|IZVORNI ZNANSTVENI ČLANCI
|Hrašovec, B., Harapin, M.
|UDK 630* 450 + 453 (001)
|Survey Methodology and Most Important Insect Pest Outbreaks in Croatian Forests pdf HR EN
|Summary: In 1980 the Croatian Ministry of Agriculture and Forestry established a Diagnose and Prognose Service located in Forest Research Institute Jastrebarsko. Since ownership structure of Croatian forest land is largely dominated by state forests, activity of the Diagnose and Prognose Service (DPS) is closely connected with closely related department in state enterprise "Hrvatske šume". Various methods are applied in order to asses the population densities of most important forest pests and these being adapted to specific biological cycles and practical aspects of survey techniques. Egg masses of Lymantria dispar are counted regularly on a yearly basis and this is done both on continental (transect method) and coastal area (sample plots). Intensity of attack is expressed by percentage of trees with egg masses for current year and these are classified into five classes, lowest having less than 1% "attacked" trees and highest having more than 50% "attacked" trees. Larvae of some insect pests are counted in some special circumstances and this applies more to forest nurseries where grubs and larvae of Scarabaeidae, Noctuidae and Elateridae are counted and if critical numbers are achieved suppressive measures have to be undertaken. On a qualitative basis, larvae are often laboratory reared and checked against parasitism or other biological agents expected to contribute the crack-down of heavy infestation period. Pupae or cocoons are sometimes being counted and used for assessment of population buildup for several pests among which more often T. pityocampa, N. sertifer, Stereonychus fraxini and Dendrolimus pini. Laboratory analysis of qualitative population elements is periodically carried out on pupae of the above mentioned pests. Adults are also targeted for population density assessment and this is done in several ways. Most often, adults are collected by some means of trapping technique. Well established method for assessment of population densities and critical population levels is used for two of the most important loopers: Erannis defoliaria and Operophthera brumata. Their wingless females are caught and counted on sticky trap-belts during the emergence period October-January. Another method for trapping the soil emerging pests is by using the wire-mesh covered cages (some Geometridae, A. abdominalis and acorn pests such as Curculio spp. and Cydia spp.). Modern methods for adult trapping in monitoring purposes involving sexual pheromones are used only fragmentary and more as an experimental addition to permanent evolvement of survey techniques (Paranthrene tabaniformis, Gypsonoma aceriana, Ostrinia nubilalis, Zeuzera pyrina).There is however one special method of branch sampling developed for specific needs of population density and defoliation risk assessment in our broadlived oak forests. Its originator, M. Harapin, introduced it by the late eighties with intention to better assess unclear situation regarding flowering and acorn production and causes for early crop loss (Harapin 1993). Another important issue were problems with population density assessment of some early defoliators which overwinter on oak twigs in egg stage. Today, this has become a widely accepted method providing fairly accurate predictions of what is going to happen in sampled forests in the following spring.
During late fall and winter, samples are taken from randomly chosen oak trees from upper strata. Usually, trees are felled down (which is mainly during the normal felling period in these forests), then 4-7 branches, 70-80 cms in length, are taken from upper parts of crown, preferably from all four aspects. Branches are transported to a centralized laboratory managed by DPS service where they are given water supply, room temperature and daily supply of sunlight. Underneath, white sheets of paper are secured on which early signs of defoliation can be recorded (excrements). Also, the defoliating fauna, their composition and densities can be easily assessed and related to available food supply, i.e. buds and sprouting leaves. Another important outcome of this method is prediction of flowering capacity. As in many neighboring countries, situation in Croatia, regarding more important insect pests is in many ways similar. There is more than twenty species of defoliators, sap suckers and xylophages screened regularly or on a periodical basis with various survey methods used in Croatian forests and forest nurseries. The most serious pests in the lowland riparian forests and partly in the hilly belt of continental Croatia are L. dispar, T. viridana and several geometrid species among which E. defoliaria and O. brumata lead in severity of attack.
During the past two decades three major peaks in L. dispar population density occurred. The highest one was by the beginning of eighties with more than 130000 hectares attacked. Second peak followed by the late eighties and the third one peaked in 1994 (Androić & Harapin 1996). Among other defoliating insects some deserve special attention due to their periodical and regional defoliating impact. Euproctis chrysorrhoea and Malacosoma neustria are good examples. Both are constantly present throughout the whole lowland region but not causing damages in larger areas. Apethymus abdominalis and A. braccatus are the two tenthredinids which were recorded in lowland oak forests by late fifties and have periodically entered gradation stages since then. Latest gradation was in mid-eighties.
Stereonychus fraxini is causing severe damages on Fraxinus angustifolia and there are several factors that hamper our efforts to efficiently survey and suppress this serious defoliator. Among other defoliating insects we would like to point out some rising populations of tenthredinid species among which Caliroa annulipes seems to have the leading role in the last three years. It started damaging young plants in some forest nurseries but in 1998 it heavily attacked some young oak stands.
Pine processionary moth is widespread and sometimes very important pest in whole coastal region. It is regularly monitored since it is always present in some parts in relatively large numbers. Abies alba, as our most important conifer species, has specific guild of defoliators. However, one lepidopterous species emerged as far most important and destructive. During mid-fifties Argyresthia fundella started its devastating gradation in the hearth of mountainous region. It remained important pest with this first eruption being the strongest. Problems regarding xylophages can be categorized in three main groups: technical damages occurring on logged and dying trees both in lowland oak forests as in fir and spruce forests of mountainous region (Trypodendron spp., Xyleborus spp. and Platypus cylindrus), physiological damages with resulting tree dyout (Agrilus spp., Pityokteines spinidens, I. typographus, Pytiogenes chalcographus) and combined technical and physiological damages in poplar and willow cultures caused by various Coleoptera and Lepidoptera.
Diagnostic and survey procedures practiced in Croatian forestry play important role in implementing the sustainable forestry principles. All the serious and most damaging insect pests are regularly monitored and their damage impact properly and aptly prognosed. Suppressive measures, coordinated from DPS service, are shifting toward ecologically more appropriate biotechnical and biological compounds. Less and less synthetic pesticides are used and when necessary, only those with lesser toxicity and shorter persistency are used.
Key words: diagnose; prognose; insect pests; population dynamics; gradation
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