Tent Caterpillar Research Articles

Changes in Nesting Density of Baltimore Orioles (1976-1995) and Other Species in the Dune- Ridge Forest, Delta Marsh, MB: Response to an Outbreak of Forest Tent Caterpillar?

Changes in Nesting Density of Baltimore Orioles (1976-1995) and Other Species in the Dune- Ridge Forest, Delta Marsh, MB: Response to an Outbreak of Forest Tent Caterpillar?

Latitudinal variation in the phenological responses of eastern tent caterpillars and their egg parasitoids

1. Eastern tent caterpillars (Malacosoma americanum) are broadly distributed within North America, with populations spanning a wide range of environmental conditions. Their egg masses are consistently attacked by a variety of wasps in the superfamily Chalcidoidea. We performed a reciprocal transplant‐type experiment to assess the performance of three populations spanning 15° of latitude when subjected to temperature regimes resembling southern or northern conditions.2. Exposure to warm temperatures and short overwintering periods (southern conditions) resulted in the increased survival of both caterpillars and parasitoids from all populations. By contrast, the ability of caterpillars to withstand starvation was maximised when exposed to conditions similar to their native region.3. Caterpillar and wasp phenology differed among populations even when exposed to the same temperature regime. Individuals exposed to novel conditions hatched 2–6 weeks later than those experiencing native conditions. Under typical conditions, the relative phenology of wasps and their hosts exhibited a latitudinal gradient consistent with growing season length, with southern, central, and northern wasps, emerging 50, 45, and 36 days, respectively, after their hosts.3. We identified four genera of primary parasitoids, which emerged within a narrow 2‐week span, and one hyperparasitoid, which emerged in distinct pulses over an approximately 5‐week span, possibly indicating the presence of a second generation.4. Caterpillars and wasps exhibited distinct phenological responses according to population of origin, indicating that not only pre‐hatching winter and spring conditions, but also historical factors, which may include local adaptation, maternal effects, and oviposition time, influence their phenological responses.

Drought and surface‐level solar radiation predict the severity of outbreaks of a widespread defoliating insect

AbstractDefoliating insects are a major factor impacting tree growth in temperate and boreal forests, but the effects of climate change on the severity and frequency of outbreaks of these insects are not well understood. Dendrochronological reconstruction of forest tent caterpillar (Malacosoma disstria) outbreaks on trembling aspen (Populus tremuloides) in southwestern Manitoba, Canada, from 1851 to 2010 was used to examine the effects of changes in temperature, incident surface‐level solar radiation, and drought severity on the cyclicity, frequency, and severity of outbreaks. Outbreak severity was determined by comparing the tree‐ring widths of trembling aspen with those of white spruce (Picea glauca), which is not a forest tent caterpillar host. Wavelet analysis suggested the cyclicity of outbreaks increased after 1930. From 1930 to 2010, the dominant period length of outbreak cycles steadily increased over time from three to seven years, coinciding with long‐term declines in drought severity. The strength of forest tent caterpillar suppression of trembling aspen growth in a given year was inversely related to incident solar radiation and drought severity during late spring (the larval feeding period) of the same year. The effect of late spring drought on the impact of forest tent caterpillar on ring growth differed from that of summer drought. There was a nonlinear relationship between growth suppression in a given year and summer drought severity in the previous year; slightly above‐average drought severity was associated with reduced impacts of the defoliator, whereas severe drought was associated with intense growth suppression. Strong suppression of aspen growth observed during outbreaks from 1983 to 2010 may be partly explained by observed long‐term trends of decreasing atmospheric transmittance of solar radiation (a phenomenon termed “global dimming”) and increasing soil moisture availability during late spring. However, as the severity and frequency of summer droughts are expected to increase over the 21st century, the nonlinear relationship between summer drought severity and growth suppression the following year suggests a future in which summer droughts precede severe outbreaks that threaten the health of boreal forest hardwood trees.

