The spider mite Tetranychus urticae poses significant threats to agriculture by destroying numerous crops and developing resistance to pesticides extremely fast. Developing efficient genetic tools is crucial for understanding adaptation mechanisms in this species. A recent breakthrough in CRISPR/CAS9 gene editing now allows to develop T. urticae into a genetic model pest. To further increase editing efficacy and allow insertions of large DNA constructs, we will map and alter the DNA repair pathway in T. urticae. By disrupting key DNA repair genes, we will study the effect on the frequency of homology-directed repair (HDR). In addition, based on a previously developed delivery formulation, we will investigate whether recombinantly expressed PiggyBac transposase/PhiC31 integrase can mediate insertions of large DNA fragments via maternal protein injection. Eventually, this tool will be used to create a transgenic mite strain expressing Cas9 in the germline, hereby further improving cost-effectiveness and efficiency. Last, we aim to knock-in the P450 CYP392A16, a main xenobiotic metabolizer, and provide evidence for its role in acaricide resistance and host plant adaptation. This project will provide important insights into DNA repair mechanisms in Acari and will develop strategies to advance gene editing in mites and other difficult to transform pest insects.