Phthorimaea operculella granulovirus (PhopGV, Baculoviridae) has the potential to serve as biological control agent against Phthorimaea operculella (Zeller) and Tecia solanivora (Povolny) in the potato production cycle and Tuta absoluta (Meyrick) in tomato production under greenhouse conditions. These three pest insect species are closely related and belong all to the Gelechiidae family of Lepidoptera. Due to the climate change and global trade these pest insects have the potential to spread worldwide. PhopGV has already successfully been used as biocontrol agent in Latin America and North Africa mainly against P. operculella in potato field and storehouses. Research on the diversity of PhopGV isolates and their interaction with the host is strongly needed, to gain knowledge which allows optimizing the use of PhopGV as active ingredient of biocontrol agents of these pest insects in food production. This study focused on PhopGV isolates and P. operculella as host system. It embraces the interaction of PhopGV isolates in case of co-infections and the interaction of PhopGV and a microsporidium when infecting the same host individual. A number of nine PhopGV isolates were tested on their biological activity against P. operculella. Median lethal concentration (LC50) and median lethal time (LT50) were determined as comparable measures of isolates´ virulence. PhopGV is a slow-killing virus which is able to inhibit pupation of infected host species. Virulence of different PhopGV isolates seems to be not only virus but also host dependent. Twelve complete genome sequences of PhopGV isolates from passages of virus isolates collected from four different continents (Africa, South America, Asia and Europe) were analysed after Illumina Next Generation Sequencing (NGS). These geographic isolates of PhopGV are genetically highly similar but were rarely genetically homogeneous and appeared in most cases as mixtures of multiple genotypes. A new grouping system (1-4) could be developed based on single nucleotide polymorphisms (SNPs) as well as insertions and deletions (Indels) spread over the PhopGV genome. Further, a highly variable gene of the superoxide dismutase (sod, ORF 54) was identified. Previously, only variability of ecdysteroid UDP–glucosyltransferase (egt, ORF 129) alone was used as a grouping system for PhopGV isolates. Virus infections of insects can easily stay undetected, without showing typically signs of a disease and do not need to be lethal. A virus named PhopGV-R could be isolated from a laboratory population of P. operculella. Crowding of larvae did not cause overt outbreak of the covert virus. An infection with a second homologue virus (PhopGV-CR3) activated the internal virus. Whereas a third isolate, namely PhopGV-GR1, was able to suppress the internal virus and showed superinfection exclusion. This research shows that stable virus infections seem to be common for insect populations and have an impact on population dynamics. It revealed that PhopGV isolates can either tolerate or block each other. A potentially new Nosema species (Nosema sp. Phop) was purified from microsporidian infected individuals of P. operculella. It was found that an infection of P. operculella larvae with Nosema sp. can reduce PhopGV-caused mortality and thus showed an antagonistic effect against PhopGV. These findings of virus-virus and virus-microsporidium interaction can help to predict the mode of action if PhopGV is applied against P. operculella field populations, where other PhopGV isolates or microsporidia can naturally occur.