Host-plant mediated effects on group cohesion and mobility in a nomadic gregarious caterpillar

Gregarious animals often live in larger groups in food-rich environments. We compare cohesion and mobility of forest tent caterpillar (Malacosoma disstria) groups on high-quality (trembling aspen, Populus tremuloides) and low-quality (sugar maple, Acer saccharum) host trees. Following pheromone trails allows forest tent caterpillars to maintain group cohesion, but can cause groups to become trapped on poor-quality food sources. Trail-following decreases in food-deprived individuals. We combine laboratory and field experiments to show that this nutritionally mediated balance between trail fidelity and exploration generates fission-fusion dynamics, leading to increased mobility and smaller group sizes on poorer quality hosts. Indeed, caterpillar groups feeding on maple spent more time in locomotion, split into smaller groups, switched feeding site more frequently and traveled farther when compared to those feeding on aspen. These fission-fusion dynamics can be explained as an emergent property of individual responses to conspecifics: Individuals drift apart in contexts where responses to cues from conspecifics are weakened. Increased mobility appears as an adaptive response that improves the chances of relocating to a superior food source, but lower group size likely decreases thermoregulation and increases predation. Finally, we show that forest tent caterpillar fission-fusion dynamics not only control the spatial distribution of individuals in a tree, but they are also linked to considerable differences in growth and mortality and therefore can play an important role in population dynamics. Many gregarious animals split up into smaller groups when food is scarce. In many species, the behaviors by which group members stay together and follow each other are well-understood; how then do these behaviors respond to changing conditions? In particular, how does hunger drive groups to split up in response to decreased food availability? We compare mobility and group size of forest tent caterpillar colonies on good- and poor-quality host plants. These caterpillars use pheromone trails to move around together, but when hungry, they tend to leave these trails. We show how caterpillars on a poor-quality host plant are not only more mobile (presumably an adaptive response that improves chances of discovering a better food source), but they also split into smaller groups—a side effect of decreased trail fidelity that possibly exposes them to greater risks.

Evening Grosbeaks Feeding on Pupae of the Forest Tent Caterpillar in the Dune- Ridge Forest, Delta Marsh, Manitoba, Spring 1976

Evening Grosbeaks Feeding on Pupae of the Forest Tent Caterpillar in the Dune- Ridge Forest, Delta Marsh, Manitoba, Spring 1976

Effects of forest tent caterpillar defoliation on carbon and water fluxes in a boreal aspen stand

Insect outbreaks can significantly influence carbon (C) and water balances of forests. Forest tent caterpillars (FTC) (Malacosoma disstria Hübner) are one of the most prominent insects found in aspen forests in Canada and have the potential to considerably influence regional C and water fluxes. In the summer of 2016, an FTC infestation occurred in a ca. 100 -year-old trembling aspen (Populus tremuloides) stand in the southern boreal forest where the longterm research site known as Old Aspen (OA) is located. The infestation led to nearly complete defoliation of the canopy during the leafing out period when photosynthesis, and thus C uptake, is progressing towards maximum levels. We used 21 years of eddy-covariance (EC) and climate measurements covering pre-infestation and infestation periods to estimate the impact of the FTC infestation on net ecosystem production (NEP), gross ecosystem production (GEP) and evapotranspiration (E). Defoliation in 2016 reduced annual NEP to −130 g C m−2 y−1 and GEP to 798 g C m−2 y−1, respectively, which were much less than their 20-year means (NEP = 118 ± 53 g C m−2 y−1, GEP = 1057 ± 74 g C m−2 y−1), and resulted in the most negative annual NEP value of the 21 years of measurements at the OA site. NEP for 2016 was even lower than values observed during three drought years (2001–2003). However, FTC infestation caused little effect on annual E. FTC infestation reduced the near-surface remotely-measured greenness index, green chromatic coordinate (GCC), to ∼0.32 on June 10 in comparison to ∼0.40 in other years. The defoliation, observable from space as reductions in normalized difference vegetation index (NDVI) values, also showed a negligible effect on E but a large effect on the C fluxes.

