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ŠUMARSKI LIST 11-12/2016 str. 26     <-- 26 -->        PDF

more on a suitable habitat around than on the vitality of the beetles. Larvae of I. gaebleri nevertheless seemed to have an affinity to the cadavers of beetle larvae or pupae (Fig. 5 A, B, G). But this is discussed further below.
Both bark beetle species, Ips cembrae and I. typographus, were mostly associated with the same species of phoretic mites. Not all of them were originally described as being associated with different carriers. Scheucher (1957) had described Histiostoma piceae as being strictly specific on I. typographus only. But several authors discovered H. piceae also on other beetle carriers, as e.g. Pernek et al. (2008, 2012), who found deutonymphs attached to Pityokteines curvidens Germar, 1824 In order to verify, whether H. piceae was correctly determined on I. cembrae and I. typographus, we reared the isolated mites. Adults and deutonymphs from both beetles were identical to the description of Scheucher (1957). Thus we could confirm that H. piceae is not specifically associated with I. typographus, but at least in Central Croatia also with I. cembrae. Due to the lack of comparative studies on other bark beetle species based on all developmental mite instars, we don’t know, whether this mite is restricted to those two bark beetle species, representing sibling species to each other, or whether H. piceae has a wider range of phoretic carriers. The correct determinations of H. piceae on other bark beetle species, based on the deutonymph only, need to be verified in the future.
The undetermined species Histiostoma sp., which was only available as adult females in the galleries of I. typographus, is assumed being adapted for carrying soil particles as tactile camouflage as common in the Histiostoma-bark-beetle-clade (Wirth, 2004). As there were no limitations in the behaviors of the observed specimens visible, the numerous fungus spores, which covered their whole bodies, were not interpreted as artefacts, but as an intended phenomenon. Being covered with substrate, which can also consist of fungus spores, might impede the tactile detection by predators. It is not known, whether this mite specifically carries spores only or whether it would cover its body with all kind of available soil. The specimens of these studies carried additionally to the two-chambered spores also the spores of different other fungal groups together with soil particles. This might support the theory that dorsal structures could function as substrate holders „on purpose“.
Although the spores were discovered attaching on different body parts, the conspicuous dorsal setation seems mainly to support the holding of particles. The fungus might benefit from these mites due to a dispersal of its spores.
Generally, different instars of histiostomatids from the bark beetle group were repeatedly observed carrying fungal spores, at least in a lower number, e.g. found on specimens of Histiostoma ovalis (Wirth & Garonna, 2015).
Scheucher (1957) assumed that very different arthropods might act as carriers for Bonomoia pini, such as chilopods, tenebrionids or coccinelids. On the other hand she stated that all these putative carriers need to visit galleries of the bark beetle Hylastes ater regularly as she only inside those galleries had discovered the stages of B. pini including the deutonymphs, which according to her findings hibernate under the bark. In case we correctly determined B. pini based on male specimens in our study about I. typographus and I. cembrae, our findings indicate a wider range of phoretic bark beetle carriers than assumed by Scheucher (1957). Based on the deutonymph, B. pini was already repeatedly discovered on numerous bark beetle species before. Penttinen et. al (2013) for example name I. typographus as a phoretic carrier. B. pini thus seems not being specific to galleries of H. ater only, but prefers at least different bark species of Curculionidae. Whether even other arthropods can carry this mite, up to date remains an unproven speculation of Scheucher (1957).
The tarsonemid mite I. gaebleri (Fig. 5 C) was already known to be carried by different bark beetle species and was here undoubtfully determined by W. Magowski. It was for example known being associated with Ips borealis and I. pilifrons (Boss et al. 1970). But also the phoretic association with I. typographus was documented in Moser et al (1989). A statistical comparison of I. gaebleri individuals between two different phoretic beetle carriers in Croatia were never performed before. The results show that there was no equipartition between mite numbers on I. typographus and I. cembrae. Our studies showed a significant higher number of mite specimens on I. cembrae. This result might be due to unknown ecological preferences of the mites. A putative argument for the missing equipartition could be the slightly bigger size of I. cembrae in relation to I typographus.
Dendrolaelaps quadrisetus (Fig. 6 B) is a common phoretic guest on different bark beetle species. According to Moser (1995), it appears in a large number of habitats and on a large number of „insect host species“. Mites of the genus Dendrolaelaps feed as predators on smaller organisms, often on nematodes (Kinn, 1984). We assume that D. quadrisetus might be a predator of juvenile histiostomatids.
The very low abundance of Urobovella sp. (Fig. 6 C) on beetles is not clearly explainable, but might be a consequence of competition between different phoretic mite groups on a beetle. Especially the very high numbers of Iponemus gaebleri specimens might act as a limitating factor for uropodids, which thus putatively were prevented to detect a suitable area for a stable attachment.
Our findings about the preferred positions of phoretic mites on their carrier beetles indicate that attachment places were not occupied accidentally. Obviously the competition between different mite groups resulted in occupying different areas on a beetle. Preferences of phoretic mites for specific areas on their carriers were for example described for Histiostoma ovalis on Ips sexdentatus (Wirth & Garonna, 2015). According to the cited paper, also movements of the beetle might influence the final positions of mites.
Our most important findings of specific interest for the forestry research was the very high abundance of I. gaebleri, whose females use to feed on bark beetle eggs (oral