids between M. sibiricum and M. spicatum in North America (LaRue et al, 2013b). However, these same traits may not be present in hybrids in the QTP due to environmental differences between the two continents.…”

Najas minor (Hydrocharitaceae) in North America: A reappraisal

“…Interspecific hybridization can enhance the invasive potential of plants (LaRue et al, 2013). However, less consideration has been given to intraspecific hybridization, which also can influence the severity of nonindigenous species invasions.…”

Divergent responses of cryptic invasive watermilfoil to treatment with auxinic herbicides in a large Michigan lake

“…Specifically, I tested the hypothesis that pure parental EWM would decrease in both distribution and abundance significantly more than HWM during the growing season following herbicide treatment, as predicted by previous laboratory studies (LaRue et al 2013;Schulte et al unpublished). My study has important implications for the debate on whether morphologically cryptic EWM and HWM warrant different treatment strategies, as well as whether incorporating genetic identifications into traditional aquatic plant survey methods could improve adaptive management of nuisance watermilfoil by providing more accurate treatment evaluations.…”