Insect pests are a major threat to many forests worldwide, from boreal to tropical forest ecosystems. Some pests exhibit periodical outbreaks, after which their populations often crash as a result of natural biological control. In this study, authors assessed the performance of aerial spraying of insecticides on pine woodland stands to control pine processionary moth Thaumetopoea pityocampa (PPM) outbreaks in southern Spain.
Herbivorous insects are integral components of forest dynamics, in which they play important roles. However, populations may occasionally grow rapidly into damaging proportions. Such sporadic outbreaks can have catastrophic impacts on forests and trees, leading to the complete destruction of large areas of natural and/or planted forests, and considerable economic losses in some cases. One such pest is the pine processionary moth (Thaumetopoea pityocampa).
In the last few decades, the area affected by processionary outbreaks in Europe has expanded northwards and upwards in the mountains, and the pest is now affecting higher altitude and latitude areas where it used to be absent. This has resulted in high attack rates in areas hardly affected by this insect in the past. Thus, the application of control methods for aggressive pests such as PPM is a key issue in Mediterranean forestry.
To control processionary outbreaks, several management techniques have been used to date, including manual cutting and burning of nests, pheromone traps/mating disruption systems, and lethal mixtures of chemical and biological insecticides. Of these, aerial spraying of pine forests with insecticides is the most widely used option in most Mediterranean countries and has proved successful in preventing the pest multiplying, with apparently limited environmental effects (Sanchis et al., 1990; Battisti et al., 1998; Demolin and Martin, 1998;Dajoz, 2000). These insecticides target larval stages and are thus applied in late summer or early autumn provided that an outbreak of PPM has been detected during the previous winter.
Application of insecticides is however implemented at a time when the moth’s population is expected to crash as a result of natural biological control (predators–parasitoids, host plant, or both). It follows to question whether processionary decline following aerial spraying of insecticides is a result of management practices or a natural consequence of the insect’s population cycle.
The study area is the region of Andalusia (southern Spain). This area covers around 87,300 km2 (ca. the size of Austria) and includes a wide variety of habitats, from lowlands and meadows on the western side to tall mountains (well above 3000 m.a.s.l.) in the east. Five main pine species, black (Pinus nigra), Aleppo (P. halepensis), maritime (P.pinaster), stone (P.pinea), and Scots (P.sylvestris), represent the bulk of pine woodlands.
In this study, insecticide spraying performance was evaluated by comparing the response of heavily infested carriers that were treated (with insecticides) or untreated. The hypothesis is that, if the outbreak is effectively controlled by biological agents, there would be no difference in the response of heavily infested forests subjected to aerial spraying and untreated forests.
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The results help us to understand it
The percentage of woodland stands that were strongly defoliated by processionary (degree of infestation ≥3) increased progressively between 2002 and 2004 from 9.5% to 11.7% respectively, suffering an abrupt decline to 6.0% in 2005 (Table 1). A similar pattern was detected for forest cover. Total treated forest area also decreased progressively from 2002 to 2004 but increased slightly in 2005, indicating that fewer but larger woodland stands were treated in this year. Over 90% of the treatments throughout the study period consisted in the aerial application of insecticides (i.e. spraying), being almost 100% during 2004 and 2005.
The authors attributed lack of difference between treated and untreated samples to natural declines of the pest populations due to exhaustion of resources and an increase of natural enemies (predators, parasitoids, etc.). Indeed, most potential pests are likely to be under natural biologi- cal control in forest and agro-ecosystems (van Lenteren, 2006), but this is rarely taken into account in forest management plans.
Fig. 1 shows no clear differences between the decrease in the degree of infestation after an out- break of PPM (degree of incidence ≥ 3 in the first year) in sprayed and non-sprayed pine stands regardless of sampling design and species.
Our results can have both economic and ecological implications for forest management practices. The profitability of aerial treatments can be questioned if the benefits obtained are not significant enough to offset such costs (Aimi et al., 2006). The management and control of insect pests entails substantial monetary investments.
The solution against the processionary is to find other alternatives
With respect to PPM, between € 1.0 and € 1.5 million are spent annually on aerial spraying to control processionary outbreaks in our study region. In addition, because aerial spraying is conducted in the winter following strong, heavy or massive defoliation, treatments are unlikely to limit growth losses or prevent further damage to trees by other organisms. In this context, authors advocate for more rigorous tests to evaluate the cost-effectiveness of pest control practices, combined with economic assessments, instead of applying a routinary and unnecessary spraying protocol. Insecticide spraying cannot be considered a prevention for outbreaks if it is applied once the outbreak explodes.
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This does not imply renouncing to control processionary when necessary. Other forest management practices implemented at different scales can also help control PPM outbreaks. First, promoting spatial heterogeneity at the landscape level. Second, fostering biodiversity in pine plantations by, for instance, increasing the proportion of broadleaved trees and shrubs in forest stands (particularly at the stand borders), would increase the resilience of these systems to pest outbreaks. This is particularly relevant in this study region, where the recuperation of woody species diversity is being considered as part of forest management plans targeting the next 50 years, forest logging is no longer a profitable activity, and forest values other than timber are gaining importance.
Spraying can be especially important in pine masses located close to or within populated areas. In such areas, aerial spraying is unpractical, but pheromone traps, hand removal, or application of high insecticide concentrations using truck or back-pack methods is justified given the risks of PPM larvae for human health (EPPO/CABI, 1997). Forest managers should shift from static to adaptive planning based on scientific evidence, monitoring systems and protocols.
Source: Cayuela, L., Hódarb J.A., Zamora R.. Is insecticide spraying a viable and cost-efficient management practice to control pine processionary moth in Mediterranean woodlands? EcoLab, Departamento de Ecología, Centro Andaluz de Medio Ambiente, Universidad de Granada y Grupo de Ecología Terrestre, Departamento de Ecología, Facultad de Ciencias, Universidad de Granada.
Main photo credit: Isaac Sanz