| Summary: | Climate warming has had significant effects on insect herbivores in Iceland, including an increased rate of establishment of new species as well as changes in outbreak patterns and distribution of insect herbivores. Many of these herbivores live on trees and shrubs. Concurrent with the onset of a warmer climate around 1990, a distinct host shift occurred in a few native insect species which started to feed on the exotic Nootka lupin (Lupinus nootkatensis), which until then had been free from any significant insect herbivory. Later, as the climate has warmed, many outbreaks of native insect species, primarily the Broom Moth (Ceramica pisi) and Satyr Pug (Eupithecia satyrata), have occurred in lupin fields. Broom Moth distribution and damage on young tree seedlings have also increased recently. The main objectives of this thesis were to study: (1) the effects of climate change on the population dynamics and distribution potential of native herbivores on Nootka lupin, using the Broom Moth as a case study and (2) the effects of insect herbivory on the fitness of the Nootka lupin and exotic trees, using seed production and annual growth as proxies for plant fitness. The effects of climate change on the population dynamics of the Broom Moth were studied in two phases: (1) the effects of warmer winters on pupal survival were studied by freezing pupae in a lab study at different sub-zero treatments and (2) the effects of warmer summers on larval development and pupal size, were studied by: (i) sampling and weighing larvae from lupin fields at different times and local climate during the larval growing season and (ii) weighing larvae just before pupation and after pupation. Additionally, the effects of herbivory intensity on the Nootka lupin seed production (fitness) were studied in a 3‐year field study at two sites at contrasting ages and successional stages, including different manipulated herbivory treatments. Winter temperatures were not found to affect Broom Moth survival as different sub‐zero treatments had no ...
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