engleski

The up rise of xylophagous insect populations in Croatia as a consequence of climatic changes

The recent decade has been characterized with extreme climatic conditions resulting with dramatic developments in forest health status and severe insect outbreaks in Croatian forests. Expectedly, hot springs and summers with prolonged dry periods enabled the multiplication of xylophagous populations, notably bark beetles, jewel beetles and loghorn beetles. The most suitable year for the insect proliferation in one hand and physiological stress on trees in other, was undoubtedly the year 2003. Almost all of the most widespread tree species in Croatia suffered from various secondary xylophages attack. In some instances, first occurrences of certain insect species attacks were recorded. The intensity of tree loss due to the attack of most harmful insect species has not decreased even in the years following 2003. The salvage cuttings are still being applied in majority of the conifer forests while in some of the riparian forest communities new complications arose due to the problems of reforestation and land conservation. Silver fir, being the most important and widespread conifer in Croatia, suffered most and the whole spectrum of proliferating xylophages can now be easily isolated from fir trees. The most important ones are bark beetle species from the genus Pityokteines. Among these, P. spinidens turned out to be the most numerous, consistent and deadliest one. First symptoms of bark beetle attack most often are top kill or branch dyout caused by the attack of P. spinidens and P. vorontzowi. Further process of attack is lead by P. spinidens attacking gradually lower parts of the bole, often together with P. curvidens and other xylophages like Pissodes piceae. Branches of still living firs or those freshly cut attract other species of bark beetles, most commonly Cryphalus piceae and Pityographus pityographus pityographus, the species considered highly damaging in some parts of Bosnia and Herzegovina. Rich spectrum of aforementioned xylophages, together with the polivoltinous character of the most damaging species, resulted with heavy tree dyout and in some locations dramatically changed standing tree species ratio in favor of broadlives. Some cases of dramatic salvage cuttings are described in which 10-year approved amounts of regular felling was reached in a couple of years. It is of equal importance to underline that majority of the wood felled in this way had lesser value, if any, and further complicated the whole process of forest management and forest protection. Spruce, being less important in Croatian forestry, still showed a serious signs of physiological stress and suffered from the attack of the two most common European spruce bark beetles, Ips typographus and Pityokteines chalcographus. Probably the most dramatic occurrences of these two bark beetle species causing tree dyout was recorded (and still is) in the mountaineous area of Lika and the two National parks, "Plitvička jezera" and "Sjeverni Velebit". Due to the strict regime and nature conservation approach, in some instances only mechanical countermeasures (debarking) and pheromone traps were deployed to stop the outbreak in the most valuable microlocations. Pines, both Pinus nigra and P. sylvestris, grow mixed in many continental and maritime forests, in many cases they form artificial cultures which are especially prone to xylophage attacks. The past decade has been important since there were several field cases of outbreaks of Tomicus piniperda which clearly acted as "tree killer". Starting points of the further developing foci usually were snow-felled or broken pines in which population found suitable substrate and than colonized surrounding trees. In three years, circular foci widened, reaching the diameters of 20 to 40 meters engulfing over 100 trees killed exclusively by this species. Off course, the whole spectrum of other bark beetles and other xylophages followed and some of the most important were: Tomicus minor, Ips acuminatus, Ips sexdentatus, Xyloterus lineatus, Acanthocinus aedilis, Rhagium inquisitor. None of these are unknown nor rare in forest practice. The important fact is that their ability to attack and develop successfully rose dramatically in the described period and in the case of T. piniperda enabled them to overcome the defensive capabilities of otherwise healthy pines. Another proof of exceptional weather conditions that spurred the xylophage populations is the first record of Tomicus destruens attack on Pinus halepensis trees in the south of the Istria peninsula. The attack was initiated by the forest fire and quickly developed into a outbreak which had to be stopped by salvage fellings and removal of infested pines. Broadlives seem to be less affected by the climatic extremes, at least considering the attack of xylophagous insects. Still, a number of cases of otherwise less known pests prove that the same process was affecting them as well. Beech has a number of xylophages but is rarely known as problematic in this respect. Due to the extremely unsuitable weather conditions it in some areas in continental Croatia a heavy attacks of Taphrorychus bicolor were recorded and sanitary fellings had to be enforced. The impact of Xyloterus domesticus also might not be neglected as well as some other wood boring bark beetles from the genus Xyleborus. Oaks were also the object of heightened xylophage attacks but generally, the effect of severe water regime and heat was somehow dampened by the availability of underground water, at least in the case of pedunculate oak. However, a number of occurrences of well known xylophages was recorded, the most characteristic ones being Scolytus intricatus, Xyloterus signatus, Xyleborus saxesenii, Agrilus biguttatus, Coraebus florentinus and several cerambycid species. Along the Mediterranean coastline and on the islands the holm oak suffered heavily from various xylophages, the most abundant probably being Agrilus angustulus that locally caused dyout of large parts of the crown and even killed some trees. Maybe the most striking example of xylophage attack triggered by the climatic effect was the case of population buildup and tree dyout in flood forests of poplar cultivars due to the attack of Trypophloeus species. This was the first record of this poplar attacking bark beetle and immediately in a dramatic way. The direct cause for the dropdown in physiology of affected poplars was a severe late frost that was followed by the attack of Trypophloeus beetles. The paper brings more details of the aforementioned xylophage occurrences, the intensity and magnitude of their attack, history of their population buildups in recent years and probable connection binds with the weather conditions in the same period. It also considers the phytosanitary situation in surrounding countries as well as some differences in respective pest occurrence and pest management strategies. It discusses the possible countermeasures and reasons why in some instances outbreaks diminished and in others did not. A projection for the following period is done on the basis of existing management policies and expected weather patterns as well as multitude of other biotic and non biotic factors affecting forest ecosystem.

xylophages; bark beetles; outbreak; climate change; drought; Croatia

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