Time-lagged intraspecific competition in temporally separated cohorts of a generalist insect.

Competition can have far-reaching consequences for insect fitness and dispersion. Time-lagged interspecific competition is known to negatively affect fitness, yet time-lagged intraspecific competition is rarely studied outside of outbreak conditions. We tested the impact of competition between larval cohorts of the western tent caterpillar (Malacosoma californicum) feeding on chokecherry (Prunus virginiana). We reared larvae on host plants that either had or did not have feeding damage from tent caterpillars the previous season to test the bottom-up fitness effects of intraspecific competition. We measured host-plant quality to test potential mechanisms for bottom-up effects and conducted field oviposition surveys to determine if female adult tent caterpillars avoided host plants with evidence of prior tent caterpillar presence. We found that time-lagged intraspecific competition impacted tent caterpillar fitness by reducing female pupal mass, which is a predictor of lifetime fitness. We found that plants that had been fed upon by tent caterpillars the previous season had leaves that were significantly tougher than plants that had not been fed upon by tent caterpillars, which may explain why female tent caterpillars suffered reduced fitness on these plants. Finally, we found that there were fewer tent caterpillar egg masses on plants that had tent caterpillars earlier in the season than plants without tent caterpillars, which suggests that adult females avoid these plants for oviposition. Our results confirm that intraspecific competition occurs among tent caterpillars and suggests that time-lagged intraspecific competition has been overlooked as an important component of insect fitness.

Latitudinal variation in responses of a forest herbivore and its egg parasitoids to experimental warming.

Disrupted biotic interactions are a predicted consequence of anthropogenic climate change when interactants differ in the magnitude or direction of phenological responses. Here, we examined the responses to artificial warming of northern, southern and central populations of the eastern tent caterpillar and its hymenopteran egg parasitoids. We subjected egg masses from each region to the typical conditions they experience in their source locality or to a warmer temperature regime, to quantify the effects of simulated warming on their relative phenology, survival and neonate starvation endurance. In addition, we characterized spring heat accumulation and cloud cover at each collection site using 30years of hourly weather station data. As predicted, degree-day accumulation rates decreased with latitude; however, the mid-latitude site experienced what we predict to be the harshest spring conditions for tent caterpillars: slow heat accumulation combined with thick cloud cover. Remarkably, caterpillars from this site exhibited the largest phenological plasticity, hatching a month earlier under warmer than under typical conditions and doubling caterpillar survival. Survival of caterpillars from all regions was enhanced at warmer temperatures, whereas parasitoid survival was unaffected. The starvation endurance of hatchlings increased under warmer conditions in the central and southern populations only. We show that phenological responses to warming differed between hosts and parasitoids, resulting in a 5-day reduction in the relative phenology of wasps and caterpillars in the northern population. Our findings caution that responses to global warming are likely to be population or region specific and cannot be readily generalized, particularly for wide-ranging organisms.

Chlorantraniliprole: Reduced-risk Insecticide for Controlling Insect Pests of Woody Ornamentals with Low Hazard to Bees

Landscape professionals need target-selective insecticides for managing insect pests on flowering woody ornamentals that may be visited by bees and other insect pollinators. Chlorantraniliprole, the first anthranilic diamide insecticide registered for use in urban landscapes, selectively targets the receptors that regulate the flow of calcium to control muscle contraction in caterpillars, plant-feeding beetles, and certain other phytophagous insects. Designated a reduced- risk pesticide by the United States Environmental Protection Agency, it has a favorable toxicological and environmental profile, including very low toxicity to bees and most types of predatory and parasitic insects that contribute to pest suppression. Chlorantraniliprole has become a mainstay for managing turfgrass pests, but little has been published concerning its performance against the pests of woody ornamentals. Researchers evaluated it against pests spanning five different orders: adult Japanese beetles, evergreen bagworm, eastern tent caterpillar, bristly roseslug sawfly, hawthorn lace bug, oleander aphid, boxwood psyllid, oak lecanium scale (crawlers), and boxwood leafminer, using real-world exposure scenarios. Chlorantraniliprole’s efficacy, speed of control, and residual activity as a foliar spray for the leaf-chewing pests was as good, or better, than provided by industry standards, but sprays were ineffective against the sucking pests (lace bugs, aphids, or scales). Basal soil drenches in autumn or spring failed to systemically control boxwood psyllids or leafminers, but autumn drenches did suppress roseslug damage and Japanese beetle feeding the following year. This study indicates that chlorantraniliprole can be an effective component of integrated pest and pollinator management programs on woody ornamentals.

