PARENT SESSION
Oral Session # 80: Urban Ecology II: Soils and Streams.
Presiding: J Ehrenfeld
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 105.

Urban soil management and its effects on insect herbivory.

LaForest, Joseph^{*,1, 2}, Herms, Daniel¹, Bonello, Pierluigi², ¹ The Ohio State University, Wooster, OH, United States of America² The Ohio State University, Columbus, OH, United States of America

ABSTRACT- Fertilizer and mulch are commonly applied to urban soils degraded by construction activities in order to ameliorate nutrient deficiencies and stimulate tree growth. Two competing hypotheses address effects of nutrient availability on tree resistance to insects. If stress weakens plant defense, fertilization may reduce herbivory by increasing tree vigor. On the other hand, if there exists an allocation trade off between growth and defense, fertilized trees will grow faster, but be less resistant to herbivores. In a two-year field study, we tested these competing hypotheses by quantifying the effects of four soil management treatments (fertilization, mulching with composted yard waste, mulching with composted bark / manure blend, and bare soil control) applied in factorial combination to soil types (topsoil, inverted subsoil) on microbial activity, nutrient cycling, as well as growth phenolic chemistry, and insect resistance of paper birch (Betula papyrifera). Trees in topsoil grew faster than those in subsoil regardless of soil treatment, but were less resistant to insects as indicated by higher larval growth rates of forest tent caterpillar [Malacosoma disstria], whitemarked tussock moth [Orgyia leucostigma], yellow-necked caterpillar [Datana ministra] and fall webworm [Hyphantria cunea]. Fertilization of subsoil plots increased tree growth but decreased foliar phenolic content, and insect resistance, with the larval growth of some species increasing to levels observed on trees in topsoil. There was a significant negative correlation between total phenolic content and tree growth. These results are consistent with the hypothesis that faster growing trees are less insect resistant due to a physiological tradeoff between growth and chemical defense.

Key words: defense, herbivory, growth, nitrogen