Deepa Pureswaran

Climate‐induced changes in host tree–insect phenology may drive ecological state‐shift in boreal forests

Climate change is altering insect disturbance regimes via temperature-mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies in boreal forests. Range expansion and increase in outbreak severity of forest insects are occurring in Europe and North America. The degree to which northern forest ecosystems are resilient to novel disturbance regimes will have direct consequences for the provisioning of goods and services from these forests and for long-term forest management planning. Among major ecological disturbance agents in the boreal forests of North America is a tortricid moth, the eastern spruce budworm, which defoliates fir (Abies spp.) and spruce (Picea spp.). Northern expansion of this defoliator in eastern North America and climate-induced narrowing of the phenological mismatch between the insect and its secondary host, black spruce (Picea mariana), may permit greater defoliation and mortality in extensive northern black spruce forests. Although spruce budworm outbreak centers have appeared in the boreal black spruce zone historically, defoliation and mortality were minor. Potential increases in outbreak severity and tree mortality raise concerns about the future state of this northern ecosystem. Severe spruce budworm outbreaks could decrease stand productivity compared with their occurrence in more diverse, southern balsam fir forest landscapes that have coevolved with outbreaks. Furthermore, depending on the proportion of balsam fir and deciduous species present and fire recurrence, changes in regeneration patterns and in nutrient cycling could alter ecosystem dynamics and replace black spruce by more productive mixed-wood forest, or by less productive ericaceous shrublands. Long-term monitoring, manipulative experiments, and process modeling of climate-induced phenological changes on herbivorous insect pests, their host tree species, and natural enemies in northern forests are therefore crucial to predicting species range shifts and assessing ecological and economic impacts.

Mate‐finding allee effect in spruce budworm population dynamics

Allee effects can cause populations to decline due to decreasing population growth rates with decreasing density and play a major role in population dynamics. Mate‐finding failure, a common mechanism contributing to demographic Allee effects, is usually difficult to demonstrate because of the arduous nature of sampling individuals at very low densities. In a rising outbreak of the eastern spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), we used caged and tethered virgin females in traps to measure mating success over population densities ranging from deep endemic to outbreak conditions. We found that mating success increased with increasing population density, and that at endemic population densities, females experienced difficulties attracting males and mating, demonstrating for the first time a mate‐finding Allee effect in the spruce budworm. The relationship between population density and mating success is nonlinear. As population density increased, the proportion of mated females eventually reached a plateau and mating success was not 100% even at the highest moth densities, probably due to female reluctance to mate and perhaps interference competition by males for access to females. Both laboratory‐reared and wild females were equally effective in synthesizing pheromone, attracting males, and mating. Our results strongly suggest that a mate‐finding Allee effect is involved in maintaining low‐density spruce budworm populations below an Allee threshold where they fail to grow. Factors such as changes in predation pressure and immigration could help populations overcome this Allee threshold.

Influence of Host Tree Condition on the Performance of Tetropium fuscum (Coleoptera: Cerambycidae)

ABSTRACT Tetropium fuscum (F.) attacks weakened Norway spruce, Picea abies (L.) Karst., in its native Europe and may colonize healthy spruce in Nova Scotia, Canada. We used manipulative field experiments to evaluate: 1) the development of T. fuscum on apparently healthy red spruce (Picea rubens Sarg.) in Nova Scotia; 2) the influence of red spruce physiological condition (healthy, girdled or cut) on T. fuscum performance; and 3) the impact of natural enemies and competitors on T. fuscum performance when developing on trees of varying condition. Tetropium fuscum successfully developed on healthy red spruce. Survival was higher on healthy than on girdled or cut trees when larvae were exposed to natural enemies and competitors. The benefits of reduced competition and parasitism on healthy trees appeared to compensate for any reductions in nutritional quality, increase in host resistance, or both. In contrast, when T. fuscum were protected from natural enemies, apparent survival was highest on girdled trees. Tetropium fuscum development took longer on healthy than on cut or girdled trees, and emerged adults were largest on healthy trees. The disparities in adult sizes among the three treatments may mean that healthy trees are more nutritious. Alternatively, the differences may indicate that a greater amount of time was spent feeding in healthy than in girdled or cut trees. Tree condition appears to have a direct impact on the success of T. fuscum, influencing survival, development time, and adult size, and may mediate the impact of natural enemies and competitors, further affecting T. fuscum performance.

