Corn syrup works well. My friend made fake blood using cornsyrup and food coloring. Some of it fell on the ground and flies were all over it we started making traps. To make the traps we took some empty 2 liters, cut the tops off, poured the corn syrup in. After those steps flip the cut off top upside down and set it in the top of the bottom half to make a funnel. Tape the edges to secure it if you want.