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Vertical stratification of leaf physical traits exerts bottom-up pressures on insect herbivory in a sugar maple temperate forest
Hakimara, Mahsa; Despland, Emma — 2024-04-10 <p>Do vertical gradients in temperate forest structure insect herbivore communities?  We tested the hypothesis that the increase in light intensity from understory to forest canopy level drives differences in leaf physical traits and budburst phenology that impact insect herbivores and thus play a role in structuring both herbivore communities and the leaf damages they cause. Twelve sugar maple <em>(Acer saccharum)</em> sites were monitored in southern Quebec, examining insect herbivore patterns from understory to the shaded and sun canopy over the summers of 2020, 2021, and 2022. Additionally, we recorded leaf physical traits, temperature, humidity, and sun exposure. Our findings revealed that leaf thickness increased along the vertical gradient in 2021, making leaves less favorable to herbivores in the canopy level. Accordingly, we recorded a consistent decrease in insect herbivory damage rates from the understory to the shaded canopy and sun canopy in 2020 and 2021, driven by leaf cutters, skeletonizers, stipplers, and leaf miners. These results support our hypothesis that variation in plant physical traits due to sun exposure contributes to the vertical stratification of insect damage. In 2022, the gradient of insect herbivore abundance corroborated the observed damage trends from the previous years. Moreover, we calculated an average annual herbivory rate of 9.1% of the leaf surface in our study site, suggesting limited evidence supporting a significant contribution of background herbivory to the decline of sugar maple forests. Overall, our study highlights the importance of vertical gradients in structuring insect herbivore communities and emphasizes the role of leaf traits in mediating these interactions.</p>
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Insect herbivore damages on white spruce growing in plantations and naturally regenerated under-canopy forest stands
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Pamela Yataco, Allison; Noor, Sabina; Girona, Miguel Montoro; Work, Timothy; Despland, Emma — 2024-04-19 <p>This data was prepared to compare insect damage on white spruce (<em>Picea glauca</em> (Moench) Voss, Pinaceae) growing in plantations with naturally regenerated trees under mature forest canopies in the boreal forest (Québec, Canada). We selected ten sites in the naturally regenerated forest and small, multispecies plantations and sampled ten young trees (per site) in late summer 2020 and again in early and late summer 2021. We recorded overall rates of damage for galls, damage by spruce budworm (<em>Choristoneura fumiferana</em> (Clemens, 1865)), spruce bud midge, spruce budmoth, spruce gall midge, cooley adelgid, defoliation from sawflies and other caterpillars.</p>
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Detoxification of host plant phenolic aglycones by the spruce budworm
Donkor, Dominic; Mirzahosseini, Zahra; Bede, Jacquie; Bauce, Eric; Despland, Emma — 2018-11-12 This study examines the post-ingestive fate of two host-plant derived small-molecule phenolics (the acetophenones piceol and pungenol) that have previously been shown to be toxic to the outbreaking forest pest, spruce budworm (Choristoneura fumiferana). We test first whether these compounds are transformed during passage through the midgut, and second whether the budworm upregulates activity of the detoxification enzyme glutathione-s-transferase (GST) in response to feeding on these compounds. Insects were reared on either foliage or artificial diet to the fourth instar, when they were transferred individually to one of two treatment diets, either control or phenolic-laced, for approximately 10 days, after which midguts were dissected out and used for Bradford soluble protein and GST enzyme activity analysis. Frass was collected and subjected to HPLC-DAD-MS. HPLC showed that the acetophenones do not autoxidize under midgut pH conditions, but that glucose- and glutathione- conjugates are present in the frass of insects fed the phenolic-laced diet. GST enzyme activity increases in insects fed the phenolic-laced diet, in both neutral pH and alkaline assays. These data show that the spruce budwom exhibits counter-adaptations to plant phenolics similar to those seen in angiosperm feeders, upregulating an important detoxifying enzyme (GST) and partially conjugating these acetophenones prior to elimination, but that these counter-measures are not totally effective at mitigating toxic effects of the ingested compounds in the context of our artifical-diet based laboratory experiment.
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Dataset for Top-down and bottom-up controls on an herbivore on a native and introduced plant in a tropical agricultural landscape
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Despland, Emma; Santacruz-Endara, Paola — 2019-10-31 The niches of herbivorous insects are best understood in a tri-trophic perspective, as top-down effects of predators and parasitoids can determine the host plant range of herbivores. The recent introduction in a tropical agricultural environment of a weedy open-habitat plant (Solanum myriacanthum) and subsequent host range expansion of a common forest-edge butterfly (Mechanitis menapis) onto that plant provides an opportunity to test hypotheses surrounding reconfiguration of tritrophic networks in anthropized environments: what is the role of bottom-up and top-down forces in this host range expansion? and does habitat breadth limit this novel trophic relationship? Field and laboratory monitoring of larval survival and performance on a native (Solanum acerifolium) and exotic host in the Mindo valley (Ecuador), combined with measurement of plant physical defenses, shows that larval mortality was mostly top-down on S. acerifolium, linked to parasitism, but mostly bottom-up on S. myriacanthum, possibly linked to observed increased plant defenses. These findings support the theory that herbivores experience little top-down regulation on exotic hosts, and suggest that, in this case, bottom-up pressure from plant defenses is stronger in the absence of a co-evolved relationship. In this system, top-down forces tend to expand rather than restrict host plant range, contrary to many previous studies. S myriacanthum was less colonized in open pastures than in semi-shaded habitats (forest edges, thickets): fewer eggs were found, suggesting limited dispersal of adult butterflies into the harsh open environments, and the survival rate of first instar larvae was lower than on ecotone plants, likely linked to the stronger defenses of sun-grown leaves. Environmental conditions thus modulate the rewiring of novel trophic networks in heavily impacted landscapes, and limit a native herbivore`s control on an invasive plant.
