Spatial Pattern of Attacks of the Invasive Woodwasp Sirex noctilio, at Landscape and Stand Scales.

M Victoria Lantschner,Juan C Corley,João Pinto

doi:10.1371/journal.pone.0127099

Abstract

Invasive insect pests are responsible for important damage to native and plantation forests, when population outbreaks occur. Understanding the spatial pattern of attacks by forest pest populations is essential to improve our understanding of insect population dynamics and for predicting attack risk by invasives or planning pest management strategies. The woodwasp Sirex noctilio is an invasive woodwasp that has become probably the most important pest of pine plantations in the Southern Hemisphere. Our aim was to study the spatial dynamics of S. noctilio populations in Southern Argentina. Specifically we describe: (1) the spatial patterns of S. noctilio outbreaks and their relation with environmental factors at a landscape scale; and (2) characterize the spatial pattern of attacked trees at the stand scale. We surveyed the spatial distribution of S. noctilio outbreaks in three pine plantation landscapes, and we assessed potential associations with topographic variables, habitat characteristics, and distance to other outbreaks. We also looked at the spatial distribution of attacked trees in 20 stands with different levels of infestation, and assessed the relationship of attacks with stand composition and management. We found that the spatial pattern of pine stands with S. noctilio outbreaks at the landscape scale is influenced mainly by the host species present, slope aspect, and distance to other outbreaks. At a stand scale, there is strong aggregation of attacked trees in stands with intermediate infestation levels, and the degree of attacks is influenced by host species and plantation management. We conclude that the pattern of S. noctilio damage at different spatial scales is influenced by a combination of both inherent population dynamics and the underlying patterns of environmental factors. Our results have important implications for the understanding and management of invasive insect outbreaks in forest systems.

Highlights

Invasive forest insects can cause important damage to planted and natural forests, which can result in widespread tree mortality when pest population outbreaks occur [1,2,3]
Seventy-five percent of the sampled stands were composed of pure Pinus ponderosa, 12% were mixed stands of P. ponderosa and P. contorta, 8% were of pure P. contorta, and 4% mixed stands of several other species (P. jeffreyi, P. radiata, Pseudotsuga menziesii)
At the stand scale, we noted a clear aggregation of the attacked trees in stands with increasing infestation levels, and again, the degree of S. noctilio attacks is strongly influenced by the host species abundance and by plantation thinning

Summary

Introduction

Invasive forest insects can cause important damage to planted and natural forests, which can result in widespread tree mortality when pest population outbreaks occur [1,2,3]. At the landscape scale, insect populations that outbreak may show a spatial pattern that has an epicenter, where the outbreak begins and spreads out through the forest or else, may occur as multiple simultaneous eruptions that tend to coalesce [3, 11, 12]. Variability in the spatial patterns observed and the variety of factors acting at different scales suggests that understanding the spatial pattern of attacks by forest pest populations is essential. This is because of its importance to improve our understanding of insect population dynamics in general and because it is critical to predicting attack risk by invasives or planning pest management strategies

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