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ŠUMARSKI LIST 11-12/2012 str. 52     <-- 52 -->        PDF

originally discovered by the describing author Scheucher (1957). Our studies might indicate that H. ulmi is switching between carriers of two beetle groups. This agrees with the original findings by Scheucher (1957) that besides Scolytinae the mite also switches to another bark-inhabiting beetle, which in our case is a cerambycid. This beetle species, as well as the Pityokteines-species, were never previously found associated with deutonymphs of H. ulmi. Our new findings might indicate that the mite has a wider range of carriers and tree hosts than assumed by Scheucher (1957).
Histiostoma ulmi was not found in samples from the T2 tree. This might suggest that most specimens had already left the tree together with newly hatched Pityokteines beetles and A. reticulatus (in case it was also there). Alternatively, it might indicate that this tree had never been colonized by H. ulmi.
However, because the real biodiversity of histiostomatid species in general has not been fully discovered worldwide, new species could also appear among these bark beetle associates. Cryptic species-groups are characterized by very similar looking species (Wirth 2004). The H. piceae-group is such a group containing cryptic species. Due to the morphology of apodeme a1 of the mite that we determined as H. ulmi, looking slightly different from that which Scheucher (1957) depicted, the mite might also represent a subspecies of H. ulmi, or could even be a new species very closely related to H. ulmi. Were doubts about the species status to remain in future, the species could in further papers be named Histiostoma cf. ulmi. Different kinds of characters such as ecological, morphological, biochemical and genetic information would help to clarify the species status.
The cuticle surface of H. ulmi appeared to be sticky. This may be due to secretions of dermal glands containing long-chain hydrocarbons and fatty acids, as was discovered by Leal and Kuwahara (1991) for some Astigmata, which produced in detail ester, medium sized hydrocarbons, higher hydrocarbons and sequestered compounds from food. This ‘sticky’ cuticle surface in deutonymphs maybe intensifies the natural adhesiveness of the fungal conidia and could ensure a permanent fixation of spores in some ventral body areas ‘full of nooks and crannies’.
Phoretic deutonymphs of H. ulmi mites were observed carrying fungal spores under culture conditions. If future studies could prove that they also carry blue stain fungi (Ophio­stomatidae), then this could be of practical importance in fir decline. Considering that H. ulmi has been vectored by bark and longhorn beetles, future studies need to show how and whether this knowledge could be used in forest protection management.
We thank J.C. Moser (USDA, Pineville, USA) for his corrections of our written English and the Croatian Forest Research Institute in Jastrebarsko, Croatia, for all kind of support.
We thank Jason Dunlop (Museum für Naturkunde Berlin) for correcting the English of our final manuscript. S. Wirth thanks J. Dunlop furthermore for offering him a working place, and he thanks W. Sudhaus (FU Berlin) generally for all kinds of biological discussions. Furthermore we would like to thank Mandica Dasović, Anamarija Petreković and Željko Kauzlarić (Croatian Forest Ltd.) for supporting our field work.
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