Defoliation Damage Research Articles

Presence of Quercus Suber Soft-Leaf Defoliators on Trees with Distinct Foliar Monoterpene Emission Profiles

The cork tree, Quercus suber L., is a characteristic species of the Dehesa agrosilvopastoral system, typical of western Spain. Defoliating insects are an important component of these ecosystems. This study assessed the presence and impact of defoliators feeding on Q. suber soft leaf tissue and their relationship with foliar monoterpene emission profiles. Samplings consisted of weekly tree beating (15 times per tree per sample) during the flight period of key species. We studied 26 cork trees with known profiles of foliar monoterpene emission (13 pinene and 13 limonene chemotypes). We identified a total of 272 larvae from 9 species. The main species were Catocala nymphagoga Esper (40.8%) and Periclista andrei Know (27.6%). Notably, 70.6% of larvae were found on trees with a pinene chemotype. The combined abundance of the four key defoliator species (C. nymphagoga L., P. andrei, Bena bicolorana L., and Cyclophora punctaria L.) was 62.7% lower on limonene-chemotype trees than pinene-chemotype trees. Significant differences were found in defoliation damage between leaves with distinct terpene emission profiles. These results suggest that both the abundance and damage caused by defoliators differ with trees’ emission profiles, and this may indicate differences in palatability and/or nutritional quality between Q. suber trees with distinct foliar monoterpene emission profiles.

The use of remote sensing indices derived from Sentinel 2 satellite images for the defoliation damage assessment of Lymantria dispar

Monitoring and assessing the phytosanitary status of forests is a crucial activity in forest protection field, and the use of satellite remote sensing has become increasingly prevalent in this regard. Satellite images offer valuable information in terms of time and space, particularly through the analysis of vegetation and biophysical indices. Within this context, the aim of the study was to explore the potential of satellite remote sensing methods to monitor and assess the extent of tree defoliation caused by the gypsy moth (Lymantria dispar). The research study was focused on a forest situated in the Oltenia region of Romania, which experienced a gradation period for the defoliator L. dispar between 2018 and 2020. To determine the extent of defoliation caused by L. dispar in 2020 and 2021, a rectangular network consisting of 70 one-hectare sample areas was implemented. Each sample area had a square shape with a side of 100 m. In each corner of the sample areas, sub-sample circles with a radius of 12.62 meters were established, in which visual observations regarding the defoliation levels were conducted. The study involved a comparison between thematic maps derived from ground observations and those based on specific remote sensing indices derived through the processing of Sentinel 2 satellite images, of the Ciuturica forest area during the years 2020 and 2021. The research study indicated that the canopy water content (CWC) index was the most suitable for assessing defoliation caused by L. dispar. Furthermore, the study highlighted that remote sensing methods could be very effective and easily applicable, complementing the field ground-based efforts in monitoring and assessing the impact of L. dispar infestations.

Tree species composition and management influence short-term resilience to defoliation by Lymantria dispar L. in oak forests

The frequency and intensity of natural disturbances such as infestations by forest insect pests are expected to increase under a scenario of climate change. Thus, tools for monitoring the impact of these disturbance events at large spatial scales are required, together with appropriate management practices for enhancing the ability of forest ecosystems to overcome affectations by insect pests. Here, we used the Enhanced Vegetation Index (EVI) combined with ‘ground truth’ measurements to assess the effects of tree species diversity and forest management on the resistance, recovery and short-term (two years) resilience in mixed oak forests after a severe Lymantria dispar L. defoliation episode. Our results showed that, although neither diversity nor management influenced forest resistance to defoliation, both factors interacted and influenced EVI recovery after the disturbance event. This interaction revealed the complex role tree diversity plays in increasing (in managed forests) or decreasing (in unmanaged forests) resilience to defoliation, thereby suggesting that tree species composition may be more relevant than tree species diversity per se. Indeed, in the field we observed a variety of species-specific degrees of defoliation (e.g. higher in Quercus ilex L.), as well as the preference of L. dispar adults for ovipositing in unmanaged areas and on particular trees (Arbutus unedo L.) that were barely defoliated. In addition, we also found significant and positive effects of tree size (dbh) on avoiding defoliation damage. We thus make three management recommendations for increasing the resilience of oak forests to L. dispar defoliation: i) the application of continuous-cover silviculture (30% thinning intensity in basal areas), ii) the retention of larger trees, and iii) the maintenance of a preferential combination of tree species in mixed stands (e.g. Q. suber and Q. pubescens).

