Insect communities on experimental mugwort (Artemisia vulgaris L.) plots along an urban gradient

We studied the ability of insect herbivores and their natural enemies to colonize exposed, potted mugwort plants (Artemisia vulgaris L.) along a rural-urban gradient in 1994 in Hamburg (northern Germany). Ectophagous insects, leafmines and galls were monitored weekly from mid-May to mid-September. Endophagous insects were counted by harvesting and dissecting the stems at the end of the growing season. The rural-urban gradient was characterized by a gradient of vegetation-free areas and increasing proportion of ground covered in concrete, tarmac, paving and other impermeable surfaces surrounding the Artemisia plots, i.e. six different zones of increasing isolation. Numbers of insect species (herbivores, parasitoids and predators) decreased along the gradient from 43 to 12. Monophagous herbivores were not more affected than polyphagous herbivores, but parasitoids, especially rare species, were more strongly affected by isolation than predators. Some dominant herbivorous species were very successful colonizers and occurred in inner city sites devoid of all natural vegetation. Sometimes their abundance increased in the inner city to significantly higher densities than in the urban fringe. Isolation appeared to be the main reason for the observed patterns, since area and soil conditions were held constant in the experiment. Microclimate and pollution were considered to play a minor role.