DRINO IMBERBIS (WIEDEMANN) (DIPTERA: TACHINIDAE), A PARASITOID OF MALACOSOMA DISSTRIA HUBNER (LEPIDOPTERA: LASIOCAMPIDAE) CATERPILLARS IN IRAN

Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae) is a widespread generalist defoliator of a wide variety of plants in Iran. This study was conducted to identify the indigenous parasitoids of the forest tent caterpillar in the Borazjan region of the Bushehr province, Iran. Samplings were conducted in different sites within a single cabbage field heavily infested with M. disstria . The forest tent caterpillars were reared in the laboratory until the emergence of parasitoids. A parasitoid species from the family Tachinidae was found, reared, and identified as Drino imberbis (Wiedemann). Malacosoma disstria is a new host record for D. imberbis in Iran.

Phylloplane bacteria increase the negative impact of food limitation on insect fitness

1. When populations of herbivorous insects increase in density, they can alter the quantity or quality of their food. The impacts of diet‐related stressors on insect fitness have been investigated singly, but not simultaneously.2. Foliage quantity and quality of red alder, Alnus rubra, were manipulated together with the presence of non‐entomopathogenic phylloplane bacteria to investigate their impacts, singly and in combination, on survival, pupal mass, growth rate, fecundity and egg quality of a cyclic forest insect, the western tent caterpillar, Malacosoma californicum pluviale.3. Food limitation (half food) had strong negative impacts on all life‐history traits. When the larvae were fed continuously, however, neither ingesting phylloplane bacteria nor eating leaves from damaged branches (induced foliage) affected survival. In the half‐food treatment, ingesting bacteria further increased mortality, while feeding on induced foliage improved survival.4. Growth rate and pupal mass of both sexes were reduced for larvae with food limitation compared with continuously fed insects and this was exacerbated when the larvae also ate bacteria‐treated leaves. A combination of bacteria and induced foliage also reduced larval growth rate by 5% in the full‐food treatment.5. Fecundity (eggs per egg mass) was 2.7 times greater in full‐food than in food‐limited treatments but neither phylloplane bacteria nor plant induction had an effect. Insects fed induced foliage produced smaller eggs. Overall, there was no evidence of a three‐way interaction between the three stressors, although there were negative synergistic effects, primarily between food limitation and the ingestion of phylloplane bacteria.

Double allomimesis of advancing and retreating individuals maintains cohesion in exploring groups of nomadic caterpillars

Group-living entails that animals maintain cohesion during collective locomotion: This cohesion often requires that each individual respond to cues from several neighbours. Social insects generally use pheromone trails to integrate information from multiple group members. We demonstrate mechanisms used by nomadic social caterpillars to maintain cohesion when exploring off-trail. Our results show that forest tent caterpillars (Malacosoma disstria) use double allomimesis, responding to cues from both advancing and retreating neighbours. A group-level experiment measured cohesion and persistence of locomotion according to caterpillar age and group size. An individual-level experiment quantified responses to cues from neighbours, and a model was used to predict the group-level patterns that emerge from these responses. We show that double allomimesis generates feedback that maintains group cohesion, but at the price of locomotion efficiency, particularly in small groups of young caterpillars. We also show that the importance of allomimesis decreases as caterpillars age and show lesser responses to social cues. Finally, we demonstrate that, especially in the early instars, efficient collective locomotion is only possible in the large group sizes typically of field colonies. Simple do-what-your-neighbour-is-doing mechanisms underlie mass collective locomotion of gregarious animals. Social insects generally use pheromone trails to follow each other. How then to stay together when exploring away from these trails? Forest tent caterpillars provide an intriguing example: They follow trails faithfully, but on unexplored territory turn back frequently to rejoin the trail behind them. We show how direct physical contact with advancing and retreating neighbours provides an explanation. An off-trail caterpillar turns back less often if it is propelled by a forward-moving neighbour, but more often if that neighbour turns back. This dual imitation keeps the group together, but at the price of efficient movement, especially in small groups. Groups as large as natural colonies seldom turn back due to the steady supply of advancing neighbours to propel forward and eventually replace vanguard individuals.