Drivers and spread of non-native pests in forests: The case of Gonipterus platensis in Spanish Eucalyptus plantations

Plantations of Eucalyptus species have been widely used in Spain to meet the high demand for wood given their rapid growth and high wood production capacity. Defoliation induced by the invasive eucalypt weevil (Gonipterus platensis (Marelli)), however, has been causing significant economic damage to Spanish Eucalyptus spp. plantations since the 1990s. G. platensis is native to Tasmania, Australia, where populations are controlled by natural enemies including the egg parasitoid Anaphes nitens Girault. In this study, spatio-temporal Universal Kriging was applied to examine the dynamics of defoliation damage caused by G. platensis in Spanish Eucalyptus spp. plantations and to identify the main factors associated with the presence and spread of the pest. The data set combines the Spanish national plots belonging to the network of the European transnational survey of forest condition in Europe (ICP Forest Level I, 16 × 16 km grid) along with regional plots, measured using similar field protocols, in which Eucalyptus spp. are present. A total of 264 Eucalyptus plots were included in the study, G. platensis being present in 167 of these plots at some time during the observed period (2005–2020). Our results show that defoliation damage > 0% and defoliation damage > 5% caused by G. platensis increased over the period 2005–2010 and then decreased between 2010 and 2020. Defoliation damage > 15% incidence steadily decreased from 2005 to 2015, but showed an upturn in 2020. Stands belonging to the Atlantic region are more affected by this pest (76% of the Atlantic sampling plots affected versus just 4% of the Mediterranean plots). The species Eucalyptus globulus Labill. and monospecific stands, as well as spring precipitation of the current year were found to be positively associated with the incidence of G. platensis whereas the relationship with summer temperature of the previous year was negative. Finally, maps showing the degree of incidence over time have been produced to support decision-making for pest prevention and control. This study puts forward a methodology which allows the spread of this pest to be better understood and simulated, thus facilitating risk prevention.

Resistance to bacterial spot (Xanthomonas gardneri) on foliage and fruit of commercial processing tomato cultivars

Bacterial spot of tomato (Xanthomonas gardneri Šutić) is an economically important disease of processing tomatoes in Ontario, Canada, resulting in premature defoliation and fruit damage. Breeding efforts for host resistance focus on assessments of foliar health as opposed to fruit health but anecdotal reports from industry suggest a poor relationship between fruit and foliar resistance. To investigate this, nine commercial cultivars were inoculated at the vegetative (foliar experiment) or reproductive (fruit experiment) stages in replicated field experiments from 2016 to 2018. In the foliar experiment, the standardized area under the disease progress curve (sAUDPC) for defoliation was 51% to 54% higher for ‘TSH18’ than ‘H9706’, ‘Hypeel 696’, and ‘H3406’, but equivalent to ‘CC337’. Fruit disease incidence was 49% and 47% lower for ‘CC337’ than ‘TSH18’ and ‘H9706’, but equivalent to ‘H3406’ and ‘Hypeel 696’. Fruit disease severity was 63% and 60% lower for ‘CC337’ than ‘H9706’and ‘H3406’, respectively, but equivalent to ‘TSH18’ and ‘Hypeel 696’. However, in the fruit experiment, fruit disease incidence was equivalent among cultivars, while the disease severity index for ‘H9706’ (3.4) was higher than ‘Hypeel 696’ (0.7). Furthermore, rank correlation analysis between sAUDPC and fruit disease variables failed to meet the criteria for a significant and strong relationship (r ≥ 0.8 or ≤ −0.8 and P ≤ 0.05). Additional research is needed to better understand the mechanisms of fruit infection by X. gardneri. In the meantime, scientists should consider the limitations of assessing only foliar damage as an evaluation method for bacterial spot management tools in tomato.

