Effets redoutables d'un grand froid hivernal sur la relation du diptere mycetophilide Sciophila sp. et du polypore Phellinus tremulae, en haute maurienne

This work provides an improved interpretation of our previous observations concerning ecology of Sciophila sp., a Mycetophilid (Diptera) living as a [much less than] symbiont [much greater than] on Phellinus tremulae which is a specific parasite of Populus tremula in subalpine forest islands. In a recent paper (2005), we have shown that Phellinus tremulae is strictly eaten by larvae of Sciophila sp. and that the Mycetophilid contributes to the dispersion of this species in aspen islets. It seems that the ecology of this insect has not been previously described. This relationship has been observed since 1978-1979 at Bonneval/Arc (Isere), as well as at other places in the Alps. It has been concluded that the relation between P. tremulae and Sciophila is mutually beneficial: Sciophila larvae are slowly digging superficial furrows in the hymenium and nibble spores of the Polypores, which, in return, find a benefit when flying Sciophila transport spores from place to place and show a great degree of specificity for their breeding habits. However, in summer 2005, we have been disappointed by the fact that Phellinus and Sciophila have strongly suffered among aspens of Bonneval. Some Polypores are seriously dried. Moreover, many larvae of Sciophila have been killed. Webs which usually protect these animals are reduced to debris scattered on hymenium of Polypores. In fact, very few larvae are remain alive. It seems interesting to determine what sort of factors may induce such a degradation. In an attempt to get some information, meteorological events could were studied in the vicinity of Bonneval (Bessans). Maximal-temperature mean in June-July was 21.4[degree]C for 2005, compared to 19.5[degree]C for the previous 10 years. This difference cannot be responsible of the observed mortality. In 2005, pluviometry was normal and storms have not been particularly violent. From 2004 October to 2005 Marsh, snowfalls were relatively limited: 332cm versus 400-450cm on average for previous 10 years. Moreover, maximum thickness was around 120-160cm against 200-230cm during previous years. This last point is striking. We note that minimal winter temperatures have been unusual; for example, -16.7[degree]C have been observed during February 2005, against -9.9[degree] for the same month of 10 previous years and even -8.8[degree]C for 20 previous years. The weather was also very cold during 4 months corresponding to winter period. A temperature of -35[degree]C has sometimes been recorded depending on exposition. It now appears that death of most Sciophila larvae may be the result of an excessive cold allied to low snow thickness: frost killed larvae. We have to note that in the past, 1979, 1991 and 1995, we observed some similar facts: Phellinus tremulae individuals and Sciophila larval-webs have been partly affected. These were exceptions, clearly depending on their location in the forest and their positioning along trunks. Moreover, in the western part of the forest, during the summer of 2005, just before metamorphosis, larvae of a cherry-tree moth (Hyponomeutidea: Yponomeuta padella L.), were climbing on trunks of Populus, covering Polypores and Sciophila larvae with dense webs. Although cyclic, such an invasion is quite unusual, ordinarily limited to the host-plant or its neighbouring bushes or rocks, but in 2005, aspens were also dressed with a sort of grey shroud. It is clear that such webs contribute to Phellinus and Sciophila degeneration. It should be noted that Yponomeutidea lay their eggs on low branches of the host Aria nivea, which is abundant around Bonneval: snow is always high enough to protect them during very cold winters. But this situation is only secondary, restricted to a small lower part of the forest island. In fact, the P. tremulae population, associated with Sciophila, is slowly climbing among masses of fallen rocks of sub-alpine zone. Aria nivea cannot follow such trees, which are able to survive in spite of drastic ecological conditions. However, we suggest that partial destruction of Phellinus and Sciophila could induce lasting disruptions in the model association described, as long as meteorological disturbances may persist from year to year.