Population stability in relation to resource availability in an introduced population of an herbivorous lady beetle

Abstract

AbstractMechanisms responsible for population stability in relation to resource availability were studied in an introduced herbivorous lady beetle, Epilachna niponica. The introduced population was relatively constant over a seven‐year study period. Egg density was related to the variation in host‐plant abundance in different years, and was highly stabilized during the period from reproductive adult to egg stage. Two density‐dependent processes were identified in the reproductive season: (1) density‐dependent reduction in fecundity and (2) density‐dependent increase in female mortality and/or emigration, all of which operated early in the season. As a result, temporal variability in cumulative egg density was greatly reduced by mid‐May, by which time approximately 40% of total eggs were laid. A field cage experiment demonstrated that egg‐laying of individual females was largely limited by resource availability even at low levels of leaf herbivory. Since movement activity of ovipositing females increased in a density‐dependent manner, inter‐plant movement is more likely to cause density‐dependent reduction in fecundity and female loss, due to enhanced energy expendiditure. The introduced population was less stable than the source population, probably because of decreased inter‐plant movement of females and the unlikelihood of egg resorption, both of which contribute significantly to the temporal stability of E. niponica population densities.