Elsevier

Forest Ecology and Management

Volume 432, 15 January 2019, Pages 1013-1021
Forest Ecology and Management

Comparing the efficacy of various aerial spraying scenarios using Bacillus thuringiensis to protect trees from spruce budworm defoliation

https://doi.org/10.1016/j.foreco.2018.10.034Get rights and content

Highlights

  • We evaluated five different Btk spraying scenarios to protect host trees from spruce budworm.

  • We hypothesized that less frequent Btk application could protect trees at a lower cost.

  • Black spruce maintained at least 54% of its residual photosynthetic capacity (RPC), even without protection.

  • Btk application every 2 years seems to be a relevant alternative to protect balsam fir and white spruce.

  • This scenario kept RPC above 39% and required 36% fewer applications over 7 years.

Abstract

Large-scale aerial spray operations against the spruce budworm (Choristoneura fumiferana (Clemens)) (SBW) with the biological insecticide Bacillus thuringiensis ssp. kurstaki (Btk) aim at maintaining trees alive during outbreaks. This objective is thought to be achieved when ≥ 50% of current-year foliage is preserved until the end of the outbreak. This protection target is associated with low balsam fir (Abies balsamea [L.] Mill.) mortality. However, it is unknown whether this approach is always needed or whether less frequent interventions could provide similar results at a lower cost. Between 2010 and 2016, we conducted field experiments in Quebec’s Côte-Nord region to determine the efficacy of five different Btk spraying scenarios for protecting balsam fir, white spruce (Picea glauca [Moench] Voss) and black spruce (P. mariana [Mill.] BSP) in mixed stands. We used the residual photosynthetic capacity (RPC) to evaluate the efficacy of the five scenarios. RPC makes it possible to take into account the impact of SBW defoliation on foliage contribution to tree photosynthetic effort over several years, and can be used as a proxy of the risk of tree mortality. We hypothesized that less frequent Btk applications could maintain the required RPC level to keep trees alive. Our results show that areas not protected resulted in great losses of RPC in balsam fir and white spruce. Btk applications every 3 years kept RPC above 50% for 2 years in balsam fir and 4 years in white spruce. RPC losses were above 62% after 4 years in both species. The strategy currently employed in Quebec (spraying every year after a first year of moderate-severe defoliation) and the intensive protection scenario (Btk applications every year) meet the protection goals for these hosts. However, their cost prevents their application at a large scale. Btk applications every 2 years seems a relevant alternative to the current strategy to protect balsam fir and white spruce stands given the adequate level of protection provided (RPC above 39%) and the reduction in the number of Btk applications required (36% fewer applications over 7 years, resulting in 36% lower cost), particularly if the objective is to maintain trees alive. Black spruce maintained at least 54% of its RPC, even without protection. Btk applications every 3 years might be a valid alternative to reduce growth losses in black spruce-dominated stands. The use of different spraying scenarios may allow us to develop cost-efficient treatment strategies to protect Quebec’s forests.

Introduction

The soil-borne bacterium Bacillus thuringiensis ssp. kurstaki (Btk) produces a proteinaceous crystal that is specifically toxic to Lepidoptera. When ingested, this bacterium provokes gut damage and larvae die within a few hours or days from septicaemia (Höfter and Whiteley, 1989). Commercial formulations of Btk are used around the world to control damage caused by forest pest insects (van Frankenhuyzen et al., 2007, van Frankenhuyzen et al., 2016, Hajek and van Frankenhuyzen, 2017). For example, the use of Btk to control various defoliators such as the pine processionary moth (Thaumetopea pityocampa (Denis and Schiff.)), nun moth (Lymantria monacha (Linnaeus)), Siberian moth (Dendrolimus superans sibiricus (Tschetv.)), and pine looper (Bupalus piniara (Linnaeus)) has grown steadily since the mid-1980s in Europe (Hajek and van Frankenhuyzen, 2017). In Canada, Btk is the most used agent for controlling damage by forest defoliators. This microbial insecticide was sprayed over about 10 million ha of forests affected mainly by spruce budworm (Choristoneura fumiferana (Clemens)), gypsy moth (Lymantria dispar (Linnaeus)) and hemlock looper (Lambdina fiscellaria fiscellaria (Guenée)) between 1985 and 2012 (van Frankenhuyzen et al., 2016).

