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) saplings and how these defenses
affect interactions with insect herbivores. Conducted at the Kenauk Nature
Reserve in Quebec, the study 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, two
Erebid caterpillars including the invasive spongy moth (Lymantria dispar)
and the native white-marked tussock moth (Orgyia leucostigma) were used in
controlled bioassays that assessed caterpillar feeding preference and
performance. The results revealed that sunlit leaves were
significantly thicker and had lower SLA compared to 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, but no significant differences were observed in
laboratory feeding initiation assays with first-instar caterpillars in
2022. These findings suggest that a tree's investment in sun
leaves to maximise 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.