Performance of Western Tent Caterpillar (Malacosoma californicum) on Two Common Host Plants, Including a New Host Plant Record

ABSTRACT. Tent caterpillars are generalists across their full host range, but display local host plant preferences. We present evidence for a new host plant record, wax currant (Ribes cereum), for western tent caterpillars (Malacosoma californicum) along the Colorado Front Range. We tested the suitability of wax currant as a host plant for western tent caterpillars as compared to chokecherry (Prunus virginiana), an abundant and commonly used host plant. We measured the density of tent caterpillar tents in areas where both host plants occur to assess host plant use. We reared tent caterpillar larvae on both host plants and measured fitness effects due to host plant quality (survival, pupal mass) and natural enemies (parasitism). We did not find a relationship between host plant abundance and use by tent caterpillars and found no evidence for a preference for either host plant. We found that western tent caterpillars do not differ in pupal mass when reared on chokecherry and on wax currant in a laboratory s...

A Forest Tent Caterpillar Outbreak Increased Resource Levels and Seedling Growth in a Northern Hardwood Forest.

In closed-canopy forests, gap formation and closure are thought to be major drivers of forest dynamics. Crown defoliation by insects, however, may also influence understory resource levels and thus forest dynamics. We evaluate the effect of a forest tent caterpillar outbreak on understory light availability, soil nutrient levels and tree seedling height growth in six sites with contrasting levels of canopy defoliation in a hardwood forest in northern lower Michigan. We compared resource levels and seedling growth of six hardwood species before, during and in the three years after the outbreak (2008–2012). Canopy openness increased strongly during the forest tent caterpillar outbreak in the four moderately and severely defoliated sites, but not in lightly defoliated sites. Total inorganic soil nitrogen concentrations increased in response to the outbreak in moderately and severely defoliated sites. The increase in total inorganic soil nitrogen was driven by a strong increase in soil nitrate, and tended to become stronger with increasing site defoliation. Seedling height growth increased for all species in the moderately and severely defoliated sites, but not in lightly defoliated sites, either during the outbreak year or in the year after the outbreak. Growth increases did not become stronger with increasing site defoliation, but were strongest in a moderately defoliated site with high soil nutrient levels. Growth increases tended to be strongest for the shade intolerant species Fraxinus americana and Prunus serotina, and the shade tolerant species Ostrya virginiana. The strong growth response of F. americana and P. serotina suggests that recurring forest tent caterpillar outbreaks may facilitate the persistence of shade intolerant species in the understory in the absence of canopy gaps. Overall, our results suggest that recurrent canopy defoliation resulting from cyclical forest insect outbreaks may be an additional driver of dynamics in temperate closed-canopy forests.

