Occurrence of Ips typographus (Col., Scolytidae) along an urbanization gradient in Brussels, Belgium

1 The distribution and dynamics of insect populations in cities is poorly understood. One approach to address this question is to explore the permeability of the urban habitat to species from surrounding rural areas, which can serve as reservoirs in source-sink dynamics. 2 Here, we present data on the distribution of the forest insect pest of spruce, Ips typographus (Coleoptera, Scolytidae), along two axes entering the city of Brussels (Belgium) from the south-east and to the town centre. 3 The insect was caught everywhere along these transects, even in heavily urbanized surroundings, and sometimes in relatively high numbers. The catches were highest near the middle of the transects and lower at both ends of them. 4 This pattern was associated, on the one hand, with an urbanization gradient with the numbers of flying individuals increasing with the distance from the city centre and, on the other hand, with lower catches at the periphery of the city where a high proportion of broadleaved trees may have disrupted the response to aggregation pheromones. 5 In addition to the probable rural origin of the beetles, high catches at the Port of Brussels indicated that some of the insects might be of foreign origin and enter the city with imported timber, highlighting a pathway for unintentional introductions of organisms, including potentially invasive species.