Variable success of biological control of Lythrum salicaria in British Columbia

Abstract

Purple loosestrife, Lythrum salicaria, has invaded North American wetlands over the last 200 years. A biological control project was started in British Columbia, Canada, in 1993 with the introduction of Galerucella calmariensis, a leaf-feeding beetle of European origin. To evaluate the success of the biological control project in the Lower Mainland of British Columbia, we monitored the defoliation levels of Lythrum at 10 release sites of the control agent, identified factors associated with among-site variability in defoliation levels, and determined the effect of defoliation on the performance and abundance of Lythrum. In tidal areas, the beetles could not establish due to tidal disturbances. The control agent established at all nontidal sites, but defoliation levels varied considerably. Defoliation levels were negatively associated with predation on Galerucella eggs, suggesting that general predators may be able to prevent or delay successful biological control of Lythrum. Eggs on Lythrum stems surrounded by water experienced lower predation rates than eggs on stems on land, indicating that standing water acts as a partial refuge against predators for Galerucella eggs. Flower-bud density, length of inflorescence, and plant biomass per m 2 were all negatively related with the intensity of beetle feeding, and stem height was marginally reduced with increased beetle grazing. Dry biomass per m 2 of Lythrum appeared to decline drastically only at sites where beetle grazing was intense.

Biological control as an invasion process: disturbance and propagule pressure affect the invasion success of Lythrum salicaria biological control agents

“…Tidal inundation, as estimated by distance to river mouth, had negative effects on colonization of agents. Denoth and Myers (2005) suggested the low persistence of G. calmariensis in estuarine environments may be linked to hydrologic regime. These negative relationships support the conclusion by Lozon and Maclsaac (1997) that plant invasions are more frequently associated with environmental variability for their establishment and range expansion than animals.…”

Should we care about purple loosestrife? The history of an invasive plant in North America

“…There is no precise evaluation of the total number of beetles that have been released for control, but available numbers (Minnesota: 7 million from 1992 to 2003; New Jersey: 1.6 million from 1997 to 2006; Ontario: 320,000 before 1998; Dech and Nosko 2002;Lindgren 2003;Scudder et al 2006) suggest that a considerable amount of money has been allocated to this program. Several biological control trials obtained spectacular results (e.g., total eradication of purple loosestrife only three to 6 years after the introduction of beetles; Landis et al 2003, Lindgren 2003, but some others failed (e.g., Dech and Nosko 2002;Denoth and Myers 2005 in intertidal areas). Carson et al (2008) estimated that ''The amount of damage caused [by introduced leaffeeding beetles] would rarely have been sufficient to allow the formerly dominant native cattails (…) to reestablish in [purple loosestrife] invaded wetlands [of New York and Pennsylvania]'' (p. 80).…”

Herbivory and novel weapons: no evidence for enhanced competitive ability or allelopathy induction of Centaurea diffusa by biological controls

“…Successful biological control programs should lessen the competitive ability of the invasive plant while leaving native plants unharmed. Although classical biological control (the use of exotic herbivores or pathogens to control exotic plant species) has had some spectacular successes against invasive plants (Coombs et al 2004;Hajek 2004), there are many examples of enemy introductions that have not resulted in measurable impacts on invasive populations (Julien and Griffiths 1998 and see Denoth et al 2002), and other examples where agents that are successful in some areas prove to be ineffective in others (LeJeune et al 2005;Denoth and Myers 2005). There are many reasons for this variable success, including unsuitable environmental conditions for biological control agent growth, geographic variation in plant resistance, and/ or attack on demographically unimportant portions or life-stages of the targeted weed.…”