Impact of light intensity on sugar maple leaf physical traits and consequences for caterpillar preference and performance

This study investigates the impact of light intensity on the physical defenses of sugar maple (Acer saccharum Marshall [Sapindaceae]) saplings and how these defenses affect interactions with insect herbivores. The study conducted at the Kenauk Nature Reserve in Quebec involved 12 sites representing both shaded and sunlit environments. Leaf traits, including thickness, specific leaf area (SLA), water content, and toughness, were measured from 216 leaves collected in 2021 and 40 in 2022. To explore herbivore interactions, we used two Erebid caterpillar species, the invasive spongy moth (Lymantria dispar (Linnaeus) [Lepidoptera: Erebidae]) and the native white-marked tussock moth (Orgyia leucostigma (J. E. Smith) [Lepidoptera: Erebidae]), in controlled bioassays that assessed caterpillar feeding preference and performance. The results revealed that sunlit leaves were significantly thicker and had lower SLA compared with shaded leaves across both years. While water content was lower in sunlit leaves in 2021, it showed no significant difference in 2022. Leaf toughness, measured only in 2022, did not significantly differ between sunlit and shaded leaves. Caterpillar feeding preference tests indicated that white-marked tussock moth caterpillars consumed more sunlit leaves, showing a significant preference for them. However, spongy moth caterpillars did not exhibit a significant preference. In terms of performance, white-marked tussock moth caterpillars had a higher survival rate and larger pupal mass on sunlit saplings in 2021. However, no significant differences were observed in laboratory feeding initiation assays with first-instar caterpillars in 2022. These findings suggest that trees' investment in sun leaves to maximize photosynthesis is not necessarily followed by increased defense against herbivory. This research contributes to our understanding of how environmental factors like light can shape plant defense strategies and impact insect herbivore dynamics in temperate forests.