Protection programs must not only take into consideration the target insect’s vulnerability to Btk; abiotic and biotic factors that may affect the efficacy of spraying operations (topography, canopy structure, weather conditions, host vulnerability) should also be considered because those factors may affect the timing of applications, application frequency and dose rate (van Frankenhuyzen et al., 2007). Indeed, plant-mediated effects may reduce the efficacy of Btk formulations (Kouassi et al., 2001, Bauce et al., 2006), which may increase the dose rate or the number of spray applications needed to achieve the protection objective in certain host species (Kouassi et al., 2001). Climatic conditions, in turn, may affect insect-host relationships (phenological synchrony, insect development, etc.) thereby affecting the achievement of protection objectives (van Frankenhuyzen et al., 2007). Furthermore, a spray program whose main objective is to eradicate an invasive pest species needs intensive treatments as opposed to a control program aimed at limiting annual defoliation. For example, the eradication program conducted in New Zealand applied a high-potency Btk product (65 BIU in 5L per ha) at weekly intervals during 9 weeks in order to eliminate the population of invasive defoliators (Glare, 2009). In contrast, limiting annual defoliation by gypsy moth to less than 60% in eastern USA can be achieved by doing one or two applications of Btk at 50-90 BIU per ha, targeting early instars (van Frankenhuyzen et al., 2007).

Spruce budworm is the most destructive forest pest in eastern North America. Periodic outbreaks of spruce budworm induce a substantial decline in vigour and increased mortality in affected trees (MacLean, 1980, MacLean, 2016). Budworm hosts include balsam fir (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), red spruce (P. rubens Sarg.), and black spruce (P. mariana [Mill.] BSP), balsam fir being the most vulnerable host tree to spruce budworm defoliation (Hennigar et al., 2008). Each year of defoliation on the aforementioned conifers has a persistent effect on the tree foliage reservoir (Clark, 1961, Fraser and McGuire, 1969). Current-year foliage contributes to 35% of the photosynthetic capacity in balsam fir (Clark, 1961) compared to 42 and 16% for white spruce and black spruce, respectively (Hom and Oechel, 1983, Hébert et al., 2011). Consequently, the continuous loss of foliage (photosynthetic capacity) through defoliation has a negative impact on budworm hosts that first results in reductions in growth and, eventually, in the death of the attacked individual (MacLean, 1980, MacLean, 2016). Tree mortality generally begins after 5 years of severe defoliation in balsam fir (Blais, 1958) and after 6 to 7 years in white spruce (Blais, 1981). In contrast, mortality of black spruce trees is normally restricted to overmature trees and suppressed saplings (Lussier et al., 2002).

The main goal of spruce budworm control programs in eastern Canada is to protect the trees’ current-year foliage (e.g. 50% in Quebec, 60% in New Brunswick) in order to ensure its survival and limit wood losses during outbreaks. In Quebec, the aerial Btk protection strategy currently implemented as part of the spruce budworm control program is to spray every single year after 1 year of moderate or severe defoliation to maintain current-year defoliation under the 50% threshold. This foliage protection target is based on the study of Hardy and Dorais (1976), which used the estimated contribution made by the various ages of foliage to the total photosynthetic capacity of balsam fir reported by Clark (1961) to evaluate the capacity of balsam fir to recover after spruce budworm defoliation. Their results suggest low balsam fir mortality when 38 to 51% of its photosynthetic capacity is maintained by the end of the outbreak. Accordingly, efforts to protect at least 50% of current-year foliage aim to maintain photosynthetic capacity above the 50% threshold to reduce the risk of tree mortality. However, it is still unknown if this approach is justified in all situations given the differences in resistance observed among host species (e.g. Blais, 1981, Blais, 1983, Hennigar et al., 2008, Fuentealba et al., 2017). Consequently, we hypothezise that protecting 50% of photosynthetic capacity might be achieved with less frequent interventions at a lower cost, especially in less vulnerable mixed fir-spruce and pure spruce stands.

