Abstract Insect population dynamics are the result of an interplay
between intraspecific competition, trophic interactions and external
forces such as weather conditions, but studying how these processes
combine to determine population change is challenging. We investigate
mechanisms of population dynamics in a natural, low density insect
population. Eggs and larvae of the noctuid moth, Abrostola asclepiadis,
develop on its host plant during summer. The population density, and
mortality, was closely monitored throughout this period during 15 years.
Densities fluctuated between one and two orders of magnitude. Egg --
larval developmental time varied substantially among years, with lower
survival in cool summers with slower development. This was presumably
due to the prolonged exposure to a large guild of polyphagous arthropod
enemies. We also found a density dependent component during this period,
that could be a result of intraspecific competition for food among old
larvae. Dynamics during the long period from pupation in late summer
through winter survival in the ground to adult emergence and oviposition
the next year displayed few clear patterns and more unexplained
variability, thus giving a more random appearance. The population hence
shows more unexplained or unpredictable variation during the long
wintering period, but seems more predictable over the summer egg-larval
period. Our study illustrates how weather - via a window of exposure to
enemies and in combination with density-dependent processes - can
determine the course of population change through the insect life cycle.