Spatial and Temporal Density Dependence of Host Plant Patch Use by the Ragwort Seed Bug, Neacoryphus Bicrucis (Hemiptera: Lygaeidae)

Abstract

Over a period of eleven years the abundance of the ragwort seed bug, Neacoryphus bicrucis, on its host plant, Senecio anonymus, varied by a factor of more than 30. Insects fed, mated and apparently oviposited primarily in larger patches of host plant during years of relatively low abundance. However, during years of relatively high abundance many insects settled in small patches, although density in small patches was still less than that in larger patches. Within-year variation in abundance in patches was associated with both patch size and ragwort flower head density. In years with relatively high mean density of adults, adults were nonrandomly distributed, indicating spatial density dependence. As with adults, the abundance of nymphs was much less in small patches than in large patches. The proportion of nymphs developing in small patches was positively correlated with population size, suggesting that small patches became more attractive to ovipositing females as large patches were more heavily utilized. Years of relatively high adult abundance were preceded and followed by years of relatively low abundance. This pattern was independent of variation in climate and host plant abundance, suggesting temporal density dependence. A model incorporating climate (temperature in February-April) and abundance (in May) the previous year accounts for 80% of variation in annual abundance (in May).