This is an example of where I think it's important to understand that the genotypes heterozygous and homozygous and how they apply beyond just recessive. Knowing that you need to make a homozygous pastel you can see that both parents must be at least heterozygous pastel. Likewise both parents must be at least heterozygous axanthic for the same reason. Knowing that you need to make a heterozygous spider you can see that only one parent is required to be at least heterozygous spider.

So the minimum parents that could make an axanthic killerbee would be bumble bee het axanthic x pastel het axanthic. Both of those parents could be produced from originally breeding a bumblebee to a homozygous axanthic. Still a daunting multi-generational project requiring some luck with the odds but I bet the results would be well worth it.