Simulation of Fall Armyworm (Spodoptera frugiperda) Attacks and the Compensative Response of Quality Protein Maize (Zea mays, var. Mudishi-1 and Mudishi-3) in Southwestern DR Congo

In this paper, to assess the varietal ability to compensate defoliation damage caused by the fall armyworm, a trial was carried out at the INERA Mvuazi research center. The aim of this study was to determine the limit threshold of damage that could cause the significant loss of the harvest of the quality protein maize distributed in the Democratic Republic of the Congo. To do this, three factors including two varieties (Mudishi-1 and Mudishi-3) of quality protein maize, four rating damage and two growth stages were used into a factorial design with 3 replications. Simulation of damage caused by FAW consisted of cutting of blades for all visible leaves of plants. Damage rates simulating leaf destroying were 0%, 25%, 50%, 75% and 100%, occurring at two growth stages: stage V3 (2 to 4 weeks after emergence) and stage V7 (flowering stage). Results showed that damage factor combined with growth stage factor significantly influenced (p < 0.05) the yield component variables including yield losses, harvest rate, ear sizes, number of ears harvested and yield as well as the market quality of the ears, based on appearance quotation. Damage rate more than 50%, at all growth stages studied, caused grain losses upper than 50% regardless of variety. However, damage less than 50% at V3 stage resulted in grain losses under 10%. At the end of this study, we showed that the varieties of maize QPM (Mudishi-1 and Mudishi-3) would be able to compensate the damage caused by the FAW and achieve its yield, if the attacks damaging 25% of the leaves occur during V3.

Overall grazing tolerance index (overall GTI) is not an ideal predictor for describing a single-species tolerance to grazing.

Plants' pattern of compensatory growth is often used to intuitively estimate their grazing tolerance. However, this tolerance is sometimes measured by the overall grazing tolerance index (overall GTI), which assumes that tolerance is a multivariate linear function of various underlying mechanisms. Because the interaction among mechanisms is not independent, the grazing tolerance expression based on overall GTI may be inconsistent with that based on compensatory growth. Through a manipulative field experiment from 2007 to 2012, we measured the responses of 12 traits of Elymus nutans to clipping under different resource availabilities in an alpine meadow and explored the compensatory aboveground biomass and the overall GTI to assess the possible differences between the two expressions of tolerance. Our results showed that these two expressions of tolerance were completely opposite. The expression based on overall GTI was over‐compensatory and did not vary with clipping and resource availability, while the expression based on compensatory aboveground biomass was under‐compensatory and altered to over‐compensation after fertilization. The over‐expression of highly variable traits with extremely high negative mean GTI to defoliation damage, the influence of random errors contained in traits considered, and the doubling weight of functional redundant traits greatly inflated the overall GTI, which leads to the inconsistency of the two tolerance expressions. This inconsistency is also associated with the different determining mechanisms of the two tolerance expressions. Our data suggest that plants' grazing tolerance is not a multivariate linear function of traits or mechanisms that determine grazing tolerance; the overall GTI is only a measure of traits' variability to defoliation damage. Our findings highlight that the tolerance of E. nutans mainly depends on the response of traits with lower variability to defoliation, and the overall GTI is not an ideal predictor for describing a single‐species tolerance to grazing.

