Abstract
Invasive species pose a major global challenge, with insects being the most frequent invaders. This is problematic, since insects’ interactions with the environment can cause heavy damage to agricultural productions. One prominent example of an invasive insect species is the Brown marmorated stink bug Halyomorpha halys (Stål, 1855) (Hemiptera: Pentatomidae), which is native to North-Eastern Asia and invaded at least 38 countries in North and South America, Africa, and Europe since the 1990s and will most likely continue to spread. H. halys is a polyphagous species that has more than 300 host plants. By feeding with its piercing-sucking mouthparts, it damages fruit, which causes severe economic losses. The most impacted crops are peach, pear, and apple, however, also many others as hazelnuts, tomatoes, or beans are under risk of feeding damage. Most studies that investigated the host plants of H. halys focus on visual surveys, which are time-consuming and do not clearly identify, if the individuals do actually feed on the plants. Therefore, in this thesis a molecular gut content analysis workflow was developed to identify H. halys’ feeding plants and to determine how long the DNA is detectable in the insect’s gut. The detectability of DNA was examined in a feeding experiment that lasted for 11 weeks. The individuals were collected in the field in November and overwintered in the laboratory, before being used for the experiment. After the first three weeks, the diet was changed, and individuals were collected after different time intervals to see how long the plant DNA from the first diet can be detected. To perform the molecular gut content analysis, the gut of the single insects was dissected and used for DNA extraction. A PCR amplifying the ITS2 region of the plant DNA was performed, and the amplicons were sequenced on the Oxford Nanopore Flongle device. Surprisingly, the experiment revealed, that plant DNA from the field, ingested before overwintering, can still be detected. This can be explained by the gut anatomy of Pentatomids. They have a constricted region in their midgut and the liquid parts of the diet are excreted via the Malpighian tubes, whereas the solid parts, which are very limited due to the feeding style, remain in the gut for several months. After confirming the suitability of the molecular gut content analysis on H. halys, the approach was implemented on individuals collected in the field across the season. This experiment allowed to elucidate what the feeding host plants of H. halys are in South Tyrol and if they differ in different seasons. A total of 65 different plant genera was detected, with Salix, Robinia, Malus, and Humulus being the plants that were most frequently fed on. Moreover, differences between the seasons were proven, showing that in summer mainly Prunus is important, whereas Phaseolus was detected in winter and Humulus and Acer serve as feeding hosts all across the year. These results did clearly highlight the importance of ornamental plants on the diet of H. halys and should be considered for future visual surveys. Moreover, the methodology can be applied in newly invaded regions, to quickly investigate the ecology of H. halys, and to apply more targeted control measures. The host plants of H. halys were not the only focus of interest in this thesis. Previous studies showed the importance of primary symbionts and the whole gut microbiota in insects. Primary symbionts are essential for the fitness and survival for their host, but also the whole gut microbiota can influence fitness parameters. H. halys harbors the primary symbiont ‘Candidatus Pantoea carbekii’ in the midgut that provides essential amino acids and vitamins to its host. H. halys is highly dependent on its symbiont and if it is missing, the development is delayed or the individuals cannot develop until the adult stage. Although the primary symbiont is well investigated, research did not focus on the composition of the whole gut bacterial community. Therefore, the whole gut microbiota was investigated from two different groups. The first group consisted of individuals collected in the field across the season and the second group from individuals from the rearing, collected in generation 0, generation 3, and generation 5. The gut was dissected, and the DNA was extracted, of which the bacterial community was investigated by a 16S rRNA gene metabarcoding approach and sequenced using the Illumina platform. The results showed a clear seasonal pattern in the composition of the gut microbiota, where the symbiont Pantoea is present in higher relative abundances in the winter months, whereas Commensalibacter is more dominant in summer. The overall diversity was higher in winter than in summer. Moreover, the comparison between field-collected and laboratory-reared individuals showed, that the gut microbiota starts to change quickly in a rearing facility, as generation 0 already showed significant differences in the community composition in comparison to the field-collected individuals. The gut microbiota further adapts to the rearing conditions, as generation 3 and generation 5 do also significantly differ compared to generation 0. These results are especially interesting in the light of parasitoid rearing. As the natural enemies need to be reared on the egg masses of H. halys a loss of fitness lowers the efficiency of the rearing. Furthermore, the function of Commensalibacter should be further investigated in future studies to understand if it is present regionally or globally and to better define its role in the life cycle of H. halys.