Effects of contrasting herbivore cues on seedlings of a long-lived woody tree: Growth, chemistry and resistance to herbivores

Abstract Plant induced defenses against herbivores have the potential to modify plant growth, fitness, and the outcome of plant-herbivore interactions. Plants use a variety of cues to initiate induction of defenses, including physical tissue damage, cues released by damaged plant tissues of neighboring plants (e.g., methyl jasmonate, MeJA), and cues that indicate herbivore presence but not associated with herbivore damage (e.g., locomotion mucus of herbivorous mollusks). Evidence shows that both herbivore-damage and herbivore-presence cues can generate induced defenses in herbaceous plants over relatively short time periods. However, the immediate and long-term effects of these cues on seedlings of deciduous broad-leaved woody plants remain poorly understood. Here, we compare the effects of two cues, MeJA and locomotion mucus of the dusky slug Arion subfuscus, on the growth and defense traits of sugar maple, Acer saccharum, seedlings at multiple timepoints. Specifically, we exposed 4-week-old seedlings to these cues in a greenhouse experiment and quantified their effects on seedling growth, total phenolics, and growth of spongy moth Lymantria dispar across the seedlings’ growing season. As expected, MeJA had a negative effect on seedling growth, a positive effect on total phenolics and no effect on the growth of L. dispar. In contrast, the slug locomotion mucus did not negatively affect growth or change total phenolic levels. Our results highlight that herbivore cues can induce responses in seedling growth and defense in deciduous broad-leaved woody plant seedlings, however, these responses vary with cue type and the induced responses decay for some seedling traits but not others.