Evaluation of the field impact of an adventitious herbivore on an invasive plant, yellow toadflax, in Colorado, USA

Abstract

Although some introduced plants arrive into their new range without their generalist and specialist herbivores, for others, their herbivores arrive prior to, with, or after the introduction of the plant, reestablishing the link between natural enemies and invaders in the introduced range. Research documenting the effects of adventitiously introduced herbivores on their target plants in the introduced range, and the mechanisms by which those effects occur, can provide insight into potential biological weed control. We studied the effects of an accidentally introduced beetle Brachypterolus pulicarius on the growth and reproduction of its host, the invasive plant Linaria vulgaris (yellow toadflax), growing under field conditions across multiple years and sites in western Colorado, USA. We found that feeding by B. pulicarius on L. vulgaris was variable among 3 years (2002)(2003)(2004) and across eight local sites. Part of the variation in damage was explained by ramet density; sites with greater ramet density experienced a higher proportion of damage. In an observational study across 2 years, damage was positively correlated with estimates of sexual reproduction, ramet growth, and clonal shoot production. However, opposite trends were observed in an experiment; damage by B. pulicarius decreased estimates of sexual reproduction. Differences between the results of the observational and experimental studies were likely driven by selective feeding by B. pulicarius on larger ramets. Nonetheless, the ability of B. pulicarius to control established L. vulgaris population growth remains uncertain under the environmental conditions we studied. In both the observational and experimental study, B. pulicarius did not affect L. vulgaris survival, and we found no evidence that established L. vulgaris populations were seed limited, suggesting that reductions in seeds may not translate into demographic changes in heavily infested populations. Interactions among insect foraging behavior, individual plant responses to damage, and the demographic consequences of seed input may help to explain the varying degrees to which herbivores affect plants and populations in this and other systems.

Exotic vertebrate and invertebrate herbivores differ in their impacts on native and exotic plants: a meta-analysis

“…The effects of herbivores on plant individuals, populations, communities, and ecosystems have been widely investigated (e.g., Bergelson 1990;Hulme 1994Hulme , 1996aKarban and Baldwin 1997;Adler et al 2001;Donlan et al 2002;Vázquez 2002;Bisigato et al 2005;Focardi and Tinelli 2005;Tierney and Cushman 2006; Baraza et al 2007;Branson and Sword 2008;Egan and Irwin 2008;Jasmin et al 2008;Joe and Daehler 2008;Nuñez et al 2008;Veen et al 2008). One important role of herbivores may be as mediators of the interaction between native and exotic plants.…”

Top-down and bottom-up controls on Dalmatian toadflax (Linaria dalmatica) performance along the Colorado Front Range, USA

“…This positive relationship may be explained by M. janthinus feeding and oviposition preferences for larger plants. Egan and Irwin (2008) found a similar positive association between Linaria vulgaris performance metrics and damage by an accidently introduced specialist beetle in a 2-year observational study, but a negative association in an experimental study and attributed the discrepancy to selective feeding on larger ramets. Herbivores commonly demonstrate oviposition and feeding preferences for larger plants (Price 1991).…”

“…Herbivores commonly demonstrate oviposition and feeding preferences for larger plants (Price 1991). As Egan and Irwin (2008) note, such selective feeding makes it difficult to evaluate the efficacy of biocontrol agents in observational and correlational studies, and may lead to equivocal findings in regard to the effects of natural enemies on target weeds. In the experimental study, we found strong negative effects of weevil attack on plant growth and reproduction.…”

Using economic instruments to develop effective management of invasive species: insights from a bioeconomic model

“…Using the model specified for owners i and j, we ran simulations to explore the optimal removal and planting strategies across a range of initial L. vulgaris densities, from 2 to 120 L. vulgaris/m 2 and assuming an initial 15 natives/m 2 . These densities of L. vulgaris and natives are representative of what is found in nature , Pauchard et al 2003, Egan and Irwin 2008, Wilke and Irwin 2010. We present simulation results from representative low, moderate, and high densities of L. vulgaris invasion (5, 30, and 60 plants/m 2 , respectively).…”