In 2007, we initiated a long-term study in which we compared five spraying scenarios (including the standard strategy currently used in Quebec, see Table 1), along a gradient of protection intensity, in a region affected by an incipient outbreak of the spruce budworm in Quebec. The long-term objective of this study is to evaluate and compare the efficacy of these scenarios in protecting Quebec’s forests in terms of their impacts on wood production (losses of wood fiber through tree mortality, growth reduction and wood quality), the cost of interventions, profitability of investments in direct protection, and effects on the forest carbon budget. Such complete assessment will be possible after the end of the outbreak. The short-term objective was to compare efficacy of these five spraying scenarios on the protection of overall photosynthetic capacity of three spruce budworm’s hosts: balsam fir, white spruce and black spruce. Using residual photosynthetic capacity (RPC) makes it possible to assess the impact of spruce budworm defoliation over multiple years. Indeed, the loss of foliage reduces the tree photosynthetic surface and, therefore, reduces the production of sugar and starch (Piene, 1980). These carbohydrates play important roles in tree metabolism, growth, defense, development of cold hardiness and survival (Pallardy, 2008). Consequently, the loss of overall photosynthetic capacity makes it possible to evaluate the capacity of trees to recover after defoliation (Clark, 1961, Hardy and Dorais, 1976) and can be used as a proxy to appraise the risk of tree mortality. The cost of each scenario is also compared in this paper.

Section snippets

Study area

The study area was located in Quebec’s Côte-Nord region (Fig. 1) where the climate is continental humid. Annual precipitation varies between 900 and 1300 mm, and mean annual temperature ranges from −1.5 °C to 2.5 °C (de Grandpré et al., 2009). The topography of this region is rugged, with high hills and deep valleys. Till is the main surface deposit, but fluvio-glacial deposit can be found at the bottom of broad valleys (de Grandpré et al., 2009). Forests in this region are dominated by balsam

Results

Defoliation level during the study period varied significantly between host species as well as among spraying scenarios and number of years after the first treatment (Table 3, Table 4) (Pseudo-R2 = 0.7802 considering fixed effects only and 0.8027 considering both fixed and random effects). Balsam fir was more affected by spruce budworm than white spruce (Fig. 2). In general, units assigned to intensive spraying scenarios sustained less defoliation, but it varied greatly through time (Fig. 2).

Discussion

The results of this study show that different Btk spraying application scenarios may be used to protect forests against spruce budworm defoliation, but the efficacy of these scenarios in protecting RPC varies according to host species and to the number of years after the first treatment. Balsam fir and white spruce lost a large portion of their RPC in unprotected stands (Fig. 3). Indeed, these host species sustained moderate to severe defoliation for at least 5 years in unprotected stands (Fig.

Conclusions

Our results show that the strategy currently employed in the province of Quebec (standard scenario) provides a good protection to balsam fir and spruce ssp. by keeping RPC above the 50% threshold that is associated with low tree mortality. However, its cost prevents its application at a large scale. Btk applications every 2 years seem to be a valid alternative to the current strategy to protect fir-spruce mixed stands given the adequate level of protection it provides to spruce species and

Acknowledgments

We thank the Board of Directors of the Société de protection des forêts contre les insectes et maladies (SOPFIM) for their sustained interest in the project. We also thank SOPFIM laboratory and field teams for all the work done over the years, two anonymous reviewers for their useful suggestions and Isabelle Lamarre from Natural Resources Canada for checking the language and editing the manuscript. We are also grateful to the Spray Efficacy Research Group International (SERG-I) members (USDA

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      This may partly explain the higher changes of C fluxes and stocks (i.e. NEP and volume) with and without treatment estimated by our TRIPLEX-Insect model for balsam fir than for any of the spruces under moderate and severe SBW outbreaks in our study areas. Fuentealba et al. (2019) also show that Btk foliage protection operations maintain foliage and keep white spruce and balsam fir alive whereas black spruce did well even in the absence of protection. Hennigar et al. (2008) showed that SBW population levels that produced 100% current year defoliation in balsam fir only resulted in an average of 28% defoliation of black spruce.

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