Root connections affect radial growth of balsam poplar trees

Connected root systems reduced the negative impact of August temperatures and insect outbreak on growth. Root connections between trees can be an ecological advantage of clonal plant species in environments with unevenly distributed resources. We investigated the effects of root connectivity in stands of balsam poplar in Quebec (Canada). We evaluated differences in growth response between groups of trees with and without root connections through climate-growth analyses, comparison of the growth dynamics, and analysis of growth response to a severe forest tent caterpillar (FTC) outbreak. Current May temperature had a positive influence on radial growth of both connected and non-connected trees. Growth of non-connected trees was negatively affected by August temperatures (r = −0.3) while connected trees did not reveal a significant relationship for that month. A mixed effect ANOVA showed a significant difference (F 1, 25 = 5.59, p = 0.02) in growth responses to FTC outbreak between connected and non-connected trees. Connected trees grew on average 16 % better than unconnected trees during the outbreak, with bootstrapped 95 % confidence range from 2.28 to 31.36 %. The study suggests a sharing of resources through root connections, affecting radial growth of connected balsam poplar trees under both average and extreme environmental conditions.

Supercooling points of diapausing forest tent caterpillar (Lepidoptera: Lasiocampidae) eggs

AbstractForest tent caterpillar (Malacosoma disstriaHübner; Lepidoptera: Lasiocampidae) is a widely distributed defoliator that undergoes intermittent outbreaks. It overwinters as pharate larvae within egg bands, is univoltine, and experiences low winter temperatures in its northern range. Little is known about how low temperatures affect winter survival and cold tolerances, their cold tolerance strategy, or how cold tolerances may vary over time and among populations. We evaluated supercooling points (SCPs) from four populations ofM. disstriaeggs collected along a 552 km latitudinal gradient from southern Wisconsin to northern Minnesota, United States of America. To test for potential effects of winter environment, we also administered three overwintering regimes (Madison, Wisconsin; Cloquet, Minnesota; Ely, Minnesota). Supercooling points were recorded in November, February, and March of 2011–2012. Supercooling points varied with maternal source (egg band), time of winter season, population source, and overwintering treatment. Means ranged from −26.8 °C (±0.5 °C) to −40.3 °C (±0.3 °C), accordingly. In a separate laboratory experiment, 89% of pharate larvae held at −20 °C (18.3 °C above coolest mean SCP) survived, but none held at −45 °C (6.7 °C below lowest mean SCP) survived. This relatively high degree of cold tolerance in its overwintering stage, due to freeze avoidance, may partially explain survival patterns and limits of overwinteringM. disstriain northern populations.

Management Of The Tent Caterpillar (Malacosoma indicum) Using Nucleopolyhedrovirus In Jammu & Kashmir, India

The tent caterpillar (Malacosoma indicum) is a serious pest that attacks a wide range of fruit and forest trees in Jammu & Kashmir (J & K) range of North-Western Himalayas. Investigations were carried out during the period March, 2012 to July, 2015 to isolate and bioprospect naturally occurring NPVs as potent biopesticides against this pest. The isolated viruses, M. indicum multiple nucleopolyhedrovirus (MainMNPV) were confirmed to be double stranded DNA virus belonging to the family Baculoviridae. Molecular identification of the viral DNA and determination of the partial sequences of the polh gene of MainMNPV isolates showed their similarity to the NPV of other insects and was more closely related to the M. californicum NPV in the NCBI taxonomy database. This is the first report from Indian subcontinent and worldwide about the occurrence of NPVs in natural larval populations of M. indicum. The field application of MainMNPV against the host populations on walnut with the pre-standardised dosage of 1 × 1012 POBs/acre reduced the larval population density up to 39-63%. In the year following application, the larval population density in the treated blocks was 24-28% less than in control blocks. Therefore, ecofriendly management of M. indicum is possible by the use of baculovirus based biopesticides in various cropping systems of J & K.

