Summary
The preference of aphids for certain potato varieties was investigated in the present study. Specifically the work dealt with the populationdynamics (reproductionrate, variety selection, lifetime, aphid infestation) of the aphid species M. persicae and M. euphorbiae on different potato varieties depending on infestation density and specific host plant characteristics (leaf hair density and plant constituents). In order to clarify the influence of the host plant on the reproductionof M. persicae and M. euphorbiae experiments were developed with eight commercial German potato cultivars (Albatros, Borwina, Fasan, Kormoran, Pirol, Romanze, Salome, Terrana) under controlled conditions (greenhouse and climate chamber) or field conditions. The above mentioned cultivars were the main varieties considered in this study. Other varieties and a wild species were also used in specific experiments. The procedures and results of the experiments are summarized as follows:

• The optimum reproductive performance of the aphids M. persicae and M. euphorbiae was investigated on eight potato cultivars, three old potato varieties and a wild species under different conditions (greenhouse, in vitro, semi-outdoor) (s. Chapter 3). The aphid species displayed different reproduction rates on the tested potato varieties under the different conditions, i.e. the susceptibility of the potato varieties (included the genetic resources in the form of old potato varieties and the wild species) was different depending on the aphid species. The in vitro studies showed that the two aphid species reproduced less on the old potato varieties and the wild species, than on the cultivars, i.e. 17% lesser by M. persicae and 24% lesser by M. euphorbiae. The comparison among the different methods showed big discrepancies on the reproduction rates within the same varieties. High reproduction rates were reported altogether on the cultivar Borwina. Both aphid species reproduced less on the cultivar Romanze.
• The effect of the leaf growth levels on the reproduction rate of M. persicae and M. euphorbia was assessed in individual leaves from the potato varieties in the greenhouse (Chapter 4). Both aphid species have a preference for a particular physiological age of the leaves. Older leaves were more suitable for M. persicae than younger leaves. In contrast, M. euphorbiae presented a higher reproduction rate on the upper leaves than on the lower ones.
• Experiments in the greenhouse and in semi-outdoor trials were developed to assess the selection of the aphid species for a potato variety (Chapter 5). Apparently M. persicae and M. euphorbiae were able to select the appropriate varieties.
• The colonization of the aphids on the potatoes was measured. M. euphorbiae was attracted by the varieties Salome, Fasan and Borwina, and M. persicae by Fasan, Borwina and Kormoran.
• M. persicae and M. euphorbiae were propagated in the greenhouse and their fecundity and longevity were investigated on eight potato varieties (Chapter 6). The reproduction rates differed between both aphid species. M. euphorbiae displayed a lower reproduction capacity than M. persicae. Regarding the longevity, the minimum and maximum lifespan was 22 days (Salome) and 30 days (Terrana) for M. persicae, and 23 days (Romanze) and 27 days (Terrana) for M. euphorbiae.
• The Chapter 7 covers the field trials developed to investigate the aphid infestation on different potato varieties at three locations. In addition, the spectral properties of the leaves were measured and their relationship to the aphids was tested. Besides the eight potato cultivars, two varieties (Alegria and Meridian) were studied in Rostock and fifteen varieties at the sites Sanitz and Gulzow. The aphid infestation differed among the locations during the years 2006 and 2007. The aphid species M. persicae, A. nasturtii, M. euphorbiae, A. solani, and A. Frangulae were observed. The results showed a correlation between the reflectance value of the different potato cultivars and the aphid infestation (M. persicae, A. nasturtii, M euphorbiae). This was especially clear for the 530 nm and 560 nm wavelengths (green area).
• The NDVI (Normalized Differenced Vegetation Index) value of the potato varieties ranged between 0.5 and 0.8. The resulting values showed a positive linear correlation between the NDVI and the aphid infestation in Gulzow and Sanitz. The trajectory of the WBI (Water Band Index) was reciprocal to that of the NDVI. There was a negative significant correlation between the WBI and the aphid colonization. The higher the water content of the potato plants the higher the aphid concentration.
• Another possible reason for the differential behavior of the aphids was the hairiness of the potato varieties (Chapter 8). The under leaf surface was more hairy than the upper leaf surface. The highest hair density of the potato varieties was observed in the greenhouse, followed by the open field and in vitro. The relationship between the hair density from the leaf underside and the abundance of aphids was significantly positive.
• In Chapter 9, the main nutritional components taken by the aphids, sugar and free amino acids, were studied on the tested varieties. The following aminoacids are underlined in descending order: alanine, threonine and valine. The variety Salome showed the highest concentration of amino acids and the varieties Alegria and Meridian the lowest ones. The sugar content of all potato varieties reached on average: 16 g * kg-1 TS fructose, 21 g * kg-1 TS glucose and 37 g kg-1 * TS sucrose. The lowest proportion of the total sugar content was detected in the Salome variety. The variety Kormoran contained the highest sugar levels, followed by Alegria. A significant positive correlation was found between the content of amino acids (Serin r= 0,48* and Threonin r= 0,71**) and the aphid infestation. At the other side, a significant negative correlation was shown between the aphid infestation and the sucrose content.

Based on the results presented here a clear influence of the potato varieties on the infestation of M. persicae and M. euphorbiae was detected. Future research approaches should involve other potato aphid species. For the potato breeding and the practical variety selection criteria should be developed that take into account the aphid susceptibility of the potato varieties and influence if possible the breeding.