So, in normal circumstances, grasshoppers are benign but under certain conditions, they can form swarms.
The rubbing of back legs in nature happens when you have high dense populations - overcrowding due to rushing for limited food in an area. Which evolving wings and flying elsewhere would be nature at play.
Real trick would be to curtail grasshopper numbers before they swarm and perhaps a more balanced approach to grasshoppers natural enemies and encouraging habits for those will do well. Chemicals for me are a desperate solution to a problem that got overlooked. Much the same way that ant do wonders with many pests for plants, yet production methods end up curtailing environments that see them thrive in area's that would benefit. So we end up with chemicals for things like aphids when ants will eat them all up for you.
Crop insurance seems like a more tractable solution.
As for stopping them, I would try covering a wide enough area with a giant electric net (just like those insects killers) also placing additional screening nets on the upper and lower side so that the screens would keep birds away, and the actual electric net would have its holes large enough so that smaller insects would remain mostly untouched. The idea is that if they're stupid enough, they will eat all crops in vicinity then eventually die attempting to attack the only remaining field.
Locusts and Grasshoppers are different insects.
"Locusts are a collection of certain species of short-horned grasshoppers in the family Acrididae that have a swarming phase."
They are grasshoppers that have decided to change behaviour and physical characteristics for some reason. They used to be considered different species.
The ants species I know of "milk" aphids for their sugary excretion, and as a counter service, protect aphids from their predators (such as ladybugs). It's... complicated. I agree with you that chemicals shouldn't be the first thing to reach for. It's interesting to try to find out how we got into this situation in the first place. The book Seeing like a State (https://en.wikipedia.org/wiki/Seeing_Like_a_State) has some interesting (but opiniated, in my opinion) notes on disastrous (on many axes - socially, economically, ecologically, ...) modernist agricultural reform programs that took place in Tanzania, Kenya and Ethiopia in the second half of the 20th century — a region much affected by these locust clouds.
If true, this seems very effective for economic terrorism. I wonder why thats not more common.
By stocking a sufficient population early on, in sufficient confinement,, it might be possible to create a perpetuating, expanding, swarm.
As a form of terrorism, well you would still need to bread and or/find a huge swarm of grasshoppers and if you found such a size large enough, they would already naturally be swarming. Though the prospect of some terrorist group breaking grasshoppers has an edge of Chris Morris about it that bemuses me.
Maybe target them with commercials for pro-depressants? Feeling restless, gregarious, like you just might take ravenous flight? Ask your locust doctor if pro-depressants are right for you.
Then we just need some sexually attractive locusts to lobby the locust doctors and problem solved!