Duh! They are garden predators. These pests want to eat your young sprouts and will destroy them quickly and efficiently. Bean sprouts are especially prone because the 2 halves of what was your bean seed become the cotyledons. In other words, the first two leaves that the plant forms upon sprouting out of the ground are still viable edible material for these plant assassins.
Bean Sprout With Cotyledons
So, logically, we must protect our young beans from attack so that their initial growth stage is strong. This is something I knew going in, as I remember mom and dad covering the pole beans with milk jug halves implanted 1-2 inches into the ground. This will lead to stronger bean plants and a better harvest. However, I was lazy and took my chances with uncovered beans. Lo and behold, half of them were destroyed within a couple of days, and I was left to replant, having learned my lesson the hard way.
Now, my second crop is just peeking out of the ground, and technology is in order. Humans, be smarter than nature; Use your tools! As I've made clear, I'm poor, and a poor-planner, so I don't have 40 milk jugs on hand with which to protect my young tender sprouts. Therefore, I have used my engineering talents to MacGyver some proper replacements. Actually, to be fair, this isn't really science....just using what's sitting around the kitchen.
Ball Jar "Devices" (hope this works!)
Appeasement Strategies Aside: Ways To Kill The Bastards.
If you have other cheap or easy solutions to this problem, please share in the comments section.