Reduced Population Control of an Insect Pest in Managed Willow Monocultures

Peter Dalin,Christer Björkman,Oskar Kindvall,Andrew Wilby

doi:10.1371/journal.pone.0005487

Peter Dalin, Christer Björkman + Show 2 more

Open Access

https://doi.org/10.1371/journal.pone.0005487

Copy DOI

Abstract

BackgroundThere is a general belief that insect outbreak risk is higher in plant monocultures than in natural and more diverse habitats, although empirical studies investigating this relationship are lacking. In this study, using density data collected over seven years at 40 study sites, we compare the temporal population variability of the leaf beetle Phratora vulgatissima between willow plantations and natural willow habitats.Methodology/Principal FindingsThe study was conducted in 1999–2005. The density of adult P. vulgatissima was estimated in the spring every year by a knock-down sampling technique. We used two measures of population variability, CV and PV, to compare temporal variations in leaf beetle density between plantation and natural habitat. Relationships between density and variability were also analyzed to discern potential underlying processes behind stability in the two systems. The results showed that the leaf beetle P. vulgatissima had a greater temporal population variability and outbreak risk in willow plantations than in natural willow habitats. We hypothesize that the greater population stability observed in the natural habitat was due to two separate processes operating at different levels of beetle density. First, stable low population equilibrium can be achieved by the relatively high density of generalist predators observed in natural stands. Second, stable equilibrium can also be imposed at higher beetle density due to competition, which occurs through depletion of resources (plant foliage) in the natural habitat. In willow plantations, competition is reduced mainly because plants grow close enough for beetle larvae to move to another plant when foliage is consumed.Conclusion/SignificanceTo our knowledge, this is the first empirical study confirming that insect pest outbreak risk is higher in monocultures. The study suggests that comparative studies of insect population dynamics in different habitats may improve our ability to predict insect pest outbreaks and could facilitate the development of sustainable pest control in managed systems.

Highlights

The push to reduce CO2 emissions from fossil fuels in favor of bioenergy together with the Earth’s growing human population leads to enhanced land transformations and intensifications of forest and agricultural systems [1,2]
One overriding hypothesis to why plant monocultures should be more susceptible to insect outbreaks is that the factors regulating insect populations in natural habitats are often altered when the habitat is transformed, which can result in larger population fluctuations and insect outbreaks
Density of the leaf beetle Phratora vulgatissima showed greater temporal variability in willow plantations than in natural willow stands during the study period (1999–2005)

Summary

Introduction

The push to reduce CO2 emissions from fossil fuels in favor of bioenergy together with the Earth’s growing human population leads to enhanced land transformations and intensifications of forest and agricultural systems [1,2]. One overriding hypothesis to why plant monocultures should be more susceptible to insect outbreaks is that the factors regulating insect populations in natural habitats are often altered when the habitat is transformed, which can result in larger population fluctuations and insect outbreaks [6]. Empirical studies testing this hypothesis are rare, as well as knowledge about the mechanisms bringing about population stability in natural systems. In this study, using density data collected over seven years at 40 study sites, we compare the temporal population variability of the leaf beetle Phratora vulgatissima between willow plantations and natural willow habitats

Objectives

Methods

Results

Conclusion