Classical biological control: exploiting enemy escape to manage plant invasions

Abstract

Practitioners of classical biological control of invasive weeds are confronted with a dual expectation: to achieve successful control of plant invaders and to avoid damage to nontarget plants and adverse indirect effects. In this paper we discuss key issues that we consider to be crucial for a safe, efficient, and successful classical biological control project, and that have also caused some recent controversy. These include selection of effective control agents, host specificity of the biological control agents, implications of the genetic population structure of the target populations, and potential impact on native food webs. With regard to improving the success rate of biological control of plant invaders, we first emphasize the importance of a clear a priori definition of success and a more ecosystembased approach to better document both negative effects of the invasive plant as well as potential positive and negative effects of introducing biological control agents. Secondly, pre-release impact assessment could be improved by better focusing on how to reach high densities of the control agents and by including tolerance to and compensation of herbivory. Thirdly, we advocate a reinforced effort to integrate and combine biological control in combination with existing or potential management options. Finally, we propose various ecological and evolutionary hypotheses in the framework of our topic to document that biological control programmes against plant invaders also offer a great opportunity to gain new insights into basic processes in ecology and evolution.

Is there rapid evolutionary response in introduced populations of tansy ragwort, Jacobaea vulgaris, when exposed to biological control?

“…We studied variation in life-history traits among introduced populations of J. vulgaris from two different parts of the introduced range and compared populations with and without exposure to L. jacobaeae, the biological control agent that is considered to be at least partly responsible for the regional decrease of this invasive weed. So far, this approach has rarely been used to explore evolutionary change in introduced species (but see Zangerl and Berenbaum 2005;Maron and Vilà 2008;Handley et al 2008;Müller-Schärer and Schaffner 2008), although it can reduce problems encountered when comparing populations from the native and the introduced range (Colautti et al 2004). Based on our results, one might assume that (re-)exposure of J. vulgaris to L. jacobaeae has led to opposite evolutionary trajectories in resistance traits in New Zealand and North America.…”

“…An alternative way to improve our understanding of evolutionary changes in introduced populations in response to different herbivore assemblages is to compare life-history traits of populations within the introduced range that have experienced successful biological control with those of populations that have not been exposed to classical biological control (Maron and Vilà 2008;Müller-Schärer and Schaffner 2008). Following the same lines of argumentation as the EICA hypothesis, and considering the specialist-generalist dilemma, populations that have been reunited with specialist herbivores through biological control measures should evolve genotypes with increased defences against specialist herbivores, reduced defences against generalist herbivores and decreased growth rate and/or reproductive output.…”

Current levels of suppression of waterhyacinth in Florida USA by classical biological control agents

“…Developing these agents can be expensive initially and, because they rarely completely control the target weed, traditional methods are often still required, albeit to a lesser degree (Müller-Scharer and Schäffner, 2008). Despite these facts, biological control is often ignored by some land managers as an asset for cutting costs, in part because of the difficulty or lack of information on integrating biological control into existing management programs.…”

Plant invasions, generalist herbivores, and novel defense weapons

“…In nonnative ranges exotic plants no longer interact with specialists and encounter a new suite of generalist natural enemies with which they have no evolutionary past (Mitchell et al 2006, Verhoeven et al 2009). Because generalist herbivores are widespread and present in both the native and nonnative ranges of invaders and apparently cannot be escaped, ecologists have focused almost exclusively on the release of invaders from specialists and the potential of reintroducing specialists to control invaders (Mu¨ller-Scha¨rer and Schaffner 2008). However, there are few if any generalist consumers of plants that do not have preferences, and the preferences of generalists can completely transform the composition and diversity of plant communities (Huntly 1990, Strauss et al 2009).…”