Protection Strategy against Spruce Budworm

Spruce budworm is one of the most significant forest insects worldwide, in terms of outbreak extent, severity, and economic impacts. As a defoliator, spruce budworm larvae are susceptible to insecticide protection, and improvements in efficacy and reductions in non-target environmental effects have made such protection attractive. In this Special Issue, 12 papers describe the advances in spruce budworm protection, most notably an ‘early intervention strategy’ approach that after six years of trials in New Brunswick, Canada, shows considerable success to date in reducing budworm outbreak occurrence and severity.


Introduction
Spruce budworm (Choristoneura fumiferana (Clem.))outbreaks are a dominant natural disturbance in forests of Canada and northeastern USA.The last major spruce budworm outbreak in eastern Canada in the 1970s-1980s peaked at 52 million hectares in 1975 [1,2].Widespread, severe defoliation by this native insect results in large-scale mortality and growth reductions of spruce (Picea sp.) and balsam fir (Abies balsamea (L.) Mill.) forests, and largely determines future age-class structure and productivity.Repeated annual defoliation typically lasts about 10 years during outbreaks, resulting in growth reductions up to 90% [3], mortality averaging 85% in mature balsam fir stands [4], and changes in regeneration and succession [5,6].Spruce budworm outbreaks also cause substantial losses in timber and economic production [7,8] and increase the risk of forest fire [9,10].Several papers have discussed spruce budworm population dynamics during outbreaks [11][12][13], tree mortality [4], and effects on stand development and ecosystem functioning [14,15].
The province of Nova Scotia, Canada decided not to protect forests with insecticide treatments during the severe 1970s-1980s spruce budworm outbreak, and suffered an average of 87% mortality in mature balsam fir stands [16].Mortality on Cape Breton Island, Nova Scotia covered 629,900 ha, reduced the growing stock of spruce and fir by 70% or 21.5 million m 3 [17], and increased the hardwood covertype from 16% to 36% [18].In total, spruce budworm defoliation during eastern Canada's last major outbreak caused timber losses estimated at 107 million m 3 year −1 from 1977-1981 and 81 million m 3 year −1 from 1982-1987 [19,20].To put these amounts in perspective, they were equivalent to 50%-70% of the total 156 million m 3 timber harvested in Canada in 2016 [21].
Management to deal with spruce budworm outbreaks has emphasized forest protection by spraying registered insecticides to prevent defoliation and keep trees alive [7].Other tactics can include salvage harvesting, altering harvest schedules to remove the most susceptible stands, or reducing future susceptibility by planting or thinning [7].Chemical insecticides are no longer used, and protection strategies use the biological insecticides Bacillus thuringiensis (B.t.) or tebufenozide, an insect-specific growth regulator.To prevent extensive tree mortality caused by spruce budworm defoliation, from 1970 to 1983, the eastern Canadian province of New Brunswick treated an average of two million hectares of forest per year with insecticide, at an average cost of $7.7 million per year [22].In comparison, it is estimated that a similar forest insecticide protection program covering two million hectares today would cost between $90 and $160 million per year, due to increased insecticide active ingredient and application costs [22].Without insecticide protection, timber harvest reductions are estimated at 18%-25% [7], equivalent to a reduction in timber supply of 2.4-3.3 million m 3 year −1 in the Atlantic Canada region [23].The direct and indirect economic losses resulting from an Atlantic Canada region outbreak would be $10.8-$15.3 billion CAD, depending on outbreak severity [8,23].Regional job losses over 30 years could total 46,000-56,000 person-years, or approximately 1500-1900 jobs per year [24].This analysis underestimates job losses during periods of temporary mill closures or in communities where mills could permanently close due to a lack of timber supply.
A large-scale spruce budworm outbreak would also have massive carbon sequestration and greenhouse gas implications [25].The total potential wood supply loss from a future spruce budworm outbreak in Atlantic Canada projected over 30 years is estimated at 96 million m 3 , which would generate approximately 66 Mt CO 2 emissions [26].On an annual basis, the emissions from dead and dying trees would be on average 2.21 MT CO 2 e, equal to the emissions of an additional 466,000 passenger vehicles [26].
In addition to the compelling economic case for forest protection intervention against spruce budworm outbreaks, there is also considerable public support, as documented in a 2007 public survey [27], which found that 94% of New Brunswick respondents supported funding research and development on pest control, and 82% supported controlling future spruce budworm outbreaks.
Over the last five years, a $30 million research project has tested another possible management tactic, termed an early intervention strategy, aimed at area-wide management of spruce budworm populations [28].This includes intensive monitoring to detect 'hotspots' of rising budworm populations before defoliation occurs, targeted insecticide treatment to prevent spread, and detailed research into effects on target and non-target insects [28,29].

