Abstract:The research on incidence of mistletoe (Viscum album ssp. abietis) on silver fir (Abies alba) was carried out in natural fir stands in Croatia. In the area of Gorski Kotar 32.8 % of the examined dominant fir trees were infected with mistletoe. The mistletoe incidence was presented according to the damage degrees of silver fir from 0 (healthy trees) to 4 (dead trees), rated by the crown defoliation and needle discoloration. With the increase of incidence, mistletoe spreads more onto more vital, i.e. less damaged trees. In the Dinaric Alps 28.6% of the examined fir trees were infected, this percentage amounting to 27.1% for the mountainous regions between the Sava and Drava rivers. The site and stand parameters (exposure, elevation, site quality, forest community and crown closure) were analysed in order to establish whether there was a correlation between these parameters and mistletoe incidence. There was a negative correlation between the elevation and mistletoe incidence. Among the compartments with a closed stands there were considerably less compartments with higher mistletoe incidence than among compartments with a sparsely closed and understocked stands. The other analysed site and stand parameters individually had no significant influence on mistletoe incidence. The correlation of silver fir mortality in 2004 and mistletoe incidence in 2002/03 was analysed, and their strong correlation was established. Mistletoe could be considered as a bioindicator of silver fir decline, and probably a significant contributor to that decline. In the areas where mistletoe incidence is great it can be presumed that silver fir is significantly damaged.

130 kg of acorns of Common Oak (Quercus robur L.) were distributed on an area of 1.44 ha in an oak forest near Zagreb (Croatia), where acorns were lacking. The influence of this surplus of food on the development of mice and voles was observed from September 1999 till February 2000 on the experimental plot and on an untreated control area before and after offering the acorns. The Redbacked Vole Clethrionomys glareolus was the most abundant and dominant species in both observed areas. After distribution of the acorns, all species of mice, e. g. the Yellownecked Fieldmouse Apodemus flavicollis, the Longtailed Fieldmouse A. sylvaticus, and the Striped Field Mouse A. agrarius, increased up to 4 times in number on the area treated with a surplus of acorns. On the same area, however, all species of voles, e. g. the Redbacked Vole C. glareolus, the Field Vole Microtus agrestis, and the Common Vole M. arvalis, decreased. The numbers of captured and recaptured rodents before and after offering acorns on the treated and the untreated control area were compared by statistical analysis (Chi 2 -test). The results show how food can influence the competitive relations between mice and voles.

The relative abundance and the population development of small rodents were investigated on 17 sites in Croatian forests from September 1999 through June 2003. Common snap traps were set for two or three nights along transect lines in different forest types. In 27,598 trap-nights, 2,151 specimens were trapped. Small rodents were present in up to 45% of the traps in pedunculate oak forests dominated by Quercus robur L. In common beech forests with a predominance of Fagus sylvatica L., the percentage of occupied traps reached 59%. At most sites the following species were dominant:Clethrionomys glareolus Schr., Apodemus agrarius Pall. and A. flavicollis Melch. Additionally, in a forest community of Carpino betuli-Quercetum roboris typicum R., the absolute number of small rodents was calculated by means of the standard minimum method. The minimum rodent density was observed in April 2003 with 11.63 n/ha (number of specimens per hectare). The maximum was reached in March 2001 with 82.03 n/ha. Near the same site, the absolute number of the small rodent population was estimated using the ''Y method'' in spring 2001 as approximately 101.34 n/ha. In general, the abundance of small rodents was dependent on the forest seed crop.

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Background and Purpose: During June 2015 in Zagreb city area (Croatia) samples of tulip tree (Liriodendron tulipifera) leaves were collected with symptoms of attack by some unknown aphid.Material and methods: Aphids were collected form leaves of tulip trees on different locations in Zagreb during July 2015. Total genomic DNA was extracted from ethanol-preserved specimens. PCR analysis was carried out and PCR products were purified from 1% agarose gel for sequencing purposes. The obtained sequences were deposited in GenBank.Results: The collected aphids were genetically determined as Illinoia liriodendri (Hemiptera, Aphididae), the North American invasive pest species. Although present in several neighboring countries on tulip trees in urban environment, this research presents the first record of Illinoia liriodendri for Croatia, confirmed on the genetic level. Interestingly enough, during genetic determination of tulip tree aphid pest, another DNA, one of parasitoid Areopraon silvestre (Hymenoptera, Braconidae) was also found in the collected samples. A. silvestere is a native European specialized solitary endoparasitoid of aphids.Conclusions: The presence of alien pest species Illinoia liriodendri was successfully determined via genetic identification. Also, genetic identification of parasitoid species on Illinoia liriodendri, Areopraon silvestre, shows the quick establishment of natural regulation of new pest species in Croatia.