DIGITALNA ARHIVA ŠUMARSKOG LISTA
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and Drosophila suzukii. Some of them are novel and only recently introduced alien species whereas some of them are present for decades in Croatia but due to the lack of a comprehensive and regularly updated inventory of alien species they have not been listed before. This also makes this up-to-date list of alien phytophagous insects in Croatia valuable. Our results have shown that Asia is the main region of origin of alien insects established in Croatia (33 %), followed by North America (27 %). The trends are similar in other European countries and Europe in general. A rapid increase in the number of new alien species introduction per year in Croatia is noticeable from the years 2007–2012 (6.4 species/year) compared to 2002–2007 (1.8 species/year) (Figure 3). In Europe, an average of 17.5 new species of insects per year was recorded between 2000 and 2007, while this value was only 8.1 from 1950 to 1974. In Europe twice as many new insect species were observed per year on trees and shrubs during the period 2000–2007 (6.3 species) compared to 1960–1979 (3.4 species). The differences between the number of new alien species/year in Europe and Croatia are probably due to differences in sampling efforts, country surface, volume of traded goods etc but the rapidly increasing trend is obvious. More than 80 % of alien insect species in Croatia (57 % on agricultural lands and 28 % in parks and gardens) have been established in man-made habitats (Figure 4). Only 15 % of alien insect species in Croatia have established themselves in natural environments (forests and woodlands) which is almost the same percentage as on European level. It is a common observation that simple, disturbed, man-made habitats are more easily invaded by insects and other invaders than complex, undisturbed, natural habitats. Alien insects linked to human environments and activities (e.g. ornamental plants, bonsais, seeds, large potted trees, cut flowers, vegetables, fruits) are more likely to be carried by human transports into a new region than insects living in natural areas. A study has shown that bonsais carry a more diverse alien insect fauna then timber and that ornamental plants constitute "miniature" ecosystems which may host a large variety of insects that have the potential to damage other woody plants as well. Almost 90 % of alien invertebrates in Europe were introduced unintentionally through human activities, mostly as contaminants of a commodity. In Europe, ornamental plant trade contributes significantly more than forestry products to the invasion of alien forest insects. As interception data have not been analysed in this paper, a research of such data for alien insect species and trade volumes in horticultural plants in Croatia is strongly needed. There is a strong suspicion that ornamental plants are one of main pathways of introduction of alien insects to Croatia due to the increase of the imported volumes from year to year. Alien insect species are known for being serious pests worldwide and they can impact habitats which they invade in several ways. Alien insects can affect native biodiversity through direct actions: phytophagous insects feeding on plants, a predator or a parasitoid attacking host, an alien species hybridizing with a native species or indirect actions: vectoring diseases, competing for food, or sharing natural enemies with native species. This research has shown that dangerous pests that can cause direct economic costs have invaded and are spreading in Croatia (Table 1). Due to high percentage of alien insect on agricultural lands (outdoor and in glasshouses) in Croatia (Figure 4) the yield losses of alien insect species on agricultural crops in Croatia must be considerable. Alien insects can have serious negative impact on forests, woodlands and urban parks. Some potentially damaging forest and urban pests have already established themselves in Croatia. In countries where the percentage of forest cover is high (Croatia around 44 %) the damage from alien insects is expected to be considerable. Most introductions of alien insects are unintentional and unpredictable. Less than 20 % of the alien invertebrates in Europe have been intercepted before their arrival. There are several harmful phytophagous alien species approaching the borders of Croatia: Agrilus planipennis which could pose serious threat to Croatian lowland oak ecosystems, Anoplophora sp. which is spreading rapidly in Northern Italy. Some of the most polyphagous alien insect species, such as Drosophila suzukii, have only recently been discovered in Croatia. This study has also shown a time lag between arrival and first record of an alien species which has direct implications on successful eradication measures. Climate change may directly influence establishment and colonisation of alien insect species in new territories-from other continents to Europe and from warmer European regions of Europe further north. Global warming is likely to influence establishment and spread of alien insect species from subtropical and tropical areas (24 % found in Croatia) especially on the Mediterranean coast. One of the main factors, globalisation, will definitely influence the upward trend of introduction and spread of new alien species in Croatia which will negatively influence economy and ecosystems.
Key words: invasive species, alien species inventory, taxonomy, geographic origin, establishment rate, habitat, damage