Effects of winter temperatures, spring degree-day accumulation, and insect population source on phenological synchrony between forest tent caterpillar and host trees

Global climate change has the potential to dramatically alter multiple ecosystem processes, including herbivory. The development rates of both plants and insects are highly sensitive to temperature. Although considerable work has examined the effects of temperature on spring phenologies of plants and insects individually, few studies have examined how anticipated warming will influence their phenological synchrony. We applied elevated temperatures of 1.7 and 3.4°C in a controlled chamberless outdoor experiment in northeastern Minnesota, USA to examine the relative responses in onset of egg eclosion by forest tent caterpillar (Malacosoma disstria Hübner) and budbreak of two of its major host trees (trembling aspen, Populus tremuloides Michaux, and paper birch, Betula papyrifera Marshall). We superimposed four insect population sources and two overwintering regimes onto these treatments, and computed degree-day models. Timing of egg hatch varied among population source, overwintering location, and spring temperature regime. As expected, the development rates of plants and insects advanced under warmer conditions relative to ambient controls. However, budbreak advanced more than egg hatch. The degree of phenological synchrony between M. disstria and each host plant was differentially altered in response to warming. The interval by which birch budbreak preceded egg hatch nearly doubled from ambient to +1.7°C. In the case of aspen, the sequence changed from egg hatch preceding, to following, budbreak at +3.4°C. Additionally, under temperature regimes simulating future conditions, some insect populations currently south of our study sites became more synchronous with the manipulated hosts than did currently coexisting insect populations. These findings reveal how climate warming can alter insect-host plant interactions, through changes in phenological synchrony, possibly driving host shifts among tree species and genotypes. They also suggest how herbivore variability, both among populations and within individual egg masses, may provide opportunities for adaptation, especially in species that are highly mobile and polyphagous.

Ecology and evolution of pathogens in natural populations of Lepidoptera.

Pathogens are ubiquitous in insect populations and yet few studies examine their dynamics and impacts on host populations. We discuss four lepidopteran systems and explore their contributions to disease ecology and evolution. More specifically, we elucidate the role of pathogens in insect population dynamics. For three species, western tent caterpillars, African armyworm and introduced populations of gypsy moth, infection by nucleopolyhedrovirus (NPV) clearly regulates host populations or reduces their outbreaks. Transmission of NPV is largely horizontal although low levels of vertical transmission occur, and high levels of covert infection in some cases suggest that the virus can persist in a nonsymptomatic form. The prevalence of a mostly vertically transmitted protozoan parasite, Ophryocystis elektroscirrha, in monarch butterflies is intimately related to their migratory behaviour that culls highly infected individuals. Virulence and transmission are positively related among genotypes of this parasite. These systems clearly demonstrate that the interactions between insects and pathogens are highly context dependent. Not only is the outcome a consequence of changes in density and genetic diversity: environmental factors, particularly diet, can have strong impacts on virulence, transmission and host resistance or tolerance. What maintains the high level of host and pathogen diversity in these systems, however, remains a question.

The effects of genetic diversity, climate and defoliation events on trembling aspen growth performance across Canada

Tree genetic makeup may provide an important control of growth dynamics; however, no studies have previously attempted to evaluate its effects in natural trembling aspen stands. In this study, we examined the relative contribution of genetics (i.e. clonal diversity, observed heterozygosity) and environmental conditions (i.e. insects, climate) on aspen growth as represented by mean inter-tree correlation (RBAR), tree basal area increment (TBAI) and inter-annual growth variability (MS). We sampled 440 trees in 22 even-aged natural stands dominated by aspen along an east-west continental gradient of decreasing annual precipitation in the Canadian boreal forest. Linear and mixed-effect models tested the relationships between tree growth, genetics and environmental factors. We showed that clonal diversity and number of years with forest tent caterpillar (FTC) defoliation (NFTC) reduced and increased the level of growth synchronicity (RBAR), respectively. Clonal diversity explained 30 % of variation in RBAR among sites. TBAI was positively influenced by high moisture conditions while NFTC and climate explained the variation in MS among trees for each site. No genetic effect could explain either TBAI or the MS variation. Climate and NFTC drive annual growth variability in trembling aspen at stand and subcontinental scales. Tree genetic makeup contributed to these dynamics, the annual growth dynamics of multi-clonal stands being less homogeneous than those of monoclonal stands. Maintaining diverse aspen stands may ensure a wider range of growth responses to environmental variability, which in turn may help maintain resilience of aspen stands under future climate.