Dynamics and Management of Rising Outbreak Spruce Budworm Populations

Jacques Régnière,Barry Cooke,Pierre Therrien,Ariane Béchard,Alain Dupont

doi:10.3390/f10090748

Abstract

Management of spruce budworm, Choristoneura fumiferana (Clem.), outbreak spread requires understanding the demographic processes occurring in low, but rising populations. For the first time, detailed observations were made in the early stages of outbreak development. We sampled populations over a three-year period in both treated and untreated populations in the Lower St-Lawrence region of Quebec, Canada, and measured the density-dependence of survival and population growth rates, and the impact of natural enemies and insecticides. Insecticides tested were Bacillus thuringiensis (Berliner 1915) and tebufenozide. We recorded strong density-dependence of survival between early larval stages and adult emergence, explained largely by the variation of natural enemy impacts and overcrowding. We also observed inverse density-dependence of apparent fecundity: net immigration into lower-density populations and net emigration from the higher, linked to a threshold of ~25% defoliation. Because of high migration rates, none of the 2013 treatments reduced egg populations at the end of summer. However lower migration activity in 2014 allowed population growth to be reduced in treated plots. This evidence lends support to the conclusion that, for a budworm population to increase to outbreak density, it must be elevated via external perturbations, such as immigration, above a threshold density of ~4 larvae per branch tip (L4). Once a population has increased beyond this threshold, it can continue growing and itself become a source of further spread by moth migration. These findings imply that populations can be brought down by insecticide applications to a density where mortality from natural enemies can keep the reduced population in check, barring subsequent immigration. While we recognize that other factors may occasionally cause a population to exceed the Allee threshold and reach outbreak level, the preponderance of immigration implies that if all potential sources of significant numbers of moths are reduced on a regional scale by insecticide applications, a widespread outbreak can be prevented, stopped or slowed down by reducing the supply of migrating moths.

Highlights

Knowing how an insect outbreak starts is central to the development of any preventative management strategy
A new cycle of spruce budworm outbreak has recently emerged in the province of Quebec [4]
Three harboured spruce budworm populations that were in the early stages of outbreak development criteria were used to select those plots: (1) Spruce budworm population density, (2) stand composition starting in 2011

Summary

Introduction

Knowing how an insect outbreak starts is central to the development of any preventative management strategy. Millions of dollars were spent during the last outbreak cycle (1975–1995) trying to predict what would happen, and to mitigate the damage [2,3]. Forests 2019, 10, 748; doi:10.3390/f10090748 www.mdpi.com/journal/forests last outbreak cycle (1975–1995) trying to predict what would happen, and to mitigate the damage [2,3]. A new cycle of spruce budworm outbreak has recently emerged in the province of Quebec [4]. Asonew ofright spruce outbreak has recently emerged in theHowever, province of Quebec [4]considerable so the time the cycle time is tobudworm ask how best to manage this emerging threat. Despite the isinvestment right to ask in how best to manage this emerging threat. Despite the considerable investment population ecology research over the last cycle, we still have only a meagre in population ecology over lastoutbreak cycle, weSBW still have only a meagre of the understanding of the research dynamics of the rising populations

Methods

Results

Discussion

Conclusion