Can I mix neem oil and biopesticides in the same spray tank?

Some combinations are fine and some destroy your costly biopesticide, so this matters. Neem oil generally mixes well with bacterial and viral biopesticides like Bt and NPV and can even improve their effect, because neem stresses the pest while Bt or NPV infects it, and many farmers tank mix neem with Bt or HaNPV for caterpillars. However, be careful mixing neem oil with living fungal biopesticides like Beauveria, Metarhizium, Trichoderma or Pseudomonas, because strong neem and especially any added soap or chemical can reduce or kill the fungal and bacterial spores, so it is safer to apply microbial agents separately or check the specific label. The firm rule is never tank mix any biopesticide with a chemical fungicide or bactericide, that will kill the microbes. Always read each product label for compatibility, do a small jar test by mixing a little and watching for clumping or curdling, and use the mix fresh, do not store it. Spray microbial products in the evening. When unsure, apply them on separate days. Your KVK or the product company can confirm specific tank mixes.