Joe, IMO you should figure out what is attracting the spiders, and get rid of them
, before you start spraying. http://whatdotheyalleat.com/whatdotheBlackwidoweat.html
Here is an excerpt from below: How to Get Rid of Black Widow Spiders
"Make the outside of your house and your yard inhospitable to black widow spiders. The first thing you will want to do is to clear away any and all vegetation that might be up close and personal with your house, especially ivy (read: how to get rid of ivy). That stuff is a haven for spiders. Cut back shrubs, plants, and decorative grasses. Get rid of rock piles, and move any wood piles as far away from your house as possible. Don't use any outdoor lighting. Outdoor lighting attracts bugs galore. Black widows eat bugs. Do the math. If you must use outdoor lights, go with yellow to minimize the number of insects you lure in."
"Go on a search and destroy mission. One of the best ways to kill a black widow is simply to squish it. At night (they're nocturnal), go around with a flashlight and a stick and let the fun begin. Inside or out, search everywhere. If you see one, put your stick to use. Just make sure you squish it good and proper. If you miss and the widow hits the floor or ground, stomp it good and leave no doubt. Keep in mind that black widows, females especially, are never far from a hiding place. If you screw up, they will go directly into hiding and you might not get another chance."
Several sites suggest sweeping them. How to Destroy a Black Widow Spider Egg Sac | eHow.com
Do you have a wet/dry garage type vacuum? If so, you could treat the inside of it with a pesticide prior to sweeping, then let them die before you dispose of it.
I just wouldn't be comfortable spraying pesticides on my horse's food. =(
Let us know how you do.