  The inhabitants of the bush lupine in coastal California have been the
subject of scientific scrutiny in recent years. Observations of a
host-parasitoid interaction in the shrub's foliage, in which victims are
significantly less motile than their exploiters, record stable spatial patterns
in a fairly homogeneous environment. Though such pattern formation has been
found in reaction-diffusion models, the correspondence of these models to
continuous-time predator-prey interactions does not reflect the reality of the
system being studied. Near the root of the lupine, another host-parasitoid
interaction is also of considerable interest. In some cases this interaction,
which promotes the health of the lupine, has been observed to be much more
persistent than suggested by mathematical models.
  In this work a discrete-time spatial model of the first host-parasitoid
system is introduced. We analyze the model, describing its transient behavior
and finding the conditions under which spatial patterns occur, as well as an
estimate of outbreak size under those conditions. We consider the feasibility
of the necessary conditions in the natural system by modeling the mechanisms
responsible for them, and discuss the effects of variable habitat on pattern
formation. We also explore one possible explanation for the persistence of the
second host-parasitoid system -- the existence of an alternate host. Under
certain surprising conditions and by means quite different from previous models
of similar situations, an alternate host can greatly enhance persistence of the
nematode parasitoid.
