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ŠUMARSKI LIST 11-12/2012 str. 50     <-- 50 -->        PDF

Dorsally, the females (Figure 1F) have three pairs of more or less distinctly elevated humps. They share this character with at least H. piceae and H. trichophorum. The posterior ringorgans (=osmoregulatory organs) are short-oval shaped (Figure 1B) and thus represent a specific H. ulmi character; they are not elongate-oval shaped as in H. piceae. The most distinct specific character of H. ulmi is the shape of the digitus fixus (Figure 1D) of the chelicera (easily visible in the females): the distal end is bulged downwards and clearly divided into three prongs with the dorsal one being longer than the following two prongs which have nearly the same length.
Voucher specimens of H. ulmi isolated from the galleries of A. reticulatus have been deposited at the Museum für Natur­kunde Berlin under ZMB 48507 (deutonymphs), ZMB 48508 (adults) and ZMB 48509 (adults).
Fungal spores attached to mite deutonymphs
Spore gljiva pričvršćene za deutonimfu grinje
The colony was exposed to conditions assumed to be unfavourable to the mites, in order to induce the development of more deutonymphs than would be produced normally. Although it was found that they do not prefer a wet or very moist habitat, but slightly drier conditions, for these purposes all kinds of moistening were ceased for about six days. The surfaces of the bark and the potatoes partly dried out.
Later, many deutonymphs were visible crawling around in larger aggregations in these drier areas of potatoes and bark. As is usual for histiostomatid deutonymphs, they preferred to occupy elevated structures such as the edges of potatoes or protruding bark splinters. There they performed – also typical for many other histiostomatid species – a behaviour during which they were fixed with their sucking plates on the ground with the whole body in an upright position and then moved alternating the body to the right and to the left. During this procedure they alternately moved the first pairs of legs. This behaviour is interpreted as olfactory carrier-searching behaviour (Wirth 2005).
Some of these deutonymphs were covered with fungal spores (Figure 1A). In the still moist areas, where the resting adults and non-deutonymphal nymphs remained, no fungus growth took place. Undetermined mold fungi grew only outside these areas. Sometimes the border between mite-development-area and fungus-area formed a sharp, distinct border. This is because histiostomatids are assumed to produce chemical fungicides in their opisthonotal glands (e.g. Wirth and Moser 2010; Koller et al. 2012). The whereabouts of the deutonymphs during the carrier-search often were inside or close to the fungal-areas, which is due to the fact that deutonymphs of Histiostomatidae in most species seem to separate themselves under laboratory conditions from the rest of their population to use elevated areas for a better perception of the olfactory particles of their carriers and better access by which to attach themselves (Wirth 2005). Some of them were visibly covered with fungal spores. Due to a ‘sticky’ cuticle surface of the mites, spores could adhere to the whole mite bodies. During the procedure of mounting such deutonymphs on light microscopic slides, many spores fell off. Lumps of spores obviously had a better hold in the areas between the dorsal shield and leg-trochanters I + II and in ventral areas, where the hysterosoma-shield laterally bulges downwards, and there remained visible also on the mounted objects (Figure 1A).
Observed deutonymphs could walk while covered with fungal spores. It is unknown whether they are able to attach to a carrier under these conditions and whether they would be successfully able to reach a new bark habitat by phoretic transport. The phenomenon of mite deutonymphs carrying fungal spores could be of an applied importance if it could be shown that they are able to disperse germinable entomopathogenous fungi such as the blue-stained fungus. Further examinations are needed.
Discovering Acanthocinus reticulatus as the first bark infesting settlers was unexpected. Usually bark beetles of the genus Pityokteines were first observed infesting the bark of silver fir, with insects such as the Cerambycidae arriving later. More rarely, Pityokteines and Cerambycidae arrived at the same time (pers. obs. M. Pernek). Our T1 tree indicated for the first time that Cerambycidae can also sometimes arrive as the first bark-damaging pests. It is unknown whether this specifically concerns silver fir and A. reticulatus. Quantitative studies as a specific investigation of this phenomenon still need to be done. It can also happen that different parts of the tree such as the crowns can have different insect invaders than the lower trunk (pers. obs. M. Pernek). This is why we observed pieces at different levels of the trunk, but different levels of the crowns remained unobserved. Besides galleries of P. curvidens and P. spinidens, galleries of A. reticulatus were not visible in our T2 tree. It is unknown whether the longhorn beetle in this tree also appeared as the first invader before Pityokteines or not.
In previous studies, deutonymphs collected directly from beetles of P. curvidens and P. spinidens were determined as H. piceae (Pernek et al. 2012). These deutonymphs were not reared to adults to provide a direct comparison with H. ulmi from A. reticulatus. Due to the morphological similarities of H. ulmi and H. piceae-deutonymphs, it is possible that both species appear in the silver fir bark of the studied area in Croatia. Perhaps H. ulmi rides on A. reticulatus and H. piceae on Pityokteines, or perhaps the deutonymphs found