Birch Leaves as a Minefield: Do Trichomes Mediate Neonate Caterpillar Site Selection?

ABSTRACT

Caterpillars are primary consumers that exert strong selective pressures on host plants. Plants, in turn, possess a range of defenses against herbivores. Leaf trichomes, for example, can act mechanically or chemically by damaging the insect's cuticle, restricting movement, disrupting attachment, or exposing insects to deterrent or toxic compounds. These defenses are especially challenging for neonate caterpillars, whose small size, soft bodies, and limited physiological capacity increase their vulnerability. To cope with these challenges, neonates have evolved multiple, non‐exclusive strategies to evade or mitigate plant defenses. Here, we tested whether neonate caterpillars selected specific leaf sites in response to the distribution of non‐glandular and glandular trichomes. We tested this with neonates of the masked birch caterpillar, Drepana arcuata Walker, 1855 (Drepanidae) on paper birch, Betula papyrifera Marshall (Betulaceae), leaves. We first quantified where neonate caterpillars established their shelter across leaf surfaces (adaxial vs. abaxial) and regions (base, middle, outer edge, and tip). Our findings showed that neonate establishment was non‐random. Larvae more frequently established on the adaxial surface, even when initially placed abaxially, and predominantly near the leaf margins (base, outer edge, and tip) rather than in the middle. We then characterized trichome densities and tested whether they were associated with the distribution of established shelters. Both trichome types were non‐uniformly distributed on paper birch leaves and negatively correlated with the number of established larvae, indicating that larvae preferentially selected sites with lower trichome densities. We also examined whether established larvae actively avoid feeding on trichomes. Larvae selectively bypassed glandular trichomes while skeletonizing leaf tissue. These results show that neonate D. arcuata selectively choose specific sites on leaves and that they avoid both non‐glandular and glandular trichomes, suggesting that variation in leaf trichomes is an important determinant of micro‐habitat selection in neonate caterpillars.