The spread of Ips typographus (L.) (Coleoptera, Scolytidae) attacks following heavy windthrow in Denmark, analysed using GIS.
The population of the spruce bark beetle Ips typographus in the forest of Rold Skov, Denmark, (dominated by Picea abies with P. sitchensis, Abies spp., and Fagus sylvatica as secondary species), was monitored in the years following a 1981 gale. All attacked standing trees, areas with windthrown trees, catches in pheromone traps, timing of salvage harvests, and stands exposed to attack were registered and mapped in 1982 and 1983. Three different indices for attack density in areas around windthrown trees, infested trees and pheromone traps were calculated using GIS (ArcView). The attack densities in 1983 around areas with windthrown trees salvaged after 1 July 1982 were significantly higher than for areas salvaged earlier. Attack densities around windthrown trees salvaged between 15 May and 1 July, just after the main spring-flight period, were the lowest. The maximum distance from an old attack to a new was 650 m and there was an old attack within 500 m of all new attacks. Attack densities around pheromone traps were not correlated with the number of beetles caught in the traps. The majority of the beetles emerging from an epidemic attack dispersed over short distances (i.e., <500 m) before entering a new host. This local dispersal may best be countered by removing old breeding sites/windthrown trees in the period between spring-flight and the emergence of the new generation, i.e., by using the windthrown trees as bait trees. It is concluded that pheromone traps are not suitable as a sole protective measure to prevent further infestation.