Preference of an exotic wood borer for stressed trees is more attributable to pre-alighting than post-alighting behaviour

Different mechanisms mediate host selection by insects before (pre‐alighting) versus after (post‐alighting) landing on potential hosts, but few studies distinguish pre‐ and post‐alighting behaviour, particularly for wood borers. This study evaluates pre‐ and post‐alighting host selection by Tetropium fuscum (F.) (Coleoptera: Cerambycidae), a Palearctic wood borer that was recently introduced to Halifax, Nova Scotia. We evaluate whether T. fuscum select stressed (i.e. girdled) over healthy red spruce, Picea rubens Sarg. (Pinaceae), trees for oviposition, as predicted by the preference–performance hypothesis (PPH). The landing rates of the native congener, Tetropium cinnamopterum Kirby (Coleoptera: Cerambycidae), and parasitoids of Tetropium spp. were also quantified. Tetropium fuscum consistently preferred girdled over healthy trees. Adults landed more than 10 times more frequently (11.0 ± 1.5 versus 0.6 ± 0.2 adults per tree), and females laid more than 3 times as many eggs (130 ± 23 versus 38 ± 13 eggs per bolt), on girdled compared with healthy trees. As T. fuscum survival and rate of development is greater on girdled than on healthy trees, these results support the PPH. Tetropium fuscum rarely made maladaptive choices pre‐alighting, but post‐alighting oviposition on hosts on which performance is relatively lower was more common. Selection of high‐quality (e.g. girdled) hosts by female T. fuscum is therefore more attributable to pre‐alighting than post‐alighting behaviour.

Paradigms in Eastern Spruce Budworm (Lepidoptera: Tortricidae) Population Ecology: A Century of Debate

Abstract Three main hypotheses have been postulated over the past century to explain the outbreaking population dynamics of eastern spruce budworm, Choristoneura fumiferana (Clemens). The Silviculture Hypothesis first arose in the 1920s, with the idea that outbreaks were driven by forestry practices favoring susceptible softwood species. In the 1960s, it was proposed that populations were governed by Multiple Equilibria, with warm weather conditions releasing low-density populations from the regulatory control of natural enemies. Dispersal from outbreak foci, or “epicenters,” was seen as causing widespread outbreaks that eventually collapsed following resource depletion. However, in the 1980s, following the re-analysis of data from the 1940s outbreak in New Brunswick, this interpretation was challenged. The alternative Oscillatory Hypothesis proposed that budworm population dynamics were governed by a second-order density-dependent process, with oscillations being driven by natural enemy–victim interactions. Under this hypothesis, weather and resource availability contribute to secondary fluctuations around the main oscillation, and weather and moth dispersal serve to synchronize population cycles regionally. Intensive, independent population studies during the peak and declining phases of the 1980s outbreak supported the principal tenet of the Oscillatory Hypothesis, but concluded that host plant quality played a more important role than this hypothesis proposed. More recent research on the early phase of spruce budworm cycles suggests that mate-finding and natural-enemy-driven Allee effects in low-density populations might be overcome by immigration of moths, which can facilitate the onset of outbreaks. Even more recent research has supported components of all three hypotheses attempting to explain spruce budworm dynamics. In the midst of a new rising outbreak (2006-present), we discuss the evolution of debates surrounding these hypotheses from a historic perspective, examine gaps in current knowledge, and suggest avenues for future research (e.g., intensive studies on low-density populations) to better understand and manage spruce budworm populations.

Evaluating seasonal variation in bottom-up and top-down forces and their impact on an exotic wood borer, Tetropium fuscum (Coleoptera: Cerambycidae).

ABSTRACT It is well understood that forces from multiple trophic levels simultaneously influence herbivore performance, but how the relative strength of these forces vary over space and time is less clear. We evaluated seasonal variation in the impact of bottom—up forces (host condition), top—down forces (natural enemies), and competition on the performance of an exotic wood borer. Tetropium fuscum (F.) (Coleoptera: Cerambycidae) eggs that were either protected or exposed were placed on healthy and stressed red spruce, Picea rubens Sargent, trees at three different times during the natural T. fuscum flight period. We also measured the length of necrotic lesions (i.e., an induced hypersensitive response) that developed in response to attacking T. fuscum. As predicted, T. fuscum performance was usually greater, and induced host defenses lower, on stressed than on healthy trees, but the impact of host condition on T. fuscum performance varied seasonally. Timing of attack was critical, influencing the strength of bottom—up forces and consequently all measures of T. fuscum performance. Survival was reduced when T. fuscum attacked too early (late-May) or too late (late-June), which may result in stabilizing selection for attack time in this species. Parasitism and competition were generally negligible during this study. Our results suggest that timing of attack is critical for this wood borer and that temporal variation in the impact of top—down and bottom—up forces should be considered in other systems.

