Temporal and spatial variability in herbivorous insect outbreaks: Impacts of parasitoids and host plant

Despite decades of study, the nature of population regulation in natural populations is still largely unknown. A question of fundamental interest is the relative importance of resources and natural enemies in this regulation. Two assumptions have hampered a clearer understanding of these issue. One, is that the effects of resources and natural enemies can be proportionally allocated and do not interact and two, that populations tend to be tightly regulated around an equilibrium density. In this dissertation, I relax these assumptions to show how the amount and distribution of food can interact with natural enemies to regulate populations focusing on cyclicly outbreaking insect herbivores that are hosts of insect parasitoids. While generally applicable to insect herbivores, the work is inspired by an outbreak of the defoliating moth Orgyia vetusta that is variable both in space and in time.; Chapter One, written in collaboration with Alan Hastings, is a theoretical examination of regulation by food and parasitoids in cyclic populations. The model analysis predicts that food limits population growth at the beginning of peak abundance while parasitoids limit populations only at the end of the peak abundance. The analysis predicts that parasitoids limit populations for two or three generations, which is independent of the length of the cycle. The patterns are robust to variation in whether food limitation is compensatory or overcompensatory.; Chapter Two, written in collaboration with John Maron and Susan Harrison, is describes several tests of assumption about parasitoid behavior that are used in mathematical models of host-parasitoid interactions. All populations show spatial heterogeneity in density. This heterogeneity can be quite important to parasitoids foraging for their hosts. The experiments demonstrated that foraging parasitoids (Tachniomyia similis Diptera:Tachinidae) aggregate to areas of relatively high host (Orgyia vetusta Lepidoptera:Lymantriidae) density and their rates of parasitism decline at high host density (a type II functional response). However, at the population level, rates of parasitism were not well described by host density.; Chapter Three, written in collaboration with Alan Hastings, is a theoretical examination of regulation in spatial spread by parasitoids and food or habitat distribution. The model shows that ability of parasitoids to slow the spread of its host is greatly altered by the habitat spacing of the host and the relative movement ability of the parasitoid. Herbivore outbreaks will spread largely unimpeded unless their habitat is widely spaced and parasitoids move at a much greater distance that the host.