Population Dynamics of a Seed Feeding Bug, Lygaeus Equestris. 2. Temporal Dynamics

Abstract

Population densities of the seed feeding bug, Lygaeus equestris have been followed for several years in three large study areas, each containing several host plant patches, in east central Sweden. Seed pod production of the perennial host plant Vincetoxicum hirundinaria as well as densities of another seed feeder, the tephritid fly Euphranta connexa, were also monitored. Densities of seed pods, E. connexa and L. equestris fluctuate about two orders of magnitude during a period of 11 yr. Population changes of L. equestris are density-independent during the non-breeding period of the year encompassing hibernation and preand posthibernation migrations. In most years mortality is low during this period, but occasionally high mortalities occur. During the breeding period, population changes are density-dependent and highly variable. Both sunshine conditions and abundance of seeds from the previous year affected population change during the breeding season. No competitive effect on L. equestris by E. connexa could be detected, despite the fact that E. connexa attacked over 90% of the pods in several years. Population density of L. equestris is determined by the sequence of weather effects of at least two years. The amount of sunshine, which affects the insect directly, also has indirect and delayed effects by affecting pod production of the host plant, and hence density of seeds on the ground in the following year. Furthermore, whereas population growth of L. equestris is positively affected by an increase in sunshine hours, pod production is negatively affected. This leads to considerable inertia in the temporal tracking of food resources by L. equestris.