Description of Papers in This Special Issue
The objective of this Protection Strategy against Spruce Budworm Special Issue of Forests was to compile recent research on protection strategies and related topics about detection, monitoring, impacts, population dynamics, and integrated pest management of spruce budworm.The issue includes 12 papers that describe the results and prospects for the use of an early intervention strategy in spruce budworm and other insect management, as well as related topics.A brief description of the content and main findings of the 12 papers in this Protection Strategy against Spruce Budworm Special Issue is as follows.
The first six papers are all directly related to the application and testing of an early intervention strategy: 1 Johns et al. [28] described a conceptual framework for an early intervention strategy against spruce budworm, including all of the core components needed for such a program to be viable.
Early intervention and foliage protection strategies against spruce budworm are not necessarily mutually exclusive and core elements are relevant to population control for other insect pests that show hotspot outbreak dynamics [28].Components required for a spruce budworm protection program to be successful include hotspot monitoring, population control, cost-benefit analyses, and proactive communications with stakeholders [28]. 2 MacLean et al. [29] reported positive results after five years of early intervention strategy trials conducted by a consortium of government, forest industry, researchers, and other partners.Following over 420,000 ha of treatments of low but increasing spruce budworm populations, second instar larvae (L2) levels across northern New Brunswick, Canada were considerably lower than populations in adjacent Québec [29].Blocks treated with Bacillus thuringiensis or tebufenozide insecticide consistently had reduced budworm levels, generally did not require treatment in the subsequent year, and areas with moderate or higher L2 populations declined by over 90% reductions in 2018, while they continued to increase in Québec.3 Liu et al. [30] investigated the potential economic impacts of future spruce budworm outbreaks on 2.8 million ha of Crown land in New Brunswick and compared early intervention and foliage protection approaches.They found that timber harvest supply from 2017 to 2067 was projected to be reduced by 29 to 43 million m 3 by uncontrolled moderate or severe budworm outbreaks, which would reduce total economic output by $25 billion (CAD) to $35 billion [30].Depending upon outbreak severity, the early intervention strategy was projected to have benefit/cost ratios of 3.8 to 6.4 and net present values of $186 million to $353 million, both higher than foliage protection strategies [30].4 Régnière et al. [31] reported on detailed observations of the dynamics of low but rising spruce budworm populations, the target for early intervention.Results showed strong density-dependent survival between early larval stages and adult emergence, explained by natural enemy impacts and overcrowding, and inverse density-dependence of apparent fecundity, with a net immigration into lower-density populations and net emigration from higher populations at a threshold of about 25% defoliation [31].This supported the conclusion that immigration, to elevate budworm above a threshold density of about four L4 larvae branch −1 was required for a population to increase to outbreak density [31], which helps set a target treatment density.5 Régnière and Nealis [32] found strong evidence of density-dependent emigration in both eastern and western spruce budworms, and concluded that migration was not random, but was density-dependent.6 Zhang et al. [33] tested the influence of a gradient of balsam fir-hardwood species composition on the defoliation of fir during the first five years of a spruce budworm outbreak.Fir defoliation was significantly lower as hardwood content increased, but the relationship varied with overall defoliation severity each year [33].Results helped to set a fir-hardwood threshold below which insecticide protection is not used.
Four papers were related to specific aspects of spruce budworm management: 7 Li et al. [34] used spatial autocorrelation analyses to determine patterns of spruce budworm defoliation of trees (clustered, dispersed, or random) and plots.About one-quarter to one-half of plots had significantly clustered defoliation, and data on plot-level defoliation and tree basal area were sufficient for modeling individual tree defoliation [34].8 Rahimzadeh-Bajgiran et al. [35] assessed the use of Landsat-5 and Landsat-MSS data to detect and map spruce budworm defoliation.A combination of three vegetation indices derived from Landsat data were able to detect and classify defoliation in three classes with an accuracy of 52%-77%.9,10 Régnière et al. [36] described the effects of temperature constraints in an individual-based model of spruce budworm moth migration that was parameterized with observations from moths captured in traps or observed migrating under field conditions.A related paper [37] incorporated crepuscular (evening) circadian rhythms of moth flight activity as influenced by evening temperatures into the model.Given the importance of density-dependent emigration [32] and the requirement for moth immigration to elevate budworm above a threshold for outbreak initiation [31], methods to model and map moth flights are important for budworm monitoring for early intervention.
The final two papers dealt with elements of integrated management of spruce budworm: 11 Régnière et al. [38] reported results of trials of aerial applications of a registered formulation of synthetic spruce budworm female sex pheromone to disrupt mating in populations.Although the pheromone application reduced the capture of male budworm moths in pheromone-baited traps by 90% and reduced mating success of virgin females held in individual cages at mid-crown, results showed that populations of eggs or overwintering larvae in the following generation were not reduced, possibly because of the immigration of mated females [38].12 Quiring et al. [39] tested the influence of a foliar endophyte and budburst phenology on budworm survival.Survival of budworm larvae to pupation and to adult emergence was 13%-17% lower on endophyte positive trees, suggesting that endopytes inoculated into spruce seedlings could limit the spruce budworm population as part of an early intervention strategy [39].

Conclusions
Collectively, the 12 papers comprising the Protection Strategy Against Spruce Budworm Special Issue of Forests describe a promising new method to reduce the occurrence or severity of defoliation in outbreaks.Early intervention strategy research continues in New Brunswick, and the most recent (autumn 2019) budworm L2 monitoring data show that populations remain at low levels (www.healthyforestpartnership.ca), while budworm populations in the adjacent province of Québec continued to increase in 2019 [40].So far, after six years of trials, the early intervention strategy appears to be working.