Impact of Abrostola asclepiadis combined with other stressors on invasive swallow-worts (Vincetoxicum species)

The moth Abrostola asclepiadis (Lepidoptera: Noctuidae) is being investigated for biological control of pale and black swallow-wort (Vincetoxicum rossicum and V. nigrum, respectively; Apocynaceae), European vines introduced into North America. Limited information existed on its potential impact on swallow-worts, and the combination of defoliation damage by A. asclepiadis and other stresses to the plants had not been studied. We conducted greenhouse studies of the impact of different frequencies (once or twice) and degrees of defoliation (50 or 100%-seedlings; one or two larvae per stem-mature plants) by larvae on seedlings and mature (flowering) plants of black and pale swallow-wort. Defoliation was combined with varying light levels (shaded or not) or competition with quackgrass (Elymus repens). Significant reductions in aboveground dry mass and seeds generally only occurred when plants were completely defoliated twice. Grass competition and, to a greater extent, shading reduced growth and reproduction of swallow-wort plants. However, no interactions between defoliation and the other stressors occurred, suggesting they would have an additive effect. Because A. asclepiadis is likely to be univoltine if released in North America, its impact appears to be limited even if severe defoliation is achieved. Therefore, A. asclepiadis should be given a lower priority for release.

Identifying critical life stage transitions for biological control of long‐lived perennial Vincetoxicum species

Abstract Demographic matrix modelling of invasive plant populations can be a powerful tool to identify life stage transitions for targeted disruption, in order to cause population decline. This approach can provide quantitative estimates of reductions in select vital rates needed to reduce population growth rate (λ) below 1 and can inform the prerelease selection of effective biological control agents, as in the case of two invasive Vincetoxicum species in North America. We parameterized a five life stage matrix model using vital rate data from six populations (field and forest) of Vincetoxicum nigrum and Vincetoxicum rossicum. Elasticity analyses were used to identify life stage transitions and associated vital rates for perturbation through the incorporation of per capita impact data of candidate agents. Several survival, growth and fecundity‐related transitions were identified that were mostly similar across species, habitats and locations, although two populations showed unique population dynamics. Reductions in associated vital rates needed to prevent population growth varied greatly among populations. Defoliation damage and predispersal seed predation can be effective against slower growing forest and field populations of Vincetoxicum species. A fly and bivoltine moth are recommended as priority agents. However, biological control with these same agents will be ineffective against other field populations as well as some forest populations if the intensity of damage is not severe. Synthesis and applications. Control of long‐lived perennial plants, such as species of Vincetoxicum, is projected to occur through disruption of some individual vital rates of survival, growth or fecundity if population increase is low to moderate (λ &lt; 1.6). However, without significant and simultaneous reductions in multiple lower level vital rates, faster growing populations will not be contained with biological control or other management tools.

How does synchrony with host plant affect the performance of an outbreaking insect defoliator?

Phenological mismatch has been proposed as a key mechanism by which climate change can increase the severity of insect outbreaks. Spruce budworm (Choristoneura fumiferana) is a serious defoliator of North American conifers that feeds on buds in the early spring. Black spruce (Picea mariana) has traditionally been considered a poor-quality host plant since its buds open later than those of the preferred host, balsam fir (Abies balsamea). We hypothesize that advancing black spruce budbreak phenology under a warmer climate would improve its phenological synchrony with budworm and hence increase both its suitability as a host plant and resulting defoliation damage. We evaluated the relationship between tree phenology and both budworm performance and tree defoliation by placing seven cohorts of budworm larvae on black spruce and balsam fir branches at different lags with tree budburst. Our results show that on both host plants, spruce budworm survival and pupal mass decrease sharply when budbreak occurs prior to larval emergence. By contrast, emergence before budbreak decreases survival, but does not negatively impact growth or reproductive output. We also document phytochemical changes that occur as needles mature and define a window of opportunity for the budworm. Finally, larvae that emerged in synchrony with budbreak had the greatest defoliating effect on black spruce. Our results suggest that in the event of advanced black spruce phenology due to climate warming, this host species will support better budworm survival and suffer increased defoliation.

