Excellent follow up questions. I will address each one itemized:
1) How long does diatomaceous earth take to kill black widows?
I am not exactly sure, as I am not a pest control expert. Feedback I have gotten is singificantly reduced spider, ticks, and other insects within one week.
2) Is any contact effective or does the powder have to be on the main body (as opposed to just walking across a powdered area?
They just need to walk across it, as it penetrats their exoskeleton on contact.
3) Are repeated applications necessary for the first kill?
Weekly unless there is a big rain or string of consecutive days with rain, then i would reapply sooner.
4) How often do you recommend future applications to prevent future infestations?
As a preventive measure, you can apply biweekly throughout the spring and summer months, bump it up to weekly if you find that pest control is wanting.
5) If the powdered area gets wet and then dries out, is the powder still effective?
It is definitely still effective, but after a good rain, it may drain off or absorb into the soil to a point that the concentration is not high enough to be effective. As such, it would be wise to re-apply after a day of torrential rains, or after 2-3 days of mild to moderate rain. You can physically see it in the grass and soil, as it is a different color than regular dirt..spot inspection after rain can help to let you know if it is time to reapply.
6) I have heard from multiple sources that none of the consumer level products is effective for black widows. Will Raid spider killer really do the job?
Raid Spider Killer should kill any spider, but again, I'm not a pest control expert.
7) Does Raid spider killer pose any risk to people or pets?
It is probably not ideal to have prolonged contact with this product human or pet, which is why I would only use it for spot treatment on the aforementioned vertical areas and corners, rely on the organic diatomaceous earth for the more broad treatment.
Hope this helps. Please let me know if you need further clarification on any points. Best regards,