While Praying Mantids are a predator they are fairly passive in that the insect needs to come within reach of the Mantis. They do not go out on search and capture missions. In some places (where you are would be of some help) insect pests were quite numerous while in other places very few of many of the normal ones appeared.
Insect pest control should never be based on just one method but should include a variety of controls.
Grasshoppers preferred diet is living plants, although they will consume dead and dry plant material as well.

I'm in northeast Kansas. Zip code is above. Insects are a pretty big problem in my backyard. I think it is due to a few factors. This is new construction that was carved out of the wilderness pretty much. There is still a green belt of forest behind my property that I am sure is teeming with insects. I also let my grass grow very high in the backyard at times. I grow it high as it is per organic guidelines, but I also have been lazy at times and skipped the back on mowing days because my yard is quite large.

At any rate, my wife insists that the pellets are a factor in our insect issues. At first I attributed her concerns to a lack of knowledge of organic lawn care, but when you think about it, alfalfa would be a pretty darn good meal for a grasshopper. Since it is ground up, wouldn't it be especially conducive to sustaining offspring?