Disruption of trophic interactions involving the heather beetle by atmospheric nitrogen deposition

Elevated nitrogen (N) deposition impacts the structure and functioning of heathland ecosystems across Europe. Calluna plants under high N-inputs are very sensitive to secondary stress factors, including defoliation attacks by the heather beetle. These attacks result in serious damage or death of Calluna, its rapid replacement by grasses, and the subsequent loss of heathland. We know very little about the mechanisms that control the populations and trigger outbreaks of the heather beetle, impeding proper management measures to mitigate the damage. We investigated the effects of N deposition on the relationships between the heather beetle, its host plant, and two arthropod predators at building (rejuvenated through fire) and mature heathlands. The study combines field manipulation experiments simulating a range of N deposition rates (0, 1, 2, 5 g N m−2 year−1 for 2 years, and 5.6 g N m−2 year−1 for 10 years), and food-choice laboratory experiments testing the preferences of adults and larvae of the heather beetle for N-treated Calluna plants, and the preferences of predators for larvae grown on plants with different N-content. The larvae of the heather beetle achieved the highest abundances after the long-term (10-year) addition of N at mature Calluna plots in the field. Contrary to the adults, the larvae foraged preferentially on the most N-rich Calluna shoots under laboratory conditions. Predators showed no aggregative numerical responses to the accumulation of heather beetle larvae at high N-input experimental plots. During the feeding trials, predators consumed a small number of larvae, both in total and per individual, and systematically avoided eating the larvae reared on high-N Calluna shoots. Our study showed that the most severe defoliation damage by the heather beetle is inflicted at the larval stage under prolonged availability of high-N inputs, and that arthropod predators might not act as effective regulators of the beetle's populations.

Aerial multispectral imaging for crop hail damage assessment in potato

Crop loss assessment after an event of hailstorm can be inaccurate, subjective, and time consuming with the conventional method. Low-altitude, high-resolution aerial imaging using an unmanned aerial vehicle can be utilized for rapid assessment of target crops in a large scale, which can potentially improve the evaluation procedure. The goal of this study was to evaluate the feasibility of rapid and accurate assessment of crop damage due to simulated hailstorms using aerial multispectral imaging. Field experiments were conducted during two seasons in two potato varieties (Russet Norkotah, Ranger Russet) with three levels of mechanical defoliation (33%, 66%, and 99%) at three growth stages (tuber initiation, early bulk, and late bulk). All defoliation treatments were compared to the non-treated control plots (0% defoliation). Aerial multispectral images were collected between 77 and 108days after planting (0–60days after damage). Vegetation indices such as green normalized difference vegetation index (GNDVI), normalized difference vegetation index, and soil-adjusted vegetation index were calculated from replicate plots of different treatments. Results from two seasons showed similar trends in GNDVI values, with maximum effect of hail damage observed in early bulk stages. The mean GNDVI value was significantly lower in crops with the severe damage (99% defoliation) than others upon hail damage at the early bulk stage, with imaging after 10days after damage. The difference in GNDVI for crops with 33–66% damage could be detected within 10days after damage, and crop regrowth after that time period removed the effects of defoliation. The 99% defoliation damage at the early bulk stage also affected the crop yield significantly. Correlation analysis between vegetation indices and yield data indicated a strong relationship (r=0.77–0.90) for damage at the early bulk stage than other stages.

Impact of the defoliating moth Hypena opulenta on invasive swallow-worts (Vincetoxicum species) under different light environments

Black and pale swallow-wort (Vincetoxicum nigrum and Vincetoxicum rossicum, Apocynaceae) are twining vines from Europe that have become invasive in eastern North America. Hypena opulenta (Lepidoptera: Erebidae), a defoliating moth from the Ukraine, has recently been released in Canada and a request for release in the United States is under review. Its potential multivoltinism, and hence repeated defoliation of swallow-worts, combined with low-light levels in an understory environment may be promising for controlling forest infestations of swallow-worts. We conducted a greenhouse study of the impact of larval defoliation on seedlings and mature (flowering) plants of black and pale swallow-wort at different frequencies (once or twice) and degrees of defoliation (50 or 100%-seedlings; three or six larvae per stem-mature plants). Defoliation occurred under two forest light-regimes: low-light conditions typical of deeply-shaded forest habitats and higher light levels representative of a more open canopy or forest edge. Plants grown under higher light conditions were generally larger and produced many more seeds than heavily-shaded plants. One round of complete defoliation by H. opulenta, but not necessarily two, generally reduced biomass and seed production for both species and life stages, except for root mass of mature plants. No mature plants died and little seedling mortality occurred apart from black swallow-wort that had been completely defoliated twice under low light. Pale and black swallow-wort seedlings and mature plants appear to be tolerant of moderate defoliation damage. Complete defoliation repeated within and across years may be needed for suppression of forest infestations of swallow-wort.

