Urban outbreaks of herbivores: Determining the effects of nutrients and drought on herbivore abundance in urban forests

Drought stress is predicted to become more prevalent in the future and affect plant-insect interactions. During drought stress, plants may increase concentrations of nutrients (amino acids, carbohydrates) in order to alleviate the effects of stress, but these nutrients may also increase herbivore abundance on stressed plants. The effects of stress on plants, therefore, may be exacerbated by stress-induced increases in herbivory.

            In Raleigh, North Carolina, herbivores outbreak on drought stressed willow oaks (Quercus phellos) and red maples (Acer rubrum) and have significant negative effects on tree health. Herbivory decreases foliage and shade cover in urban forests and recreational areas. With increasing temperatures and drought with climate change, understanding the effects of temperature and drought stress on herbivore abundance is critical to preserving urban forests.

            In previous work we found that herbivore abundance was greater in hotter areas than in cooler areas of the city. In this study, herbivore abundance was surveyed on willow oaks and red maples in Raleigh in 20 different sites. We measured herbivore abundance, turgor pressure in plants (water pressure) to determine stress severity, and concentrations of amino acids and carbohydrates. We found that herbivore abundance increased on stressed trees with greater concentrations of nutrients regardless of stress severity. Amino acids had greater effects on herbivores than carbohydrates. Temperature increased the prevalence of drought stress to a greater degree in red maples, but severe stress decreased herbivore abundance overall. Our study suggests that nutrients influence herbivore abundance on stressed trees and considerations of nutrients are necessary to accurately predict herbivore abundance on drought stressed plants.