Displacement of Tetropium cinnamopterum (Coleoptera: Cerambycidae) by Its Invasive Congener Tetropium fuscum

Abstract We examined the native community of insects interacting with an invasive species, Tetropium fuscum (F.) (Coleoptera: Cerambycidae), in its new range to explore reasons for the invaders relatively slow spread. Tetropium fuscum is a European spruce borer established in Nova Scotia since at least 1990, but it has spread only about 125 km from its site of introduction. We compared the densities of Tetropium spp., their known parasitoids, and the community of wood-boring insects at sites located within the invasion zone in Nova Scotia versus well outside this zone, in New Brunswick, Canada. Using red spruce trees stressed by girdling or felling, we tested whether: 1) T. fuscum had altered the native wood-boring community; 2) T. fuscum displaced a native congener, Tetropium cinnamopterum (Kirby); and 3) parasitism rates of Tetropium spp. differed between the invaded and noninvaded zones. Both Tetropium spp. and their parasitoid wasps emerged exclusively from felled trees as opposed to girdled trees. We found no difference in community diversity inside versus outside the invasion zone. The combined densities of both Tetropium spp. and their overall parasitism rates also did not differ between zones, but T. cinnamopterum density was significantly greater outside the invasion zone, suggesting T. fuscum may displace the native congener where they are sympatric. Our results suggest that the native and invasive Tetropium spp. act as a single functional species in the invasion zone. We speculate that natural control agents (predators, parasitoids, and competitors) might be limiting the rate of spread of T. fuscum.

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.

Impact of an Invasive Longhorned Beetle, Tetropium fuscum (Coleoptera: Cerambycidae), on Community Structure of Subcortical and Wood-Associated Insects in Eastern Canada

Abstract Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae), a phloem-feeding and wood-boring beetle introduced from Eurasia, attacks spruce in eastern Canada alongside its native congener Tetropium cinnamopterum Kirby. We reared phloem- and wood-feeding insects (and their predators) from bolts of red and Norway spruce (Picea rubens and Picea abies) in Nova Scotia, comparing insect communities between bolts with added eggs of T. fuscum or T. cinnamopterum and bolts without added Tetropium (controls). We tested for impacts of each Tetropium on insect community structure (Simpsons diversity, richness, and evenness). We also asked whether, consistent with Darwins Naturalization Hypothesis, Tetropium spp. would have greater impacts on emergence of its closer relatives (which might be most likely to compete and/or share natural enemies). Addition of Tetropium eggs (either species) to bolts lowered insect diversity in both host trees. Both richness and evenness components of diversity were always lower in +Tetropium treatments, although different components reached statistical significance in different Tetropium species × host combinations. Addition of Tetropium spp. significantly reduced emergence of some species: Evodinus monticola (Randall) (Coleoptera: Cerambycidae) was reduced by T. fuscum on both hosts and by T. cinnamopterum on Norway spruce; Hylobius congener Dalla Torre, Schenkling, and Marshall was reduced by T. fuscum on red spruce; and Xylophagus sp. (Diptera: Xylophagidae) was reduced by T. cinnamopterum on Norway spruce. However, there was no relationship between Tetropiums impact on a community member and their phylogenetic relatedness, and the overall impacts of Tetropium presence were not very different between T. fuscum and T. cinnamopterum.

Forest Insects and Climate Change

Purpose of ReviewClimate change affects populations of forest insect pests in a number of ways. We reviewed the most recent literature (2013–2017) on this subject including previous reviews on the topic. We provide a comprehensive discussion of the subject, with special attention to insect range expansion, insect abundance, impacts on forest ecosystems, and effects on forest insect communities. We considered forest insects according to their major guilds and biomes.Recent FindingsEffects of climate change on forest insects are demonstrated for a number of species and guilds, although generalizations of results available so far are difficult because of species-specific responses to climate change. In addition, disentangling direct and indirect effects of climate change is complex due to the large number of variables affected. Modeling based on climate projections is useful when combined with mechanistic explanations.SummaryExpansion of either the true range or the outbreak range is observed in several model species/groups of major insect guilds in boreal and temperate biomes. Mechanistic explanations are provided for a few species and are mainly based on increase in winter temperatures. In relation to insect abundance, climate change can either promote outbreaks or disrupt trophic interactions and decrease the severity of outbreaks. There is good evidence that some recent outbreaks of bark beetles and defoliating insects are influenced by climate change and are having a large impact on ecosystems as well as on communities of forest insects.