They might be two different issues. General insect collapse is likely dude to the overuse of broad spectrum insecticides. Monarch collapse is more likely due to reductions in milkweed population.

Milkweed is often found on the edges of corn/soy fields so there is definitely an add on effect, but its probably not the root cause. The solution is pretty clear though, if you care about monarchs, plant some milkweed.