Delayed efficacy of Beauveria bassiana foliar spray applications against Colorado potato beetle: Impacts of number and timing of applications on larval and next-generation adult populations

Spray programs comprising multiple or single foliar applications of the fungal pathogen Beauveria bassiana strain GHA (Bb-GHA) made during morning (AM) vs. evening (PM) hours were tested against Colorado potato beetle Leptinotarsa decemlineata (CPB) in small research plots of potatoes over multiple field seasons, and efficacy against larval and first-generation adult populations was evaluated. The pathogen (formulated as BotaniGard WP®) was applied at the rate of 2.5×1013 conidia in a spray volume of 468–480Lperha. Modes of efficacy were investigated by collecting pre-pupal larvae and holding them in soil cages in the field and laboratory. Results revealed that larval control achievable with multiple-spray programs was poor regardless of environmental moisture conditions, but that emerging adult populations were reduced by an average of 80% (range 58–93%) relative to control populations. Programs comprising a single application of Bb targeted against late-instar larvae reduced adult populations by an average of 60% (range 36–75%). Overall mean differences in efficacy of PM vs. AM applications were not significant in terms of larval control and defoliation damage. Emergence of next-generation adults was significantly lower in the PM vs. AM treatments, but differences were small in terms of percent control. Average yields from the Bb-GHA multiple-spray programs were 18% greater than those from the controls; the mean 5% increase resulting from the Bb-GHA single-spray programs was not significant. Observations indicated that moderate to high rates of inoculation and infection were achieved under a broad range of environmental conditions in the crop canopy, but larvae persisted until they entered the soil to pupate. Most deaths due to mycosis occurred among pre-pupal larvae in their pupation cells in the soil. These results, in combination with similar findings of delayed efficacy reported by other researchers, indicate that foliar applications of Bb could be a significant component of an IPM program aimed at suppressing CPB populations on large farms or over large areas.

Monitoring of damage from cedar shoot moth Dichelia cedricola Diakonoff (Lep.: Tortricidae) by multi-temporal Landsat imagery

Abstract: In this study defoliation damage in Taurus cedar (Cedrus libani A. Rich) stands in Turkey (Isparta region) caused by cedar shoot moth (Dichelia cedricola Diakonoff - Lep.: Tortricidae) was examined using multi-temporal Landsat data. Undamaged, low-damaged and heavily-damaged areas were located by assessing the variation of the Normalized Difference Vegetation Index derived from satellite imagery. Threshold boundaries for different damage levels were defined using mean NDVI values obtained from sub-areas spanning over the whole range of NDVI values. The reliability of the classification based on damages was statistically tested by comparing mean annual ring widths measured on increment cores extracted from sample trees exposed to different damage levels. Significant differences were found in mean annual ring widths among different areas previously classified based on NDVI data. Mean annual ring width in 2001 (an outbreak year) was 1.64 mm for undamaged area, 1.04 mm for low-damaged area, and 0.54 mm for heavily-damaged area. These findings indicate that damage mapping and monitoring mass damage caused by insect defoliation in Taurus cedar stands can be performed remotely by using NDVI values and Landsat TM data.