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Ontogenetic shift from aposematism and gregariousness to crypsis in a Romaleid grasshopper - dataset
Despland, Emma — 2020-05-13 Traits of chemically-defended animals can change as an individual grows and matures, and both theoretical and empirical evidence favour a direction of change from crypsis to aposematism. This study examines the suite of traits involved in an unusual opposite shift from aposematism to crypsis in a neotropical toxic-plant-feeding Romaleid grasshopper, Chromacris psittacus (Gerstaecker, 1873). Field surveys, behavioural observations and a rearing experiment compare host plant choice, aggregation, locomotion and thermoregulation between life history stages. Results showed that both nymphs and adults fed exclusively on a narrow range of Solanaceae plants, suggesting that the shift in defensive syndrome is not due to a change in chemical defense. Instead, nymphal aposematism appears linked to aggregation in response to plant-based selection pressures. Slow nymphal development suggests a cost to feeding on toxic plant compounds, and grouping could mitigate this cost. Grouping also increases conspicuousness, and hence can favour warning colourating in chemically-defended insects. The role of diet breadth in aposematism is poorly understood, and these results suggest how constraints imposed by feeding on toxic plants can generate bottom-up selection pressures shaping the adaptive suites of traits of chemically-defended animals.
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Improved performance of the eastern spruce budworm on black spruce as warming temperatures disrupt phenological defenses
Bellemin-noel, Bastien; Bourassa, Stéphane; Despland, Emma; De Grandpré, Louis; Pureswaran, Deepa — 2020-12-21 Phenological shifts, induced by global warming, can lead to mismatch between closely interacting species. The eastern spruce budworm, Choristoneura fumiferana, an important outbreaking insect defoliator in North America, mainly feeds on balsam fir, Abies balsamea, which has historically been well synchronized with the insect. But as climate change pushes the northern range limit of the budworm further north towards the boreal forest, the highly valuable black spruce, Picea mariana, historically protected against the budworm by its late budburst phenology, is suffering increased defoliation during the current outbreak. We tested the hypothesis that rising temperatures can lead, not to a mismatch, but to an improved match between the budworm and black spruce through differential phenological advancement. For three years, eastern spruce budworm larvae were reared from instar 2 to pupae, on both black spruce and balsam fir, in a temperature free-air enhancement experiment (T-FACE) consisting in 24 field plots, half of which were heated at +2°C from March to October. Our results show that every year, larval development was faster on heated trees and pupation was earlier than on unheated trees. Bud development was also accelerated in heated trees of both species. However, there was no difference in mass between pupae that developed at +2°C and controls at the end of the season. Finally, we found no difference either in development rate or pupal mass between larvae reared on black spruce and those reared on balsam fir. This suggests that under higher temperature eastern spruce budworm will be as successful on black spruce as on balsam fir, as black spruce budburst becomes better synchronized with the insect’s emergence from diapause. This could lead to critical changes in outbreak dynamics and severity with important ecological state shifts at the landscape level.
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Delayed density dependence of various early-instar mortality sources during the crash of a forest tent caterpillar outbreak
Caron, Anne-Sophie; Jarry, Joshua Joseph; Lafleur, Benoit; Despland, Emma — 2021-03-19 Predation can be a major source of mortality in outbreaking pest insects and can help regulate the high densities of individuals at the peak of the outbreak. However, other mechanisms could be at play in the years of population crashing, contributing to the endemic levels of these populations. Forest tent caterpillars (Malacosoma disstria Hubner) are outbreaking pest species defoliating mixed wood boreal forests in eastern Canada, including Quebec. This species presents periodic population dynamics with peaks every 10 years and outbreaks lasting 1–3 years which can highly impact the health of its host tree. Different sources of mortality can play a role in the dynamics of these outbreaks such as extrinsic agents (natural enemies) and intrinsic ones (pathogens and maternal effect). Extrinsic mortality was considered in terms of natural enemies that can fly to reach the colony and those that have to walk to reach the prey. This study seeks to answer questions concerning the relative importance of these sources of mortality during and after the crash of an outbreak. We also investigated the difference between sites that had been defoliated during the last outbreak and those that were not to test for density dependence. For this experiment, we use a triad set-up of complete predator exclusion, partial exclusion and free colonies. We found that intrinsic mortality increases after the crash of the outbreak while only flighted mortality (related to parasitoids and other flying predators) did, and not walking mortality. There was no difference between the defoliated and control sites. These findings support our hypothesis that predation would increase after the outbreak and would lead to regulation of the population. However, we had underestimated the importance of intrinsic mortality for this regulation. This finding is important when thinking out potentially control agents to limit the spread of future outbreaks.

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(Author: Despland, Emma)

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