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ŠUMARSKI LIST 1-2/2015 str. 69     <-- 69 -->        PDF

Bacillus thuringiensis kurstaki, commonly used for the suppression of the local populations of L. dispar, does not always produce the desired impact. Consequently, the highly selective, host specific, fungal pathogen that could be put in use as a biological control agent of the L. dispar populations is found to be very interesting for both science and professional application thus deminishing the economical and ecological negative feedback of this indigenous defoliator. The research was carried out during July 2013 throughout the north-east part of Bosnia and Herzegovina where gypsy moth populations continued an ongoing outbreak. From five selected localities, where excessive mortality was observed, larval cadavers were sampled from tree trunks 0,5 – 1,5 m above the ground. Only older larval stages (L4 – L6) were sampled due to the late sampling period. Cadavers were inspected, and larval tissue samples were prepared under the stereo microscope (LEICA Leitz MZ8) and light stereo microscope (Motic SMZ – 168 TLED). Process was digitaly documented with Olympus SP – 500UZ digital camera equipped with the Olympus QuickPHOTO CAMERA 2.3 digital imaging software. Larval tissue samples were inspected under light microscope (Olympus BX53) and images were recorded by digital camera Motic MoticamPro 252A. Measurements of azygospores and conidia (conidiospores) were made via digital imaging software Motic Images Plus 2.0 and Motic Images Advanced 3.2. associated with a compound microscope. Microscopic analysis of the dead tissue larvae, in all five locations, confirmed both conidia and azygospores or azygospores only, of the E. maimaiga species. Spore dimensions were as follows: pear-shaped conidia crosswise 25,7–35,1 µm and 34,6–43,7 µm lenghtwise; azygospores 32,2–47,9 µm in diameter (Figure 2, Figure 3). The type of spores (conidia or azygospores or both) that will form after host death is determined by the pathogen and the type of host infection, host-related factors and environmental conditions. Macroscopic symptoms of E. maimaiga attack were easily recognizable on the tree trunks along with some signs of larval mortality caused by Lymantria dispar multicapsid nuclear polihedrosis virus (LdMNPV) (Figure 4). There were very few signs of parasitoid mortality, but unlike in the Croatian sites (Hrašovec et al. 2013), with a great abundant presence of gypsy moth predators like Calosoma sycophanta L. (Figure 5), which could be an indicator that the pathogen has emerged when the L. dispar population was already starting its descent into a retrogradation phase. Just like in the Croatian localities where the sampling took place (Hrašovec et al. 2013), dead larvae were hanging from the tree trunks head down all through the sites and no living larva or viable pupa could be found in the area. Based on the field collections and microscopic analysis, entomopathogenic fungus E. maimaiga, a pathogen of L. dispar introduced on the European continent, has been confirmed in Bosnia and Herzegovina. Extensive monitoring of the fungal pathogen in the following years will give us the information on pathogen spatial spread, its speed and the possibility for establishing its permanent position among the other local indigenous species. Nevertheless, there are some concerns whether E. maimaiga may show some direct and indirect impacts on non-targeted and beneficial organisms in the future, or change the basic community structure of folivore insect guilds on oaks, maybe resulting in increasing populations of other defoliating insect groups such as tortricids, geometrids and sawflies. These concerns will demand more scientific attention in the future.
Key words: Entomophaga maimaiga, Lymantria dispar, biological control agent, fungal pathogen, spatial spread, mortality, impact, natural enemies, beneficial organisms, defoliators