[I]The D50 volume for humans for this toxin is between 0.43 and 1.39mg/kg, which is more than a single spider can usually produce, but is considered more than enough to kill a small dog or cat and is still enough to occasionally create a life-threatening condition in some people.[/I] -Wikipedia

Yup. Should totally leave the spider the can kill your dog and almost certainly hospitalize you alone because it's totally not in a position to bite MDK's dog on the off chance it would eat a lesser pest.

Makes sense to me! You should go Google black widow bites and let us know if it inspires you to not remove them from your yard.