Yep, Industrious can be a tricky one to get. Here's what I normally do in that situation:
- Have a few different breeding pairs of Diligent and Unweary going at the same time, all with max fertility.
- When I get an Industrious hybrid or pure drone out of the mix but the princess is still Diligent and/or Unweary, squirrel the drone away and keep breeding.
- During breeding, try to keep the number of species traits the same on both sides - IE, Diligent-pure goes with Unweary-pure, Diligent-Unweary goes with Unweary-Diligent. I don't think whether the species is active or inactive makes a difference (breeding two Diligent-Unweary bees is the same as breeding two Unweary-Diligent ones), but not 100% sure on this one.
- If I happen to get a purebred princess, save her away until I have a purebred drone to go with her and just continue with my other breeding pairs. Not worth risking losing the species by breeding with hybrids.
- If I get a hybrid princess, put her with a hybrid drone of the other type (Diligent-Industrious goes with Unweary-Industrious), still using soul frames. Theoretically that'll increase chances of getting the target species a bit more, because you could get another mutation from the Diligent/Unweary side during breeding. Might not work well for breeding combinations where more than one type of mutation is possible, though!