The effects of flooding, plant traits, and predation on purple loosestrife leaf‐beetles

Abstract

Classical biological weed control is based on the premise that introducing specialized natural enemies from the native range re‐establishes herbivore control of plant invaders, ultimately leading to negative population growth rates. Evidence from past biocontrol programs suggests that herbivores are not solely responsible for shaping plant demography. Diverse environmental conditions in the introduced range may not only affect demography, but also influence top‐down control of target plants. We investigated how flooding affects impacts of predators (top‐down) and plant quality (bottom‐up) on performance of two leaf‐beetles, Galerucella calmariensis L. and Galerucella pusilla Duftschmid (Coleoptera: Chrysomelidae: Galerucini), released in North America as biocontrol agents of purple loosestrife, Lythrum salicaria L. (Lythraceae). Predation and flooding regime have been linked to low leaf‐beetle recruitment at sites where insects failed to attain outbreak populations. Predator exclusion experiments at adjacent flooded and non‐flooded sites indicated a positive effect of flooding on leaf‐beetle survival for all developmental stages, whereas predator exposure had little effect. There was no difference in predation rates at sites with successful or failed purple loosestrife control, questioning the importance of predation in limiting growth and impact of these biocontrol agents’ populations. Effect of flooding on purple loosestrife quality was evaluated in a common garden study where plants were grown under different flooding treatments. Plants grown in flooded soil had higher water content and lower tannic acid concentration than plants grown in well‐drained soil. Consistent with field observations, leaf‐beetle oviposition rate and survival were higher on flooded plants. Results indicate that both bottom‐up and top‐down forces operate on Galerucella populations, yet their relative strength is mediated by flooding regime. Ignoring intricacies of plant‐herbivore and trophic interactions in the introduced range appears to be a major handicap for the improvement of weed biocontrol programs.

Habitat restoration alters adult butterfly morphology and potential fecundity through effects on host plant quality

“…To increase the rate of CO 2 acquisition, submerged plants often elongate their shoots to keep their leaves above water (Voesenek et al 2006). Compared to leaves produced by non-submerged plants, these leaves are thin with a high surface area to enhance gas exchange, have a high water content, and have lower secondary plant compounds (Davalos and Blossey 2010). These traits suggest an increase in host plant quality for herbivores.…”

“…The reduced plant quality of sun leaves resulted in longer caterpillar development times compared to those that fed on shade leaves (Jansen andStamp 1997, Henriksson et al 2003). Similarly, leaf beetles fed purple loosestrife (Lythrum salicaria) grown in flooded soils had higher survival and oviposited more compared to those fed leaves from well-drained soils (Davalos and Blossey 2010). One possible mechanism behind these fitness differences is that changes in plant quality alter the amount and allocation of nutrients available for reproduction, survival, and dispersal.…”

Retention of immunolabels by Diorhabda carinulata, a biological control agent of saltcedar

“…At these times, natural enemies can impact population dynamics of agents by acting as a drain on the agent’s local population, and thus limiting its population increase, and thereby affecting establishment or persistence. The specific circumstances of the system in question appear to affect the degree to which top‐down forces negatively impact biological control agents, and in turn, weed suppression (Hunt‐Joshi et al., 2005; Dávalos & Blossey, 2010).…”

Does dimethyl sulfoxide increase protein immunomarking efficiency for dispersal and predation studies?

“…Natural enemies of weed biological control agents can impact establishment and population growth, as well as population dynamics, and thus can be a detriment to weed suppression (Goeden & Louda, ). The role that top‐down forces play in weed suppression appears to vary with the system in question, but their occurence is not uncommon (Hunt‐Joshi et al., ; Dávalos & Blossey, ; Paynter et al., ). Nearly half of the failures of exotic herbivores to establish in weed biological control projects worldwide have been attributed to natural enemies (Julien & Griffiths, ).…”