Effects of phenological synchronization on caterpillar early-instar survival under a changing climate

Abstract

Early-instar caterpillars experience very high and often very variable mortality; if it is density dependent, it can be a key factor in outbreak dynamics. Plant physical and chemical defenses can be extremely effective against young caterpillars, even of specialists. Phenological asynchrony with host plants can lead to dispersal and mortality in the early instars and increased predation or poor nutrition in later instars. Predation on early-instar larvae (including cannibalism) can be extremely high, parasitism appears generally low, and pathogens acquired early in larval development can lead to high mortality in later stadia. Four well-studied species reveal very different roles of early-instar mortality in population dynamics. In spruce budworm and gypsy moth, early-instar mortality rates can be very high; they do not drive outbreak cycles because density dependence is weak, but can modulate cycles and contribute to outbreak size and duration. For the autumnal moth, early-instar survival depends on host plant synchrony, but may or may not be density dependent. For monarch butterflies, the relative importance of larval mortality rates in population dynamics remains unclear. Tritrophic interactions between herbivores, host plants, natural enemies, and microbes play complex and species-specific roles in early-instar ecology, leading to emergent dynamics in population fluctuations. The phenology of these relationships is often poorly understood, making their responses to climate change unpredictable.Key words: Lepidoptera, hatchlings, synchronization, outbreaks.Résumé : Les chenilles des premiers stades larvaires connaissent des taux de mortalité très élevés et souvent très variables; si ces taux varient avec la densité, ils peuvent devenir un facteur clé dans la dynamique des épidémies. Les défenses physiques et chimiques des plantes sont souvent extrêmement efficaces contre les jeunes chenilles, même celles des espèces spécialisées. L'asynchronie phénologique avec les plantes hôtes peut entraîner la dispersion et la mortalité chez les premiers stades larvaires, et une augmentation de la prédation ou une mauvaise nutrition chez les derniers stades larvaires. La prédation lors des premiers stades larvaires (incluant le cannibalisme) peut être extrêmement élevée, le parasitisme semble généralement faible et les pathogènes qui infectent tôt dans le développement des larves peuvent entraîner un taux de mortalité élevé chez les stades ultérieurs. Quatre espèces bien étudiées révèlent différents rôles de la mortalité chez les premiers stades larvaires dans la dynamique des populations. Chez la tordeuse des bourgeons de l'épinette et la spongieuse, les taux de mortalité peuvent être très élevés chez les premiers stades larvaires; ils n'ont pas d'effet déterminant sur les cycles épidémiques parce que la dépendance à la densité est faible, mais ils peuvent moduler les cycles et contribuer à l'ampleur et à la durée des épidémies. Dans le cas de l'épirrite automnale, la survie des premiers stades larvaires dépend ...

Resistance, Resilience or Change: Post-disturbance Dynamics of Boreal Forests After Insect Outbreaks

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Monarch Butterfly Distribution and Breeding Ecology in Idaho and Washington

“…Over the 2 years of our study, we observed phenological synchrony between adult monarch arrival (i.e., egg-laying) and bud burst/young expanding foliage of showy milkweed. Research on eastern monarchs has shown that phenological asynchrony with milkweed host plants can lead to high mortality of early instars and increased predation or poor nutrition in later instars (Zalucki et al, 2011), though how this mortality contributes to overall population dynamics remains unclear (Despland, 2017).…”

Caterpillars Cooperate to Overcome Plant Glandular Trichome Defenses

“…In general, the leaf surface influences the ability of insect herbivores to establish a feeding site, especially in the early larval instars when mandibles are small (Despland, 2018) as newly emerged larvae must pierce the leaf cuticle to access the nutritious tissues underneath (Clissold, 2008). Tough leaves, or additional surface features such as waxes or trichomes, can increase the food processing time required by caterpillars to remove or circumvent these obstructions in order to access the nutritious leaf parenchyma (Voigt et al, 2007;Shelomi et al, 2010).…”

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