Remote Monitoring of Forest Insect Defoliation -A Review-

Aim of study: This paper reviews the global research during the last 6 years (2007-2012) on the state, trends and potential of remote sensing for detecting, mapping and monitoring forest defoliation caused by insects.Area of study: The review covers research carried out within different countries in Europe and America.Main results: A nation or region wide monitoring system should be scaled in two levels, one using time-series with moderate to coarse resolutions, and the other with fine or high resolution. Thus, MODIS data is increasingly used for early warning detection, whereas Landsat data is predominant in defoliation damage research. Furthermore, ALS data currently stands as the more promising option for operative detection of defoliation.Vegetation indices based on infrared-medium/near-infrared ratios and on moisture content indicators are of great potential for mapping insect pest defoliation, although NDVI is the most widely used and tested.Research highlights: Among most promising methods for insect defoliation monitoring are Spectral Mixture Analysis, best suited for detection due to its sub-pixel recognition enhancing multispectral data, and use of logistic models as function of vegetation index change between two dates, recommended for predicting defoliation.Key words: vegetation damage; pest outbreak; spectral change detection.

Interactions between stand thinning, site quality and host tree species on spruce budworm biological performance and host tree resistance over a 6 year period after thinning

Forest management activities such as thinning have been proposed to reduce defoliation damage incurred by spruce budworm (Choristoneura fumiferana (Clem.)). Yet, information regarding thinning effects on tree and stand vulnerability to spruce budworm is equivocal. A better understanding of thinning with respect to host tree resistance and budworm performance could be useful to reduce its impacts while respecting forest ecological integrity. Our objectives were (1) to evaluate the effects of thinning and site drainage quality on balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss) and black spruce (P. mariana (Mill.) BSP) resistance to spruce budworm through time (6years) and (2) to produce a resistance classification model that could be used by forest managers to determine when and where to apply this treatment. Field-rearing experiments of spruce budworm were conducted, together with foliar chemical analyses, along a gradient of stand thinning intensity (0%, control; 25%, light; 40%, heavy) and drainage class (rapidly drained, class 2; mesic with seepage, class 3; subhygric, class 4; hydric, class 5). Budworm performance was followed throughout the insect’s life cycle, including its winter biology. Balsam fir resistance, unlike that of white and black spruce, was significantly reduced 1year after thinning. Fir response was related to increased defoliation linked to reduction of certain monoterpenes on mesic sites and to decreased foliage production on subhygric sites. On hydric sites, thinning increased fir resistance by increasing foliage production. We observed an opposite response 3years after treatment. Heavy thinning (40%) positively affected balsam fir and white spruce tolerance and, therefore, tree resistance, by increasing foliage production and the amount remaining after budworm feeding throughout the study, except 6years after thinning in white spruce growing on mesic sites. From the fourth year onward, black spruce resistance was positively affected by thinning. This response was observed despite thinning also having favoured insect performance (high female pupal mass), which resulted in greater foliage destruction in thinned stands. These findings suggest that thinning may be used to increase tree and stand resistance to spruce budworm. Our resistance classification system could be useful to forest managers in planning thinning that would reduce budworm impacts at the stand level.

Relationships between Pikonema alaskensis larval density and shoot growth and production in young black spruce

Six years of field experiments were carried out to establish relationships between the density of larval yellowheaded spruce sawfly, Pikonema alaskensis (Roh.) (Hymenoptera: Tenthredinidae), and current-year shoot growth and production in juvenile, open-grown black spruce, Picea mariana [Mill.] B.S.P. In manipulative sleeve-cage experiments, larval density explained 36–65% of defoliation on branches and 27–37% of variation in shoot length in the year following defoliation. The negative impact of larval feeding on shoot elongation increased with each year of herbivory, resulting in a nearly 31% reduction in tree height after 5years of severe (i.e., c. 70% or more) defoliation. Production of current-year shoots was not influenced until 2o years following the initial bout of larval feeding and generally declined each year thereafter. Although a small but variable number of dormant (i.e., epicormic) shoots were produced each year, there was no apparent association with defoliation intensity. In field surveys, egg, mid-instar, and late-instar larval density explained, respectively 8%, 18%, and 33% of variation in shoot length growth in the following year. Despite some branches and trees sustaining multiple years of severe defoliation, there were no instances of either terminal shoot mortality or top kill (i.e., upper stem and branch mortality). Our study provides essential insect density–defoliation and defoliation–damage relationships for P. alaskensis in black spruce that may aid in establishing the first economic injury level for this destructive insect pest in Atlantic Canada.

