Establishment and spread of non-native bark- and wood-boring beetles

Sammanfattning: One of the biggest threats to forest ecosystems globally is the introduction and establishment of non-native species. In Sweden alone, several bark- and woodboring beetles have established populations in recent years. Three of the recently established species are the larch bark beetle Ips cembrae and the larch longhorn beetle Tetropium gabrieli in southern Sweden, as well as the small spruce bark beetle I. amitinusin northern Sweden. This thesis aims to assess their current distribution in Sweden and evaluate the factors that influence their local colonization and further spread.Pheromone-baited traps were used to assess if T. gabrieli is attracted to congeneric pheromone. Subsequently such traps were also used to estimate the local population sizes of I. cembrae and T. gabrieli. A survey method was developed for delimiting the distribution of I. amitinus based on inspection of logging waste on fresh cuttings. The method was used for establishing the southern range limit and the inter-annual spread. Field and laboratory experiments were also used to assess factors that influence the species local colonization.We found that T. gabrieli was attracted to the congeneric pheromone but not to the host volatiles. We found that local population sizes of both I. cembrae and T. gabrieli were related to the amount of suitable habitat in the landscape. We found that I. amitinus is already spread over large parts of northern Sweden and is one of the most common bark beetle species found in the region. Our results indicate that factors such as their reproductive strategy, forage for food and escape the enemy pressure influenced the probability of colonization. Allee effects, operating through the above mentioned components, might explain the low colonization probability for small local populations in the southern area and the abrupt range limit for I. amitinus in the north.In conclusion, species life history traits should be considered when planning risk assessments and monitoring programs of non-native species as it is a strong indicator for establishment. At the same time, the use of a correct method for surveying no nnative species is fundamental for an accurate conclusion of establishment and distribution.

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