Fast replenishment of initial carbon stores after defoliation by the pine processionary moth and its relationship to the re-growth ability of trees

Defoliation by herbivores may alter the source:sink balance of trees leading to transient decreases in carbon (C) stores. When C stores are replenished concurrently with re-growth both processes may compete, store formation proceeding at the expenses of growth. However, the interactions between both processes are not fully understood. We investigated the effects of defoliation by the pine processionary moth (PPM, Thaumetopoea pityocampa Dennis and Schiff.) on the non-structural carbohydrate (NSC) and nitrogen (N) stores and the growth of Pinus nigra Arnold trees. Short-term effects were evaluated immediately after a PPM outbreak and at the end of the first growing season in trees suffering a range of defoliation damage. Long-term effects were explored by a 17-year-long PPM defoliation experiment, with 11 years of repeated defoliation treatments followed by 6 years of recovery. Defoliation by PPM was followed by transient NSC decreases, but trees were able to exceed initial NSC pools and compensate growth in just one growing season. Such recovery was linked to increased foliage N. Repeated severe defoliations decreased growth and survival of trees in the long-term, but trees increased starch allocation to stems. Defoliation led to an accumulation of C storage compounds in P. nigra trees irrespective of their ability to re-grow. In trees included in the short-term experiment, the accumulation of stores proceeded concurrently with re-growth. However, the repeated severe defoliations included in our long-term experiment impaired the growth of trees, surplus C being accumulated as stores. These results indicate that, growth declines in pines defoliated by PPM are not due to C (source) limitation but may respond to the reduced sink strength of growing meristems due to defoliation, and thus, a decrease in C allocation to growth.

Comparing modern and presettlement forest dynamics of a subboreal wilderness: Does spruce budworm enhance fire risk?

Insect disturbance is often thought to increase fire risk through enhanced fuel loadings, particularly in coniferous forest ecosystems. Yet insect disturbances also affect successional pathways and landscape structure that interact with fire disturbances (and vice-versa) over longer time scales. We applied a landscape succession and disturbance model (LANDIS-II) to evaluate the relative strength of interactions between spruce budworm (Choristoneura fumiferana) outbreaks and fire disturbances in the Boundary Waters Canoe Area (BWCA) in northern Minnesota (USA). Disturbance interactions were evaluated for two different scenarios: presettlement forests and fire regimes vs. contemporary forests and fire regimes. Forest composition under the contemporary scenario trended toward mixtures of deciduous species (primarily Betula papyrifera and Populus spp.) and shade-tolerant conifers (Picea mariana, Abies balsamea, Thuja occidentalis), with disturbances dominated by a combination of budworm defoliation and high-severity fires. The presettlement scenario retained comparatively more "big pines" (i.e., Pinus strobus, P. resinosa) and tamarack (L. laricina), and experienced less budworm disturbance and a comparatively less-severe fire regime. Spruce budworm disturbance decreased area burned and fire severity under both scenarios when averaged across the entire 300-year simulations. Contrary to past research, area burned and fire severity during outbreak decades were each similar to that observed in non-outbreak decades. Our analyses suggest budworm disturbances within forests of the BWCA have a comparatively weak effect on long-term forest composition due to a combination of characteristics. These include strict host specificity, fine-scaled patchiness created by defoliation damage, and advance regeneration of its primary host, balsam fir (A. balsamea) that allows its host to persist despite repeated disturbances. Understanding the nature of the three-way interaction between budworm, fire, and composition has important ramifications for both fire mitigation strategies and ecosystem restoration initiatives. We conclude that budworm disturbance can partially mitigate long-term future fire risk by periodically reducing live ladder fuel within the mixed forest types of the BWCA but will do little to reverse the compositional trends caused in part by reduced fire rotations.